Tag Archives: photography techniques

What Shutter Speed?

Posted on by
5

Shutter speed, and the choices we make over it, can have a profound effect on the outcome of the final image.

Now everyone has a grasp of shutter speed and how it relates to subject movement – at least I hope they do!

We can either use a fast shutter speed to freeze constant action, or we can use a slow shutter speed to:

  • Allow us to capture movement of the subject for creative purposes
  • Allow us to use a lower ISO/smaller aperture when shooting a subject with little or no movement.

 

Fast Shutter Speed – I need MORE LIGHT Barry!

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels

1/8000th sec @ f8, Nikon D4 and 500mm f4

Good strongish sunlight directly behind the camera floods this Red Kite with light when it rolls over into a dive.  I’m daft enough to be doing this session with a 500mm f4 that has very little in the way of natural depth-of-field so I opt to shoot at f8.  Normally I’d expect to be shooting the D4 at 2000iso for action like this but my top end shutter speed is 1/8000th and this shutter speed at f8 was slightly too hot on the exposure front, so I knocked the ISO down to 1600 just to protect the highlights a little more.

Creative Slow Shutter Speed – getting rid of light.

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels

1/5th sec @ f22

I wanted to capture the movement in a flock of seagulls taking off from the water, so now I have to think the opposite way to the Kite shot above.

Firstly I need to think carefully about the length of shutter speed I choose: too short and I won’t capture enough movement; and too long will bring a vertical movement component into the image from me not being able to hold the camera still – so I opt for 1/5th sec.

Next to consider is aperture.  Diffraction on a deliberate motion blur has little impact, but believe it or not focus and depth of field DO – go figure!

So I can run the lens at f16/20/22 without much of a worry, and 100 ISO gets me the 1/5th sec shutter speed I need at f22.

 

Slow Shutter  Rear Curtain Synch Flash

We can use a combination of both techniques in one SINGLE exposure with the employment of flash, rear curtain synch and a relatively slow shutter speed:

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels

6/10th sec @ f3.5 -1Ev rear curtain synch flash

A technique the “Man Cub” uses to great effect in his nightclub photography, here he’s rotated the camera whilst the shutter is open, thus capturing the glowing LEDs and other highlights as circular trails.  As the shutter begins to close, the scene is lit by the 1/10,000th sec burst of light from the reduced power, rear curtain synched SB800 flash unit.

But things are not always quite so cut-and-dried – are they ever?

Assuming the lens you use is tack sharp and the subject is perfectly focused there are two factors that have a direct influence upon how sharp the shot will be:

  • System Vibration – caused by internal vibrations, most notably from the mirror being activated.
  • Camera Shake – caused by external forces like wind, ground vibration or you not holding the camera properly.

Shutter Speed and System Vibration

There was a time when we operated on the old adage that the slowest shutter speed you needed for general hand held shooting was equal to 1/focal length.

So if you were using a 200mm lens you shot with a minimum shutter speed of 1/200th sec, and, for the most part, that rule served us all rather well with 35mm film; assuming of course that 1/200th sec was sufficient to freeze the action!

Now this is a somewhat optimistic rule and assumes that you are hand holding the camera using a good average technique.  But put the camera on a tripod and trigger it with a cable or remote release, and it’s a whole new story.

Why?  Because sticking the camera on a tripod and not touching it during the exposure means that we have taken away the “grounding effect” of our mass from the camera and lens; thus leaving the door open to for system vibration to ruin our image.

 

How Does System Vibration Effect an Image?

Nowadays we live in a digital world with very high resolution sensors instead of film. and the very nature of a sensor – its pixel structure (to use a common parlance) has a direct influence on minimum shutter speed.

So many camera owners today have the misguided notion that using a tripod is the answer to all their prayers in terms of getting sharp images – sadly this ain’t necessarily so.

They also have the other misguided notion that “more megapixels” makes life easier – well, that definitely isn’t true!

The smallest detail that can be recorded by a sensor is a point of light in the projected image that has the same dimensions a one photosite/pixel on that sensor. So, even if a point is SMALLER than the photosite it strikes, its intensity or luminance will effect the whole photosite.

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images.

A point of light smaller than 1 photosite (left) has an effect on the whole photosite (right).

If the lens is capable of resolving this tiny detail, our sensor – in this case (right) – isn’t, and so the lens out-resolves the sensor.

But let’s now consider this tiny point detail and how it effects a sensor of higher resolution; in other words, a sensor with smaller photosites:

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images

The same detail projected onto a higher resolution sensor (right). Though not shown, the entire photosite will be effected, but its surface area represents a much small percentage of the whole sensor area – the sensor now matches the lens resolution.

Now this might seem like a good thing; after all, we can resolve smaller details.  But, there’s a catch when it comes to vibration:

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images

A certain level of vibration causes the small point of light to vibrate. The extremes of this vibration are represented by the the outline circles.

The degree of movement/vibration/oscillation is identical on both sensors; but the resulting effect on the exposure is totally different:

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images

The same level of vibration has more effect on the higher resolution sensor.

If you read the earlier post on sensor resolution and diffraction HERE you’ll soon identify the same concept.

The upshot of it all is that “X” level of internal system vibration has a greater effect on a higher resolution sensor than it does on a lower resolution sensor.

Now what’s all this got to with shutter speed I hear you ask.  Well, whereas 1/focal length used to work pretty well back in the day, we need to advance the theory a little.

Let’s look at four shots from a Nikon D3, shot with a 300mm f2.8, mounted on a tripod and activated by a remote (so no finger-jabbing on the shutter button to effect the images).

Also please note that the lens is MANUALLY FOCUSED just once, so is sharply on the same place for all 4 shots.

These images are full resolution crops, I strongly recommend that you click on all four images to open them in new tabs and view them sequentially.

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images

Shutter = 1/1x (1/320th) Focal Length. No VR, No MLU (Mirror Lock Up). Camera on Tripod+remote release.

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images

Shutter = 1/2x (1/640th) Focal length. No VR. No MLU. Camera on Tripod+remote release.

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images

Shutter = 1/2x Focal length + VR. No MLU. Camera on Tripod+remote release.

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images

Shutter = 1/2x Focal length. Camera on Tripod+remote release + MLU – NO VR + Sandbag.

Now the thing is, the first shot at 1/320th looks crap because it’s riddled with system vibration – mainly a result of what’s termed ‘mirror slap’.  These vibrations travel up the lens barrel and are then reflected back by the front of the lens.  You basically end up with a packet of vibrations running up and down the lens barrel until they eventually die out.

These vibrations in effect make the sensor and the image being projected onto it ‘buzz, shimmy and shake’ – thus we get a fuzzy image; and all the fuzziness is down to internal system vibration.

We would actually have got a sharper shot hand holding the lens – the act of hand holding kills the vibrations!

As you can see in shot 2 we get a big jump in vibration reduction just by cranking the shutter speed up to 2x focal length (actually 1/640th).

The shot would be even sharper at 3x or 4x, because the vibrations are of a set frequency and thus speed of travel, and the faster the shutter speed we use the sooner we can get the exposure over and done with before the vibrations have any effect on the image.

We can employ ‘mirror up shooting’ as a technique to combat these vibrations; by lifting the mirror and then pausing to give the vibrations time to decay; and we could engage the lens VR too, as with the 3rd shot.  Collectively there has been another significant jump in overall sharpness of shot 3; though frankly the VR contribution is minimal.

I’m not a very big fan of VR !

In shot 4 you might get some idea why I’m no fan of VR.  Everything is the same as shot 3 except that the VR is OFF, and we’ve added a 3lb sandbag on top of the lens.  This does the same job as hand holding the lens – it kills the vibrations stone dead.

When you are shooting landscapes with much longer exposures/shutter speeds THE ONLY way to work is tripod plus mirror up shooting AND if you can stand to carry the weight, a good heavy sand bag!

Shot 4 would have been just as sharp if the shutter had been open for 20 seconds, just as long as there was no movement at all in the subject AND there was no ground vibration from a passing heavy goods train (there’s a rail track between the camera and the subject!).

For general tripod shooting of fairly static subjects I was always confident of sharp shots on the D3 (12Mb) at 2x focal length.

But since moving to a 16Mp D4 I’ve now found that sometimes this let’s me down, and that 2.5x focal length is a safer minimum to use.

But that’s nothing compared to what some medium format shooters have told me; where they can still detect the effects of vibration on super high resolution backs such as the IQ180 etc at as much as 5x focal length – and that’s with wide angle landscape style lenses!

So, overall my advice is to ALWAYS push for the highest shutter speed you can possibly obtain from the lighting conditions available.

Where this isn’t possible you really do need to perfect the skill of hand holding – once mastered you’ll be amazed at just how slow a shutter speed you can use WITHOUT employing the VR system (VR/IS often causes far more problems than it would apparently solve).

For long lens shooters the technique of killing vibration at low shutter speeds when the gear is mounted on a tripod is CRITICAL, because without it, the images will suffer just because of the tripod!

The remedy is simple – it’s what your left arm is for.

So, to recap:

  • If you shot without a tripod, the physical act of hand holding – properly – has a tendency to negate internal system vibrations caused by mirror slap etc just because your physical mass is in direct contact with the camera and lens, and so “damps” the vibrations.
  • If you shoot without a tripod you need to ensure that you are using a shutter speed fast enough to negate camera shake.
  • If you shoot without a tripod you need to ensure that you are using a shutter speed fast enough to FREEZE the action/movement of your subject.

 

Camera Shake and STUPID VR!

Now I’m going to have to say at the outset that this is only my opinion, and that this is pointed at Nikons VR system, and I don’t strictly know if Canons IS system works on the same math.

And this is not relevant to sensor-based stabilization, only the ‘in the lens’ type of VR.

The mechanics of how it works are somewhat irrelevant, but what is important is its working methodology.

Nikon VR works at a frequency of 1000Hz.

What is a “hertz”?  Well 1Hz = 1 full frequency cycle per second.  So 1000Hz = 1000 cycles per second, and each cycle is 1/1000th sec in duration.

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images

Full cycle sine wave showing 1,0.5 & 0.25 cycles.

Now then, here’s the thing.  The VR unit is measuring the angular momentum of the lens movement at a rate of 1000 times per second. So in other words it is “sampling” movement every 1/1000th of a second and attempting to compensate for that movement.

But Nyquist-Shannon sampling theory – if you’re up for some mind-warping click HERE – says that effective sampling can only be achieved at half the working frequency – 500 cycles per second.

What is the time duration of one cycle at a frequency of 500Hz?  That’s right – 1/500th sec.

So basically, for normal photography, VR ceases to be of any real use at any shutter speed faster than 1/500th.

Remember shot 3 with the 300mm f2.8 earlier – I said the VR contribution at 1/640th was minimal?  Now you know why I said it!

Looking again at the frequency diagram above, we may get a fairly useful sample at 1/4 working frequency – 1/250th sec; but other than that my personal feelings about VR is that it’s junk – under normal circumstances it should be turned OFF.

What circumstances do I class as abnormal? Sitting on the floor of a heli doing ariel shots out of the open door springs to mind.

If you are working in an environment where something is vibrating YOU while you hand hold the camera then VR comes into its own.

But if it’s YOU doing the vibrating/shaking then it’s not going to help you very much in reality.

Yes, it looks good when you try it in the shop, and the sales twat tells you it’ll buy you three extra stops in shutter speed so now you can get shake-free shots at 1/10th of a second.

But unless you are photographing an anaesthetized Sloth or a statue, that 1/10th sec shutter speed is about as much use to you as a hole in the head. VR/IS only stabilizes the lens image – it doesn’t freeze time and stop a bird from flapping its wings, or indeed a brides veil from billowing in the breeze.

Don’t get me wrong; I’m not saying VR/IS is a total waste of time in ALL circumstances.  But I am saying that it’s a tool that should only be deployed when you need it, and YOU need to understand WHEN that time is; AND you need to be aware that it can cause major image problems if you use it in the wrong situation.

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.

In Conclusion

shutter speed,Red Kite,Andy Astbury,action photography,Wildlife in Pixels,vibration reduction,camera shake,mirror slap,sharp images

1/2000th sec is sufficient to pretty much freeze the forward motion of this eagle, but not the downward motion of the primary feathers.

This rather crappy shot of a White-tailed eagle might give you food for thought, especially if compared with the Red Kite at the start of the post.

The primary feathers are soft because we’ve run out of depth of field.  But, notice the motion blur on them too?  Even though 1/2000th sec in conjunction with a good panning technique is ample to freeze the forward motion of the bird, that same 1/2000th sec is NOT fast enough to freeze the speed of the descending primary feathers on the end of that 4 foot lever called a wing.

Even though your subject as a whole might be still for 1/60th sec or longer, unless it’s dead, some small part of it will move.  The larger the subject is in the frame then more apparent that movement will be.

Getting good sharp shots without motion blur in part of the subject, or camera shake and system vibration screwing up the entire image is easy; as long as you understand the basics – and your best tool to help you on your way is SHUTTER SPEED.

A tack sharp shot without blur but full of high iso noise is vastly superior to a noiseless shot full of blur and vibration artefacting.

Unless it’s done deliberately of course – “H-arty Farty” as my mate Ole Martin Dahle calls it!

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.

Metering Modes Explained

Posted on by
3

Camera Metering Modes

Become a Patron!

I always get asked about which camera metering mode I use,  and to be honest, I think sometimes the folk doing the asking just can’t get their heads around my simplistic, and sometimes quite brutal answers!

“Andy, it’s got to be more complicated than that surely….otherwise why does the camera give me so many options…?”

Well, I always like to keep things really simple, mainly because I’m not the brightest diamond in the jewellery shop, and because I’m getting old and most often times my memory keeps buggering off on holiday without telling me!

But before I espouse on “metering the Uncle Andy way” let’s take a quick look at exactly how the usual metering options work and their effects on exposure.

The Metering Modes

  • Average (a setting usually buried in the center-weighted menu)
  • Spot
  • Center-weighted
  • 3D Matrix (Nikon) or Evaluative (Canon)
Metering Mode Icons

Metering Mode Icons

You can continue reading this article FREE over on my public Patreon posts pages.  Just CLICK HERE

Auto Focus & Shooting Speed

Posted on by
3

Auto Focus & Shooting Speed

Firstly, an apology to my blog followers for the weird blog post notification this morning – I had one of those “senior moments” where I confused the Preview button with Publish – DOH!

There is truly no hope………..!  But let’s get on….

The effectiveness of auto focus and its ability to track and follow a moving subject IS INFLUENCED by frame rate.

Why is this I here you ask.

Well, it’s simple, and logical if you think about it – where are your AF sensors?

They’re in the bottom of your cameras mirror box.

Most folk thing that the mirror just sits there, reflecting at 45 degrees all the light that comes through the lens up to the focus screen and viewfinder.  The fact that the mirror is still DOWN when they are using the auto focus leads most people into thinking the AF sensor array is elsewhere – that’s if they can be bothered to think about it in the first place.

 

So how does the AF array SEE the scene?

Because the center area of the main mirror is only SEMI silvered, and in reality light from the lens does actually pass through it.

 

auto focus,how auto focus works,main mirror,dslr mirror,mirror box,photography,camera

Main mirror of a Nikon D2Xs in the down position.

 

Now I don’t recommend you jam a ball point pen under your own main mirror, but in the next image:

 

auto focus,how auto focus works,main mirror,dslr mirror,mirror box,photography,camera

Main mirror of a Nikon D2Xs lifted so you can see the secondary mirror.

 

Now there’s a really good diagram of the mechanics at http://www.reikan.co.uk/ – makers of FoCal software, and I’ll perhaps get my goolies cut of for linking to it, but here it is:

 

This image belongs to Reikan

 

As you can now hopefully understand, light passes through the mirror and is reflected downwards by the secondary mirror into the AF sensor array.

As long as the mirror is DOWN the auto focus sensor array can see – and so do its job.

Unless the MAIN mirror is fully down, the secondary mirror is not in the correct position to send light to the auto focus sensor array – SO GUESS WHAT – that’s right, your AF ain’t working; or at least it’s just guessing.

So how do we go about giving the main mirror more “down time”?  Simply by slowing the frame rate down is how!

When I’m shooting wildlife using a continuous auto focus mode then I tend to shot at  5 frames per second in Continuous LOW (Nikon-speak) and have the Continuous HIGH setting in reserve set for 9 frames per second.

 

The Scenario Forces Auto Focus Settings Choices

From a photography perspective we are mainly concerned with subjects CROSSING or subjects CLOSING our camera position.

Once focus is acquired on a CROSSING subject (one that’s not changing its distance from the camera) then I might elect to use a faster frame rate as mirror-down-time isn’t so critical.

But subjects that are either CLOSING or CROSSING & CLOSING are far more common; and head on CLOSING subjects are the ones that give our auto focus systems the hardest workout – and show the system failures and short-comings the most.

Consider the focus scale on any lens you happen to have handy – as you focus closer to you the scale divisions get further apart; in other words the lens focus unit has to move further to change from say 10 meters to 5 meters than it does to move from 15 meters to 10 meters – it’s a non-linear scale of change.

So the closer a subject comes to your camera position the greater is the need for the auto focus sensors to see the subject AND react to its changed position – and yes, by the time it’s acquired focus and is ready to take the next frame the subject is now even closer – and things get very messy!

That’s why high grade dSLR auto focus systems have ‘predictive algorithms’ built into them.

Also. the amount of light on the scene AND the contrast between subject and background ALL effect the ability of the auto focus to do its job.  Even though most pro-summer and all pro body systems use phase detection auto focus, contrast between the subject to be tracked and its background does impact the efficiency of the overall system.

A swan against a dark background is a lot easier on the auto focus system than a panther in the jungle or a white-tailed eagle against a towering granite cliff in Norway, but the AF system in most cameras is perfectly capable of acquiring, locking on and tracking any of the above subjects.

So as a basic rule of thumb the more CLOSING a subject is then the LOWER your frame rate needs to be if you are looking for a sharp sequence of shots.  Conversely the more CROSSING a subject is then the higher the frame rate can be and you might still get away with it.

 

Points to Clarify

The mechanical actions of an exposure are:

  1. Mirror lifts
  2. Front shutter curtain falls
  3. Rear shutter curtain falls
  4. Mirror falls closed (down)

Here’s the thing; the individual time taken for each of these actions is the same ALL the time – irrespective of whether the shutter speed is 1/8000th sec or 8 sec; it’s the gap in between 2. & 3. that makes the difference.

And it’s the ONLY thing shutter-related we’ve got any control over.

So one full exposure takes t1 + t2 + shutter speed + t3 +t4, and the gap between t4 and the repeat of t1 on the next frame is what gives us our mirror down time between shots for any given frame rate.  So it’s this time gap between t4 and the repeat of t1 that we lengthen by dropping the shooting speed frame rate.

There’s another problem with using 10 or 11 frames per second with Nikon D3/D4 bodies.

10 fps on a D3 LOCKS the exposure to the values/settings of the first frame in the burst.

11 fps on a D3 LOCKS both exposure AND auto focus to the values/settings of the first frame in the burst.

11 fps on a D4 LOCKS both exposure AND auto focus* to those of the first frame in the burst – and it’s one heck of a burst to shoot where all the shots can be out of focus (and badly exposed) except the first one!

*Page 112 of the D4 manual says that at 11fps the second and subsequent shots in a burst may not be in focus or exposed correctly.

That’s Nikon-speak for “If you are photographing a statue or a parked car ALL your shots will be sharp and exposed the same; but don’t try shooting anything that’s getting closer to the camera, and don’t try shooting things where the frame exposure value changes”.

 

There’s a really cool video of 11 fps slowed right down with 5000fps slo-mo  HERE  but for Christ’ sake turn your volume down because the ST is some Marlene Dietrich wannabe!

So if you want to shoot action sequences that are sharp from the first frame to the last then remember – DON’T be greedy – SLOW DOWN!

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.

Flash Photography

Posted on by
4

Flash Photography

 

Red Squirrel,Andy Astbury,Flash,flash photography,fill flash,photography techniques

Really Cute Red Squirrel

 

On Sunday myself and my buddy Mark Davies made a short foray up to the Lake District and our small Red Squirrel site.  The weather was horrible, sleet, sun. rain, cloudy, sunny then rain again – in other words just not conducive to a half-descent session on the D4.

The one Achilles Heal with this site is the fact that it’s hard to get a descent background for your shots – it’s in the middle of a small wooded valley and you just can’t get away from tree trunks in the background.

This is further complicated by the fact that the “Squidgers” have a propensity for keeping in the ‘not so sunny’ bits, so frequently you end up with a scenario where backgrounds are brighter than foregrounds – which just won’t DO!

So what’s needed is some way to switch the lighting balance around to give a brighter foreground/subject AND a darker background.

Now that sounds all very well BUT; how do we achieve it?

Reflectors perhaps?  They’d do the trick but have one big problem; they rely on AMBIENT light  – and in the conditions we were shooting in the other day the value of the ambient light was up and down like a Yo-Yo.

Wouldn’t it be cool if we could have a consistent level of subject/foreground illumination AND at the same time have some degree of control over the exposure of the background?

Well with flash we can do just that!

Let’s look at a shot without flash:

 

No FLASH

No FLASH, AMBIENT light only – 1/320th @ f7.1

 

I don’t suppose this shot is too bad because the background isn’t strongly lit by the sun (it’s gone behind a cloud again!) but the foreground and background are pretty much the same exposure-wise.  For me there is not enough tonal separation between the two areas of the image, and the lighting is a bit flat.

If we could knock a stop or so out of the background; under expose it, then the image would have more tonal separation between foreground and background, and would look a lot better, but of course if we’re just working with ambient light then our adjusted exposure would under expose the foreground as well, so we’d be no better off.

Now look at the next image – we’ve got a background that’s under exposed by around  -1.5Ev, but the subject and foreground are lit pretty much to the same degree as before, and we’ve got a little more shape and form to the squirrel itself – it’s not quite so flat-looking.

 

With FLASH

With FLASH added – 1/800th @ f7.1

 

The image also has the slight sense that it’s been shot in more sunny conditions – which I can promise you it wasn’t !

And both images are basically straight off the camera, just with my neutral camera profile applied to them on import.

 

The Set Up

The Setup - shocking iPhone 3 quality!

The Setup – shocking iPhone 3 quality!

 

The first secret to good looking flash photography OF ANY KIND is to get the damn flash OFF the camera.

If we were in a totally dark studio with the sexiest looking model on the planet we’d NOT be lighting her with one light from the camera position now would we?

So we use basic studio lighting layouts where ever we can.

There are two other things to consider too:

  •   It’s broad daylight, so our exposure will contain both FLASH and an element of AMBIENT light – so we are working along the premise of ADDING to what’s already there.
  •   If we put the flash closer to the subject (off camera) then the output energy has less distance to travel in order to do its job – so it doesn’t have to have as much power behind it as it would have if emanating from the camera position.

 

You can see in the horrible iPhone 3 shot I took of the setup that I’m using two flash guns with white Lambency diffusers on them; one on a stand to the left and slightly in front of the log where the squirrels will sit, and one placed on the set base (Mr. Davies old knackered Black & Decker Workmate!) slightly behind the log and about the same distance away from where I anticipate a squirrel will sit on the log as the left flash.

The thing to note here is that I’m using the SIDE output of these Lambency diffuser domes and NOT the front – that’s why they are pointed up at the sky. The side output of these diffusers is very soft – just what the flash photography doctor ordered in terms of ‘keeping it real’.

The left light is going to be my MAIN light, the right is my FILL light.

The sun, when & if it decides to pop its head out, will be behind me and to my left so I place my MAIN light in a position where it will ‘simulate’ said ball in the sky.

The FILL light basically exists to ‘counter balance’ the ‘directionality’ of the MAIN light, and to weaken any shadows thrown by the MAIN light.

Does this flash bother a subject? For the most part NOT SO YOU’D NOTICE!

Take a look at the shot below – the caption will be relevant shortly.

This SB800 has just fired in "front curtain synch" and the balance of the exposure is from the ambient light - the shutter is still open after the flash has died. Does the squirrel look bothered?

This SB800 has just fired in “front curtain synch” and the balance of the exposure is from the ambient light. Does the squirrel look bothered?

Settings & The Black Art!

Before we talk about anything else I need to address the shutter curtain synch question.

We have two curtain synch options, FRONT & REAR.

Front Curtain (as in the shot above) – this means that the flash will fire as the front curtain starts to move, and most likely, the flash will be finished long before the rear curtain closes. If your subject reacts to the flash then some element of subject movement might be present in the shot due to the ambient light part of the exposure.

Rear Curtain Synch – my recommended ‘modus operandi’ – the ‘ambient only’ part of the exposure gets done first, then the flash fires as the rear curtain begins to close the exposure. This way, if the subject reacts to the flash the exposure will be over before it has chance to – MOSTLY!

The framing I want, and the depth of field I want dictates my camera position and aperture – in this case f7 or f8 – actually f7.1 is what I went for.

 

I elect to go with 2000 iso on the D4.

So now my only variable is shutter speed.

Ambient light dictates that to be 1/320th on average, and I want to UNDER EXPOSE that background by at least a stop and a bit (technical terms indeed!) so I elect to use a shutter speed of 1/800th.

So that’s it – I’m done; seeing as the light from the flashes will be constant my foreground/subject will ALWAYS be exposed correctly. In rear curtain synch I’ll negate the risk of subject movement ‘ghosting’ in the image, and at 1/800th I’ll have a far better chance of eliminating motion blur caused by a squirrel chewing food or twitching its whiskers etc.

 

Triggering Off-Camera Flashes

 

We can fire off-camera flashes in a number of ways, but distance, wet ground, occasional rain and squirrels with a propensity for chewing everything they see means CORDS ain’t one of ’em!

With the Nikon system that I obviously use we could employ another flash on-camera in MASTER/COMMANDER mode, with the flash pulse deactivated; or a dedicated commander such as the SU800; or if your camera has one, the built-in flash if it has a commander mode in the menu.

The one problem with Nikon CLS triggering system, and Canons as far as I know, is the reliance upon infra-red as the communication band. This is prone to a degree of unreliability in what we might term ‘dodgy’ conditions outdoors.

I use a Pocket Wizard MiniTT1 atop the camera and a FlexTT5 under my main light. The beauty of this system is that the comms is RADIO – far more reliable outdoors than IR.

Because a. I’m poor and can’t afford another TT5, and b. the proximity of my MAIN and FILL light, I put the SB800 FILL light in SU mode so it gets triggered by the flash from the MAIN light.

What I wouldn’t give for a dozen Nikon SB901’s and 12 TT5s – I’d kill for them!

The MAIN light itself is in TTL FP mode.

The beauty of this setup is that the MAIN light ‘thinks’ the TT5 is a camera, and the camera ‘thinks’ the miniTTL is a flash gun, so I have direct communication between camera and flash of iso and aperture information.

Also, I can turn the flash output down by up to -3Ev using the flash exposure compensation button without it having an effect on the background ambient exposure.

Don’t forget, seeing as my exposure is always going to 1/800th @ f7.1 at 2000 iso the CAMERA is in MANUAL exposure mode. So as long as the two flashes output enough light to expose the subject correctly at those settings (which they always will until the batteries die!) I basically can’t go wrong.

When shooting like this I also have a major leaning towards shooting in single servo – one shot at a time with just one AF point active.

 

Flash Photography – Flash Duration or Burn Time

Now here’s what you need to get your head around. As you vary the output of a flash like the SB800 the DURATION of the flash or BURN TIME of the tube changes

Below are the quoted figures for the Nikon SB800, burn time/output:

1/1050 sec. at M1/1 (full) output
1/1100 sec. at M1/2 output
1/2700 sec. at M1/4 output
1/5900 sec. at M1/8 output
1/10900 sec. at M1/16 output
1/17800 sec. at M1/32 output
1/32300 sec. at M1/64 output
1/41600 sec. at M1/128 output

On top of that there’s something else we need to take into account – and this goes for Canon shooters too; though Canon terminology is different.

Shutter Speed & The FP Option

35mm format cameras all have a falling curtain shutter with two curtains, a front one, and a rear one.

As your press the shutter button the FRONT curtain starts to fall, then the rear curtain starts to chase after it, the two meet at the bottom of the shutter plane and the exposure is over.

The LONGER or slower the shutter speed the greater head-start the front curtain has!

At speeds of 1/250th and slower the front curtain has reached the end of its travel BEFORE the rear curtain wakes up and decides to move – in other words THE SENSOR is FULLY exposed.

The fastest shutter speed that results in a FULLY EXPOSED film plane/sensor is the basic camera-to-flash synch speed; X synch as it used to be called, and when I started learning about photography this was usually 1/60th; and on some really crap cameras it was 1/30th!

But with modern technology and light weight materials these curtains can now get moving a lot faster, so basic synch now runs at 1/250th for a full frame DSLR.

If you go into your flash camera menu you’ll find an AUTO FP setting for Nikon, Canon refer to this as HSS or High Speed Synch – which makes far more sense (Nikon please take note, Canon got something right so please replicate!).

There’s something of an argument as to whether FP stands for Focal Plane or Flash Pulse; and frankly both are applicable, but it means the same as Canon’s HSS or High Speed Synch.

At speeds above/faster than 1/250th the sensor/film plane is NOT fully exposed. The gap between the front and rear curtains forms a slot or ‘letter box’ that travels downwards across the face of the sensor, so the image is, if you like, ‘scanned’ onto the imaging plane.

Obviously this is going to cause on heck of an exposure problem if the flash output is ‘dumped’ as a single pulse.

So FP/HSS mode physically pulses or strobes the flash output to the point where it behaves like a continuous light source.

If the flash was to fire with a single pulse then the ‘letterbox slot’ would receive the flash exposure, but you’d end up with bands of under exposure at the bottom or top of the image depending on the curtain synch mode – front or rear.

In FP/HSS mode the power output of each individual pulse in the sequence will drop as the shutter speed shortens, so even though you might have 1:1 power selected on the back of the flash itself (which I usually do on the MAIN light, and 1/2 on the FILL light) the pulses of light will be of lower power, but their cumulative effect gives the desired result.

By reviewing the shot on the back of the camera we can compensate for changes in ambient in the entire scene (we might want to dilute the effect of the main light somewhat if the sun suddenly breaks out on the subject as well as the background) by raising the shutter speed a little – or we might want to lighten the shot globally by lowering the shutter speed if it suddenly goes very gloomy.

We might want to change the balance between ambient and flash; this again can be done from the camera with the flash exposure compensation controls; or if needs be, by physically getting up and moving the flash units are little nearer or further away from the subject.

All in all, using flash is really easy, and always has been.

Except nowadays manufacturers tend to put far more controls and modes on things then are really necessary; the upshot of which is to frighten the uninitiated and then confuse them even further with instruction manuals that appear to be written by someone under the influence of Class A drugs!

 

"Trouble Brewing.." Confrontation over the right to feed between two Red Squirrels.

“Trouble Brewing..” Confrontation over the right to feed between two Red Squirrels.

 

The whole idea of flash is that it should do its job but leave no obvious trace to the viewer.

But its benefits to you as the photographer are invaluable – higher shutter speeds, more depth of field and better isolation of the subject from its background are the three main ones that you need to be taking advantage of right now.

If you have the gear and don’t understand how to use it then why not book a tuition day with me – then perhaps I could afford some more TT5s!

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.

Gamma Encoding – Under the Hood

Posted on by
Reply

Gamma, Gamma Encoding & Decoding

Gamma – now there’s a term I see cause so much confusion and misunderstanding.

So many people use the term without knowing what it means.

Others get gamma mixed up with contrast, which is the worst mistake anyone could ever make!

Contrast controls the spatial relationship between black and white; in other words the number of grey tones.  Higher contrast spreads black into the darker mid tones and white into the upper mid tones.  In other words, both the black point and white point are moved.

The only tones that are not effected by changes in image gamma are the black point and white point – that’s why getting gamma mixed up with contrast is the mark of a “complete idiot” who should be taken outside and summarily shot before they have chance to propagate this shocking level of misunderstanding!

What is Gamma?

Any device that records an image does so with a gamma value.

Any device which displays/reproduces said image does so with a gamma value.

We can think of gamma as the proportional distribution of tones recorded by, or displayed on, a particular device.

Because different devices have different gamma values problems would arise were we to display an image that has a gamma of X on a display with a gamma of Y:

Ever wondered what a RAW file would look like displayed on a monitor without any fancy colour & gamma managed software such as LR or ACR?

gamma,gamma encoding,Andy Astbury

A raw file displayed on the back of the camera (left) and as it would look on a computer monitor calibrated to a gamma of 2.2 & without any colour & gamma management (right).

The right hand image looks so dark because it has a native gamma of 1.0 but is being displayed on a monitor with a native gamma of 2.2

RAW file Gamma

To all intents and purposes ALL RAW files have a gamma of 1.0

gamma,gamma encoding,Andy Astbury

Camera Sensor/Linear Gamma (Gamma 1.0)

Digital camera sensors work in a linear fashion:

If we have “X” number of photons striking a sensor photosite then “Y” amount of electrons will be generated.

Double the number of photons by doubling the amount of light, then 2x “Y” electrons will be generated.

Halve the number of photons by reducing the light on the scene by 50% then 0.5x “Y” electrons will be generated.

We have two axes on the graph; the horizontal x axis represents the actual light values in the scene, and the vertical y axis represents the output or recorded tones in the image.

So, if we apply Lab L* values to our graph axes above, then 0 equates to black and 1.0 equates to white.

The “slope” of the graph is a straight line giving us an equal relationship between values for input and output.

It’s this relationship between input and output values in digital imaging that helps define GAMMA.

In our particular case here, we have a linear relationship between input and output values and so we have LINEAR GAMMA, otherwise known as gamma 1.0.

Now let’s look at a black to white graduation in gamma 1.0 in comparison to one in what’s called an encoding gamma:

gamma,gamma encoding,Andy Astbury

Linear (top) vs Encoded Gamma

The upper gradient is basically the way our digital cameras see and record a scene.

There is an awful lot of information about highlights and yet the darker tones and ‘shadow’ areas are seemingly squashed up together on the left side of the gradient.

Human vision does not see things in the same way that a camera sensor does; we do not see linearly.

If the amount of ambient light falling on a scene suddenly doubles we will perceive the increase as an unquantifiable “it’s got brighter”; whereas our sensors response will be exactly double and very quantifiable.

Our eyes see a far more ‘perceptually even’ tonal distribution with much greater tonal separation in the darker tones and a more compressed distribution of highlights.

In other words we see a tonal distribution more like that contained in the gamma encoded gradient.

Gamma encoding can be best illustrated with another graph:

gamma,gamma encoding,Andy Astbury

Linear Gamma vs Gamma Encoding 1/2.2 (0.4545)

Now sadly this is where things often get misunderstood, and why you need to be careful about where you get information from.

The cyan curve is NOT gamma 2.2 – we’ll get to that shortly.

Think of the graph above as the curves panel in Lightroom, ACR or Photoshop – after all, that’s exactly what it is.

Think of our dark, low contrast linear gamma image as displayed on a monitor – what would we need to do to the linear slope  to improve contrast and generally brighten the image?

We’d bend the linear slope to something like the cyan curve.

The cyan curve is the encoding gamma 1/2.2.

There’s a direct numerical relationship between the two gamma curves; linear and 1/2.2. and it’s a simple power law:

  •  VO = VIγ where VO = output value, VI = input value and γ = gamma

Any input value (VI) on the linear gamma curve to the power of γ equals the output value of the cyan encoding curve; and γ as it works out equals 0.4545

  •  VI 0 = VO 0
  •  VI 0.25 = VO 0.532
  •  VI 0.50 = VO 0.729
  •  VI 0.75 = VO 0.878
  •  VI 1.0 = VO 1.0

Now isn’t that bit of maths sexy………………..yeah!

Basically the gamma encoding process remaps all the tones in the image and redistributes them in a non-linear ratio which is more familiar to our eye.

Note: the gamma of human vision is not really gamma 1/2.2 – gamma 0.4545.  It would be near impossible to actually quantify gamma for our eye due to the behavior of the iris etc, but to all intents and purposes modern photographic principles regard it as being ‘similar to’..

So the story so far equates to this:

gamma,gamma encoding,Andy Astbury

Gamma encoding redistributes tones in a non-linear manner.

But things are never quite so straight forward are they…?

Firstly, if gamma < 1 (less than 1) the encoding curve goes upwards – as does the cyan curve in the graph above.

But if gamma > 1 (greater than 1) the curve goes downwards.

A calibrated monitor has (or should have) a calibrated device gamma of 2.2:

gamma,gamma encoding,Andy Astbury

Linear, Encoding & Monitor gamma curves.

As you can now see, the monitor device gamma of 2.2 is the opposite of the encoding gamma – after all, the latter is the reciprocal of the former.

So what happens when we apply the decoding gamma/monitor gamma of 2.2 to our gamma encoded image?

gamma,gamma encoding,Andy Astbury

The net effect of Encode & Decode gamma – Linear.

That’s right, we end up back where we started!

Now, are you thinking:

  • Don’t understand?
  • We are back with our super dark image again?

Welcome to the worlds biggest Bear-Trap!

The “Learning Gamma Bear Trap”

Hands up those who are thinking this is what happens:

gamma,gamma encoding,Andy Astbury

If your arm so much as twitched then you are not alone!

I’ll admit to being naughty and leading you to edge of the pit containing the bear trap – but I didn’t push you!

While you’ve been reading this post have you noticed the occasional random bold and underlined text?

Them’s clues folks!

The super dark images – both seascape and the rope coil – are all “GAMMA 1.0 displayed on a GAMMA 2.2 device without any management”.

That doesn’t mean a gamma 1.0 RAW file actually LOOKS like that in it’s own gamma environment!

That’s the bear trap!

gamma,gamma encoding,Andy Astbury

Gamma 1.0 to gamma 2.2 encoding and decoding

Our RAW file actually looks quite normal in its own gamma environment (2nd from left) – but look at the histogram and how all those darker mid tones and shadows are piled up to the left.

Gamma encoding to 1/2.2 (gamma 0.4545) redistributes and remaps those all the tones and lightens the image by pushing the curve up BUT leaves the black and white points where they are.  No tones have been added or taken away, the operation just redistributes what’s already there.  Check out the histogram.

Then the gamma decode operation takes place and we end up with the image on the right – looks perfect and ready for processing, but notice the histogram, we keep the encoding redistribution of tones.

So, are we back where we started?  No.

Luckily for us gamma encoding and decoding is all fully automatic within a colour managed work flow and RAW handlers such as Lightroom, ACR and CapOnePro etc.

Image gamma changes are required when an image is moved from one RGB colour space to another:

  • ProPhoto RGB has a gamma of 1.8
  • Adobe RGB 1998 has a gamma of 2.2
  • sRGB has an oddball gamma that equates to an average of 2.2 but is nearly 1.8 in the deep shadow tones.
  • Lightrooms working colour space is ProPhoto linear, in other words gamma 1.0
  • Lightrooms viewing space is MelissaRGB which equates to Prophoto with an sRGB gamma.

Image gamma changes need to occur when images are sent to a desktop printer – the encode/decode characteristics are actually part and parcel of the printer profile information.

Gamma awareness should be exercised when it comes to monitors:

  • Most plug & play monitors are set to far too high a gamma ‘out the box’ – get it calibrated properly ASAP; it’s not just about colour accuracy.
  • Laptop screen gamma changes with viewing position – God they are awful!

Anyway, that just about wraps up this brief explanation of gamma; believe me it is brief and somewhat simplified – but hopefully you get the picture!

Become a Patron!

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.

Photoshop CC Update

Posted on by
2

Photoshop CC Update

Installing a new Photoshop CC update is supposed to be a simple matter of clicking a button and the job gets done.

This morning both my Mac systems were telling me to update from v14.1.2 to v14.2

I have two Macs, a late 2012 iMac and a mid 2009 Mac Pro.  The Mac Pro used to run Snow Leopard but was upgraded to Mountain Lion because of Lightroom 5 dropping Snow Leopard support.

Now I never have any problems with Cloud Updates from Adobe on the iMac, but sometimes the Mac Pro can do some strange things – and this morning was no exception!

The update installed on the iMac without a hitch, but when the update was complete on the Mac Pro I was greeted with a message telling me that some components had not installed correctly.  On opening Photoshop CC I was greeted with the fact that the version had rolled back to v14.0 and that hitting UPDATE in both the app and my CC control panel simply informed me that my software was up to date and no updates were available!

So I just thought I’d do a blog entry on what to do if this ever happens to you!

 

Remove Photoshop CC

The first thing to do is UNINSTALL  Photoshop CC with the supplied uninstaller.

You’ll find this in the main Photoshop CC root directory:

Photoshop CC Update

Locate the Photoshop CC Uninstaller.

Take my advice and put a tick in the check box to “Remove Preferences” – the Photoshop preferences file can be a royal pain in the ass sometimes, so dump it – a new one will get written as soon as your fire Photoshop up after the new install.

Click UNINSTALL.

Once this action is complete YOU MUST RESTART THE MACHINE.

 

After the restart wait for the Creative Cloud to connect then open your CC control panel.

Under the Apps tab you’ll see that Photoshop CC is no longer listed.

Scroll down past all the apps Adobe have listed and you’ll come to Photoshop CC;  it’ll have an INSTALL button next to it – click the install button:

Photoshop CC Update

Install Photoshop CC from the Cloud control panel.

If you are installing the 14.1.2 to 14.2 update (the current one as of today’s date) you might find a couple of long ‘stick bits’ during the installation process – notably between 1 and 20% and a long one at 90% – just let the machine do it’s thing.

When the update is complete I’d recommend you do a restart – it might not be necessary, but I do it anyway.

Once the machine has restarted fire up Photoshop, click on ‘About Photoshop’ and you should see:

Photoshop CC Update

Photoshop “about screen” showing version number.

Because we dumped the preferences file we need to go and change the defaults for best best working practice:

Photoshop CC Update

Preferences Interface tab.

If you want to change the BG colour then do it here.

Next, click File Handling:

Photoshop CC Update

File handling tab in Photoshop Preferences

Remove the tick from the SAVE IN BACKGROUND check box – like the person who put it there, you too might think background auto-save is a good idea – IT ISN’T – think about it!

Finally, go to Performance:

Photoshop CC Update

Photoshop preferences Performance tab

and change the Scratch Disc to somewhere other than your system drive if you have the internal drives fitted.  If you only have 1 internal drive then leave “as is”.  You ‘could’ use an external drive as a scratch disk, but to be honest it really does need to be a fast drive over a fast connection – USB 2 to an old 250Gb portable isn’t really going to cut it!

You can go and check your Colour Settings, though these should not have changed – assuming you had ’em set right in the first place!

Here’s what they SHOULD look like:

Photoshop CC Update

Photoshop PROPER COLOUR SETTINGS!

That’s it – you’re done!

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.

Please consider supporting this blog.

This blog really does need your support. All the information I put on these pages I do freely, but it does involve costs in both time and money.

If you find this post useful and informative please could you help by making a small donation – it would really help me out a lot – whatever you can afford would be gratefully received.

Your donation will help offset the costs of running this blog and so help me to bring you lots more useful and informative content.

Many thanks in advance.

 

Accurate Camera Colour within Lightroom

Posted on by
8

Obtaining accurate camera colour within Lightroom 5, in other words making the pics in your Lr Library look like they did on the back of the camera; is a problem that I’m asked about more and more since the advent of Lightroom 5 AND the latest camera marks – especially Nikon!

UPDATE NOTE: Please feel free to read this post THEN go HERE for a further post on achieving image NEUTRALITY in Lightroom 6/CC 2015

Does this problem look familiar?

Accurate Camera Colour within Lightroom

Back of the camera (left) to Lightroom (right) – click to enlarge.

The image looks fine (left) on the back of the camera, fine in the import dialogue box, and fine in the library module grid view UNTIL the previews have been created – then it looks like the image on the right.

I hear complaints that the colours are too saturated and the contrast has gone through the roof, the exposure has gone down etc etc.

All the visual descriptions are correct, but what’s responsible for the changes is mostly down to a shift in contrast.

Let’s have a closer look at the problem:

Accurate Camera Colour within Lightroom

Back of the camera (left) to Lightroom (right) – click to enlarge.

The increase in contrast has resulted in “choking” of the shadow detail under the wing of the Red Kite, loss of tonal separation in the darker mid tones, and a slight increase in the apparent luminance noise level – especially in that out-of-focus blue sky.

And of course, the other big side effect is an apparent increase in saturation.

You should all be aware of my saying that “Contrast Be Thine Enemy” by now – and so we’re hardly getting off to a good start with a situation like this are we…………

So how do we go about obtaining accurate camera colour within Lightroom?

Firstly, we need to understand just what’s going on inside the camera with regard to various settings, and what happens to those settings when we import the image into Lightroom.

Camera Settings & RAW files

Let’s consider all the various settings with regard to image control that we have in our cameras:

  • White Balance
  • Active D lighting
  • Picture Control – scene settings, sharpening etc:
  • Colour Space
  • Distortion Control
  • Vignette Control
  • High ISO NR
  • Focus Point/Group
  • Uncle Tom Cobbly & all…………..

All these are brought to bare to give us the post-view jpeg on the back of the camera.

And let’s not forget

  • Exif
  • IPTC

That post-view/review jpeg IS subjected to all the above image control settings, and is embedded in the RAW file; and the image control settings are recorded in what is called the raw file “header”.

It’s actually a lot more complex than that, with IFD & MakerNote tags and other “scrummy” tech stuff – see this ‘interesting’ article HERE – but don’t fall asleep!

If we ship the raw file to our camera manufacturers RAW file handler software such as Nikon CapNX then the embedded jpeg and the raw header data form the image preview.

However, to equip Lightroom with the ability to read headers from every digital camera on the planet would be physically impossible, and in my opinion, totally undesirable as it’s a far better raw handler than any proprietary offering from Nikon or Canon et al.

So, in a nutshell, Lightroom – and ACR – bin the embedded jpeg preview and ignore the raw file header, with the exception of white balance, together with Exif & IPTC data.

However, we still need to value the post jpeg on the camera because we use it to decide many things about exposure, DoF, focus point etc – so the impact of the various camera image settings upon that image have to be assessed.

Now here’s the thing about image control settings “in camera”.

For the most part they increase contrast, saturation and vibrancy – and as a consequence can DECREASE apparent DYNAMIC RANGE.  Now I’d rather have total control over the look and feel of my image rather than hand that control over to some poxy bit of cheap post-ASIC circuitry inside my camera.

So my recommendations are always the same – all in-camera ‘picture control’ type settings should be turned OFF; and those that can’t be turned off are set to LOW or NEUTRAL as applicable.

That way, when I view the post jpeg on the back of the camera I’m viewing the very best rendition possible of what the sensor has captured.

And it’s pointless having it any other way because when you’re shooting RAW then both Lightroom and Photoshop ACR ignore them anyway!

Accurate Camera Colour within Lightroom

So how do we obtain accurate camera colour within Lightroom?

We can begin to understand how to achieve accurate camera colour within Lightroom if we look at what happens when we import a raw file; and it’s really simple.

Lightroom needs to be “told” how to interpret the data in the raw file in order to render a viewable preview – let’s not forget folks, a raw file is NOT a visible image, just a matrix full of numbers.

In order to do this seemingly simple job Lightroom uses process version and camera calibration settings that ship inside it, telling it how to do the “initial process” of the image – if you like, it’s a default process setting.

And what do you think the default camera calibration setting is?

Accurate Camera Colour within Lightroom

The ‘contrasty’ result of the Lightroom Nikon D4 Adobe Standard camera profile.

Lightroom defaults to this displayed nomenclature “Adobe Standard” camera profile irrespective of what camera make and model the raw file is recorded by.

Importantly – you need to bare in mind that this ‘standard’ profile is camera-specific in its effect, even though the displayed name is the same when handling say D800E NEF files as it is when handling 1DX CR2 files, the background functionality is totally different and specific to the make and model of camera.

What it says on the tin is NOT what’s inside – so to speak!

So this “Adobe Standard” has as many differing effects on the overall image look as there are cameras that Lightroom supports – is it ever likely that some of them are a bit crap??!!

Some files, such as the Nikon D800 and Canon 5D3 raws seem to suffer very little if any change – in my experience at any rate – but as a D4 shooter this ‘glitch in the system’ drives me nuts.

But the walk-around is so damned easy it’s not worth stressing about:

  1. Bring said image into Lightroom (as above).
  2. Move the image to the DEVELOP module
  3. Go to the bottom settings panel – Camera Calibration.
  4. Select “Camera Neutral” from the drop-down menu:
    Accurate Camera Colour within Lightroom

    Change camera profile from ‘Adobe Standard’ to ‘Camera Neutral’ – see the difference!

    You can see that I’ve added a -25 contrast adjustment in the basics panel here too – you might not want to do that*

  5. Scoot over to the source panel side of the Lightroom GUI and open up the Presets Panel

    Accurate Camera Colour within Lightroom

    Open Presets Panel (indicated) and click the + sign to create a new preset.

  6. Give the new preset a name, and then check the Process Version and Calibration options (because of the -25 contrast adjustment I’ve added here the Contrast option is ticked).
  7. Click CREATE and the new “camera profile preset” will be stored in the USER PRESETS across ALL your Lightroom 5 catalogs.
  8. The next time you import RAW files you can ADD this preset as a DEVELOP SETTING in the import dialogue box:
    Accurate Camera Colour within Lightroom

    Choose new preset

    Accurate Camera Colour within Lightroom

    Begin the import

  9. Your images will now look like they did on the back of the camera (if you adopt my approach to camera settings at least!).

You can play around with this procedure as much as you like – I have quite a few presets for this “initial process” depending on a number of variables such as light quality and ISO used to name but two criteria (as you can see in the first image at 8. above).

The big thing I need you to understand is that the camera profile in the Camera Calibration panel of Lightroom acts merely as Lightroom’s own internal guide to the initial process settings it needs to apply to the raw file when generating it’s library module previews.

There’s nothing complicated, mysterious or sinister going on, and no changes are being made to your raw images – there’s nothing to change.

In fact, I don’t even bother switching to Camera Neutral half the time; I just do a rough initial process in the Develop module to negate the contrast in the image, and perhaps noise if I’ve been cranking the ISO a bit – then save that out as a preset.

Then again, there are occasions when I find switching to Camera Neutral is all that’s needed –  shooting low ISO wide angle landscapes when I’m using the full extent of the sensors dynamic range springs to mind.

But at least now you’ve got shots within your Lightroom library that look like they did on the back of the camera, and you haven’t got to start undoing the mess it’s made on import before you get on with the proper task at hand – processing – and keeping that contrast under control.

Some twat on a forum somewhere slagged this post off the other day saying that I was misleading folk into thinking that the shot on the back of the camera was “neutral” – WHAT A PRICK…………

All we are trying to do here is to make the image previews in Lr5 look like they did on the back of the camera – after all, it is this BACK OF CAMERA image that made us happy with the shot in the first place.

And by ‘neutralising’ the in-camera sharpening and colour/contrast picture control ramping the crappy ‘in camera’ jpeg is the best rendition we have of what the sensor saw while the shutter was open.

Yes, we are going to process the image and make it look even better, so our Lr5 preview starting point is somewhat irrelevant in the long run; but a lot of folk freak-out because Lr5 can make some really bad changes to the look of their images before they start.  All we are doing in this article is stopping Lr5 from making those unwanted changes.

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.

MTF, Lens & Sensor Resolution

Posted on by
2

MTF, Lens & Sensor Resolution

I’ve been ‘banging on’ about resolution lens performance and MTF over the last few posts so I’d like to start bringing all these various bits of information together with at least a modicum of simplicity.

If this is your first visit to my blog I strongly recommend you peruse HERE and HERE before going any further!

You might well ask the question “Do I really need to know this stuff – you’re a pro Andy and I’m not, so I don’t think I need to…”

My answer is “Yes you bloody well do need to know, so stop whinging – it’ll save you time and perhaps stop you wasting money…”

Words used like ‘resolution’ do tend to get used out of context sometimes, and when you guys ‘n gals are learning this stuff then things can get a mite confusing – and nowhere does terminology get more confusing than when we are talking ‘glass’.

But before we get into the idea of bringing lenses and sensors together I want to introduce you to something you’ve all heard of before – CONTRAST – and how it effects our ability to see detail, our lens’s ability to transfer detail, and our camera sensors ability to record detail.

Contrast & How It Effects the Resolving of Detail

In an earlier post HERE I briefly mentioned that the human eye can resolve 5 line pairs per millimeter, and the illustration I used to illustrate those line pairs looked rather like this:

5 line pairs per millimeter with a contrast ratio of 100% or 1.0

5 line pairs per millimeter with a contrast ratio of 100% or 1.0

Now don’t forget, these line pairs are highly magnified – in reality each pair should be 0.2mm wide.  These lines are easily differentiated because of the excessive contrast ratio between each line in a pair.

How far can contrast between the lines fall before we can’t tell the difference any more and all the lines blend together into a solid monotone?

Enter John William Strutt, the 3rd Baron Rayleigh…………

5 line pairs at bottom threshold of human vision - a 9% contrast ratio.

5 line pairs at bottom threshold of human vision – a 9% contrast ratio.

The Rayleigh Criterion basically stipulates that the ‘discernability’ of each line in a pair is low end limited to a line pair contrast ratio of 9% or above, for average human vision – that is, when each line pair is 0.2mm wide and viewed from 25cms.  Obviously they are reproduced much larger here, hence you can see ’em!

Low contrast limit for Human vision (left) & camera sensor (right).

Low contrast limit for Human vision (left) & camera sensor (right).

However, it is said in some circles that dslr sensors are typically limited to a 12% to 15% minimum line pair contrast ratio when it comes to discriminating between the individual lines.

Now before you start getting in a panic and misinterpreting this revelation you must realise that you are missing one crucial factor; but let’s just recap what we’ve got so far.

  1. A ‘line’ is a detail.
  2. but we can’t see one line (detail) without another line (detail) next to it that has a different tonal value ( our line pair).
  3. There is a limit to the contrast ratio between our two lines, below which our lines/details begin to merge together and become less distinct.

So, what is this crucial factor that we are missing; well, it’s dead simple – the line pair per millimeter (lp/mm) resolution of a camera sensor.

Now there’s something you won’t find in your cameras ‘tech specs’ that’s for sure!

Sensor Line Pair Resolution

The smallest “line” that can be recorded on a sensor is 1 photosite in width – now that makes sense doesn’t it.

But in order to see that line we must have another line next to it, and that line must have a higher or lower tonal value to a degree where the contrast ratio between the two lines is at or above the low contrast limit of the sensor.

So now we know that the smallest line pair our sensor can record is 2 photosites/pixels in width – the physical width is governed by the sensor pixel pitch; in other words the photosite diameter.

In a nutshell, the lp/mm resolution of a sensor is 0.5x the pixel row count per millimeter – referred to as the Nyquist Rate, simply because we have to define (sample) 2 lines in order to see/resolve 1 line.

The maximum resolution of an image projected by the lens that can be captured at the sensor plane – in other words, the limit of what can be USEFULLY sampled – is the Nyquist Limit.

Let’s do some practical calculations:

Canon 1DX 18.1Mp

Imaging Area = 36mm x 24mm / 5202 x 3533 pixels/photosites OR LINES.

I actually do this calculation based on the imaging area diagonal

So sensor resolution in lp/mm = (pixel diagonal/physical diagonal) x 0.5 = 72.01 lp/mm

Nikon D4 16.2Mp = 68.62 lp/mm

Nikon D800 36.3Mp = 102.33 lp/mm

PhaseOne P40 40Mp medium format = 83.15 lp/mm

PhaseOne IQ180 80Mp medium format = 96.12 lp/mm

Nikon D7000 16.2mp APS-C (DX) 4928×3264 pixels; 23.6×15.6mm dimensions  = 104.62 lp/mm

Canon 1D IV 16.1mp APS-H 4896×3264 pixels; 27.9×18.6mm dimensions  = 87.74 lp/mm

Taking the crackpot D800 as an example, that 102.33 lp/mm figure means that the sensor is capable of resolving 204.66 lines, or points of detail, per millimeter.

I say crackpot because:

  1. The Optical Low Pass “fights” against this high degree of resolving power
  2. This resolving power comes at the expense of S/N ratio
  3. This resolving power comes at the expense of diffraction
  4. The D800E is a far better proposition because it negates 1. above but it still leaves 2. & 3.
  5. Both sensors would purport to be “better” than even an IQ180 – newsflash – they ain’t; and not by a bloody country mile!  But the D800E is an exceptional sensor as far as 35mm format (36×24) sensors go.

A switch to a 40Mp medium format is BY FAR the better idea.

Before we go any further, we need a reality check:

In the scene we are shooting, and with the lens magnification we are using, can we actually “SEE” detail as small as 1/204th of a millimeter?

We know that detail finer than that exists all around us – that’s why we do macro/micro photography – but shooting a landscape with a 20mm wide angle where the nearest detail is 1.5 meters away ??

And let’s not forget the diffraction limit of the sensor and the incumbent reduction in depth of field that comes with 36Mp+ crammed into a 36mm x 24mm sensor area.

The D800 gives you something with one hand and takes it away with the other – I wouldn’t give the damn thing house-room!  Rant over………

Anyway, getting back to the matter at hand, we can now see that the MTF lp/mm values quoted by the likes of Nikon and Canon et al of 10 and 30 lp/mm bare little or no connectivity with the resolving power of their sensors – as I said in my previous post HERE – they are meaningless.

The information we are chasing after is all about the lens:

  1. How well does it transfer contrast because its contrast that allows us to “see” the lines of detail?
  2. How “sharp” is the lens?
  3. What is the “spread” of 1. and 2. – does it perform equally across its FoV (field of view) or is there a monstrous fall-off of 1. and 2. between 12 and 18mm from the center on an FX sensor?
  4. Does the lens vignette?
  5. What is its CA performance?

Now we can go to data sites on the net such as DXO Mark where we can find out all sorts of more meaningful data about our potential lens purchase performance.

But even then, we have to temper what we see because they do their testing using Imatest or something of that ilk, and so the lens performance data is influenced by sensor, ASIC and basic RAW file demosaicing and normalisation – all of which can introduce inaccuracies in the data; in other words they use camera images in order to measure lens performance.

The MTF 50 Standard

Standard MTF (MTF 100) charts do give you a good idea of the lens CONTRAST transfer function, as you may already have concluded. They begin by measuring targets with the highest degree of modulation – black to white – and then illustrate how well that contrast has been transferred to the image plane, measured along a corner radius of the frame/image circle.

MTF 1.0 (100%) left, MTF 0.5 (50%) center and MTF 0.1 (10%) right.

MTF 1.0 (100%) left, MTF 0.5 (50%) center and MTF 0.1 (10%) right.

As you can see, contrast decreases with falling transfer function value until we get to MTF 0.1 (10%) – here we can guess that if the value falls any lower than 10% then we will lose ALL “perceived” contrast in the image and the lines will become a single flat monotone – in other words we’ll drop to 9% and hit the Rayleigh Criterion.

It’s somewhat debatable whether or not sensors can actually discern a 10% value – as I mentioned earlier in this post, some favour a value more like 12% to 15% (0.12 to 0.15).

Now then, here’s the thing – what dictates the “sharpness” of edge detail in our images?  That’s right – EDGE CONTRAST.  (Don’t mistake this for overall image contrast!)

Couple that with:

  1. My well-used adage of “too much contrast is thine enemy”.
  2. “Detail” lies in midtones and shadows, and we want to see that detail, and in order to see it the lens has to ‘transfer’ it to the sensor plane.
  3. The only “visual” I can give you of MTF 100 would be something like power lines silhouetted against the sun – even then you would under expose the sun, so, if you like, MTF would still be sub 100.

Please note: 3. above is something of a ‘bastardisation’ and certain so-called experts will slag me off for writing it, but it gives you guys a view of reality – which is the last place some of those aforementioned experts will ever inhabit!

Hopefully you can now see that maybe measuring lens performance with reference to MTF 50 (50%, 0.5) rather than MTF 100 (100%, 1.0) might be a better idea.

Manufacturers know this but won’t do it, and the likes of Nikon can’t do it even if they wanted to because they use a damn calculator!

Don’t be trapped into thinking that contrast equals “sharpness” though; consider the two diagrams below (they are small because at larger sizes they make your eyes go funny!).

A lens can transfer full contrast but be unsharp.

A lens can have a high contrast transfer function but be unsharp.

A lens can have low contrast transmission (transfer function) but still be sharp.

A lens can have low contrast transfer function but still be sharp.

In the first diagram the lens has RESOLVED the same level of detail (the same lp/mm) in both cases, and at pretty much the same contrast transfer value; but the detail is less “sharp” on the right.

In the lower diagram the lens has resolved the same level of detail with the same degree of  “sharpness”, but with a much reduced contrast transfer value on the right.

Contrast is an AID to PERCEIVED sharpness – nothing more.

I actually hate that word SHARPNESS; and it’s a nasty word because it’s open to all sorts of misconceptions by the uninitiated.

A far more accurate term is ACUTANCE.

How Acutance effects perceived "sharpness" and is contrast independent.

How Acutance effects perceived “sharpness”.

So now hopefully you can see that LENS RESOLUTION is NOT the same as lens ACUTANCE (perceived sharpness..grrrrrr).

Seeing as it is possible to have a lens with a higher degree resolving power, but a lower degree of acutance you need to be careful – low acutance tends to make details blur into each other even at high contrast values; which tends to negate the positive effects of the resolving power. (Read as CHEAP LENS!).

Lenses need to have high acutance – they need to be sharp!  We’ve got enough problems trying to keep the sharpness once the sensor gets hold of the image, without chucking it a soft one in the first place – and I’ll argue this point with the likes of Mr. Rockwell until the cows have come home!

Things We Already Know

We already know that stopping down the aperture increases Depth of Field; and we already know that we can only do this to a certain degree before we start to hit diffraction.

What does increasing DoF do exactly; it increases ACUTANCE is what it does – exactly!

Yes it gives us increased perceptual sharpness of parts of the subject in front and behind the plane of sharp focus – but forget that bit – we need to understand that the perceived sharpness/acutance of the plane of focus increases too, until you take things too far and go beyond the diffraction limit.

And as we already know, that diffraction limit is dictated by the size of photosites/pixels in the sensor – in other words, the sensor resolution.

So the diffraction limit has two effects on the MTF of a lens:

  1. The diffraction limit changes with sensor resolution – you might get away with f14 on one sensor, but only f9 on another.
  2. All this goes “out the window” if we talk about crop-sensor cameras because their sensor dimensions are different.

We all know about “loss of wide angles” with crop sensors – if we put a 28mm lens on an FX body and like the composition but then we switch to a 1.5x crop body we then have to stand further away from the subject in order to achieve the same composition.

That’s good from a DoF PoV because DoF for any given aperture increases with distance; but from a lens resolving power PoV it’s bad – that 50 lp/mm detail has just effectively dropped to 75 lp/mm, so it’s harder for the lens to resolve it, even if the sensors resolution is capable of doing so.

There is yet another way of quantifying MTF – just to confuse the issue for you – and that is line pairs per frame size, usually based on image height and denoted as lp/IH.

Imatest uses MTF50 but quotes the frequencies not as lp/mm, or even lp/IH; but in line widths per image height – LW/IH!

Alas, there is no single source of the empirical data we need in order to evaluate pure lens performance anymore.  And because the outcome of any particular lens’s performance in terms of acutance and resolution is now so inextricably intertwined with that of the sensor behind it, then you as lens buyers, are left with a confusing myriad of various test results all freely available on the internet.

What does Uncle Andy recommend? – well a trip to DXO Mark is not a bad starting point all things considered, but I do strongly suggest that you take on board the information I’ve given you here and then scoot over to the DXO test methodology pages HERE and read them carefully before you begin to examine the data and draw any conclusions from it.

But do NOT make decisions just on what you see there; there is no substitute for hands-on testing with your camera before you go and spend your hard-earned cash.  Proper testing and evaluation is not as simple as you might think, so it’s a good idea to perhaps find someone who knows what they are doing and is prepared to help you out.   Do NOT ask the geezer in the camera shop – he knows bugger all about bugger all!

Do Sensors Out Resolve Lenses?

Well, that’s the loaded question isn’t it – you can get very poor performance from what is ostensibly a superb lens, and to a degree vice versa.

It all depends on what you mean by the question, because in reality a sensor can only resolve what the lens chucks at it.

If you somehow chiseled the lens out of your iPhone and Sellotaped it to your shiny new 1DX then I’m sure you’d notice that the sensor did indeed out resolve the lens – but if you were a total divvy who didn’t know any better then in reality all you’d be ware of is that you had a crappy image – and you’d possibly blame the camera, not the lens – ‘cos it took way better pics on your iPhone 4!

There are so many external factors that effect the output of a lens – available light, subject brightness range, angle of subject to the lens axis to name but three.  Learning how to recognise these potential pitfalls and to work around them is what separates a good photographer from an average one – and by good I mean knowledgeable – not necessarily someone who takes pics for a living.

I remember when the 1DX specs were first ‘leaked’ and everyone was getting all hot and bothered about having to buy the new Canon glass because the 1DX was going to out resolve all Canons old glass – how crackers do you need to be nowadays to get a one way ticket to the funny farm?

If they were happy with the lens’s optical performance pre 1DX then that’s what they would get post 1DX…duh!

If you still don’t get it then try looking at it this way – if lenses out resolve your sensor then you are up “Queer Street” – what you see in the viewfinder will be far better than the image that comes off the sensor, and you will not be a happy camper.

If on the other hand, our sensors have the capability to resolve more lines per millimeter than our lenses can throw at them, and we are more than satisfied with our lenses resolution and acutance, then we would be in a happy place, because we’d be wringing the very best performance from our glass – always assuming we know how to ‘drive the juggernaut’  in the first place!

Become a Patron!

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.

Lens Performance

Posted on by
2

I have a friend – yes, a strange concept I know, but I do have some – we’ll call him Steve.

Steve is a very talented photographer – when he’ll give himself half a chance; but impatience can sometimes get the better of him.

He’ll have a great scene in front of him but then he’ll forget things such as any focus or exposure considerations the scene demands, and the resulting image will be crap!

Quite often, a few of Steve’s character flaws begin to emerge at this juncture.

Firstly, Steve only remembers his successes; this leads to the unassailable ‘fact’ that he couldn’t possibly have ‘screwed up’.

So now we can all guess the conclusive outcome of that scenario can’t we……..that’s right; his camera gear has fallen short in the performance department.

Clairvoyance department would actually be more accurate!

So this ‘error in his camera system’ needs to be stamped on – hard and fast!

This leads to Steve embarking on a massive information-gathering exercise from various learned sources on ‘that there inter web’ – where another of Steve’s flaws shows up; that of disjointed speed reading…..

The terrifying outcome of these situations usually concludes with Steve’s confident affirmation that some piece of his equipment has let him down; not just by becoming faulty but sometimes, more worryingly by initial design.

These conclusions are always arrived at in the same manner – the various little snippets of truth and random dis-associated facts that Steve gathers, all get forcibly hammered into some hellish, bastardized ‘factual’ jigsaw in his head.

There was a time when Steve used to ask me first, but he gave up on that because my usual answer contravened the outcome of his first mentioned character flaw!

Lately one of Steve’s biggest peeves has been the performance of one or two of his various lenses.

Ostensibly you’ll perhaps think there’s nothing wrong in that – after all, the image generated by the camera is only as good as the lens used to gather the light in the scene – isn’t it?

 

But there’s a potential problem, and it  lies in what evidence you base your conclusions on……………

 

For Steve, at present, it’s manufacturers MTF charts, and comparisons thereof, coupled with his own images as they appear in Lightroom or Photoshop ACR.

Again, this might sound like a logical methodology – but it isn’t.

It’s flawed on so many levels.

 

The Image Path from Lens to Sensor

We could think of the path that light travels along in order to get to our camera sensor as a sort of Grand National horse race – a steeplechase for photons!

“They’re under starters orders ladies and gentlemen………………and they’re off!”

As light enters the lens it comes across it’s first set of hurdles – the various lens elements and element groups that it has to pass through.

Then they arrive at Becher’s Brook – the aperture, where there are many fallers.

Carefully staying clear of the inside rail and being watchful of any lose photons that have unseated their riders at Becher’s we move on over Foinavon – the rear lens elements, and we then arrive at the infamous Canal Turn – the Optical Low Pass filter; also known as the Anti-alias filter.

Crashing on past the low pass filter and on over Valentines only the bravest photons are left to tackle the the last big fence on their journey – The Chair – our camera sensor itself.

 

Okay, I’ll behave myself now, but you get the general idea – any obstacle that lies in the path of light between the front surface of our lens and the photo-voltaic surface of our sensor is a BAD thing.

Andy Astbury,Wildlife in Pixels,lens,resolution,optical path,sharpness,resolution,imaging pathway

The various obstacles to light as it passes through a camera (ASIC = Application Specific Integrated Circuit)

The problems are many, but let’s list a few:

  1. Every element reduces the level of transmitted light.
  2. Because the lens elements have curved surfaces, light is refracted or bent; the trick is to make all wavelengths of light refract to the same degree – failure results in either lateral or longitudinal chromatic aberration – or worse still, both.
  3. The aperture causes diffraction – already discussed HERE

We have already seen in that same previous post on Sensor Resolution that the number of megapixels can effect overall image quality in terms of overall perceived sharpness due to pixel-pitch, so all things considered, using photographs of any 3 dimensional scene is not always a wise method of judging lens performance.

And here is another reason why it’s not a good idea – the effect on image quality/perceived lens resolution of anti-alias, moire or optical low pass filter; and any other pre-filtering.

I’m not going to delve into the functional whys and wherefores of an AA filter, save to say that it’s deemed a necessary evil on most sensors, and that it can make your images take on a certain softness because it basically adds blur to every edge in the image projected by the lens onto your sensor.

The reasoning behind it is that it stops ‘moire patterning’ in areas of high frequency repeated detail.  This it does, but what about the areas in the image where its effect is not required – TOUGH!

 

Many photographers have paid service suppliers for AA filter removal just to squeeze the last bit of sharpness out of their sensors, and Nikon of course offer the ‘sort of AA filter-less’ D800E.

Side bar note:  I’ve always found that with Nikon cameras at least, the pro-body range seem to suffer a lot less from undesirable AA filtration softening than than their “amateur” and “semi pro” bodies – most notably the D2X compared to a D200, and the D3 compared to the D700 & D300.  Perhaps this is due to a ‘thinner’ filter, or a higher quality filter – I don’t know, and to be honest I’ve never had the desire to ‘poke Nikon with a sharp stick’ in order to find out.

 

Back in the days of film things were really simple – image resolution was governed by just two things; lens resolution and film resolution:

1/image resolution = 1/lens resolution + 1/film resolution

Film resolution was a variable depending on the Ag Halide distribution and structure,  dye coupler efficacy within the film emulsion, and the thickness of the emulsion or tri-pack itself.

But today things are far more complicated.

With digital photography we have all those extra hurdles to jump over that I mentioned earlier, so we end up with a situation whereby:

1/Image Resolution = 1/lens resolution + 1/AA filter resolution + 1/sensor resolution + 1/image processor/imaging ASIC resolution

Steve is chasing after lens resolution under the slightly misguided idea the resolution equates to sharpness, which is not strictly true; but he is basing his conception of lens sharpness based on the detail content and perceived detail ‘sharpness’ of his  images; which are ‘polluted’ if you like by the effects of the AA filter, sensor and imaging ASIC.

What it boils down to, in very simplified terms, is this:

You can have one particular lens that, in combination with one camera sensor produces a superb image, but in combination with another sensor produces a not-quite-so-superb image!

On top of the “fixed system” hurdles I’ve outlined above, we must not forget the potential for errors introduced by lens-to-body mount flange inaccuracies, and of course, the big elephant-in-the-room – operator error – ehh Steve.

So attempting to quantify the pure ‘optical performance’ of a lens using your ‘taken images’ is something of a pointless exercise; you cannot see the pure lens sharpness or resolution unless you put the lens on a fully equipped optical test bench – and how many of us have got access to one of those?

The truth of the matter is that the average photographer has to trust the manufacturers to supply accurately put together equipment, and he or she has to assume that all is well inside the box they’ve just purchased from their photographic supplier.

But how can we judge a lens against an assumed standard of perfection before we part with our cash?

A lot of folk, including Steve – look at MTF charts.

 

The MTF Chart

Firstly, MTF stands for Modulation Transfer Function – modu-what I hear your ask!

OK – let’s deal with the modulation bit.  Forget colour for a minute and consider yourself living in a black & white world.  Dark objects in a scene reflect few photons of light – ’tis why the appear dark!  Conversely, bright objects reflect loads of the little buggers, hence these objects appear bright.

Imagine now that we are in a sealed room totally impervious to the ingress of any light from outside, and that the room is painted matte white from floor to ceiling – what is the perceived colour of the room? Black is the answer you are looking for!

Now turn on that 2 million candle-power 6500k searchlight in the corner.  The split second before your retinas melted, what was the perceived colour of the room?

Note the use of the word ‘perceived’ – the actual colour never changed!

The luminosity value of every surface in the room changed from black to white/dark to bright – the luminosity values MODULATED.

Now back in reality we can say that a set of alternating black and white lines of equal width and crisp clean edges represent a high degree of contrast, and therefore tonal modulation; and the finer the lines the higher is the modulation frequency – which we measure in lines per millimeter (lpmm).

A lens takes in a scene of these alternating black and white lines and, just like it does with any other scene, projects it into an image circle; in other words it takes what it sees in front of it and ‘transfers’ the scene to the image circle behind it.

With a bit of luck and a fair wind this image circle is being projected sharply into the focal plane of the lens, and hopefully the focal plane matches up perfectly with the plane of the sensor – what used to be refereed to as the film plane.

The efficacy with which the lens carries out this ‘transfer’ in terms of maintaining both the contrast ratio of the modulated tones and the spatial separation of the lines is its transfer function.

So now you know what MTF stands for and what it means – good this isn’t it!

 

Let’s look at an MTF chart:

Nikon 500mm f4 MTF chart

Nikon 500mm f4 MTF chart

Now what does all this mean?

 

Firstly, the vertical axis – this can be regarded as that ‘efficacy’ I mentioned above – the accuracy of tonal contrast and separation reproduction in the projected image; 1.0 would be perfect, and 0 would be crappier than the crappiest version of a crap thing!

The horizontal axis – this requires a bit of brain power! It is scaled in increments of 5 millimeters from the lens axis AT THE FOCAL PLANE.

The terminus value at the right hand end of the axis is unmarked, but equates to 21.63mm – half the opposing corner-to-corner dimension of a 35mm frame.

Now consider the diagram below:

Andy Astbury,image circle,photography,frame,full frame,dimensions,radial,

The radial dimensions of the 35mm format.

These are the radial dimensions, in millimeters, of a 35mm format frame (solid black rectangle).

The lens axis passes through the center axis of the sensor, so the radii of the green, yellow and dashed circles correspond to values along the horizontal axis of an MTF chart.

Let’s simplify what we’ve learned about MTF axes:

Andy Astbury,image circle,photography,frame,full frame,dimensions,radial,

MTF axes hopefully made simpler!

Now we come to the information data plots; firstly the meaning of Sagittal & Meridional.   From our perspective in this instance I find it easier for folk to think of them as ‘parallel to’ and ‘at right angles to’ the axis of measurement, though strictly speaking Meridional is circular and Sagittal is radial.

This axis of measurement is from the lens/film plane/sensor center to the corner of a 35mm frame – in other words, along that 21.63mm radius.

Andy Astbury,image circle,photography,frame,full frame,dimensions,radial,

The axis of MTF measurement and the relative axial orientation of Sagittal & Meridional lines. NOTE: the target lines are ONLY for illustration.

Separate measurements are taken for each modulation frequency along the entire measurement axis:

Andy Astbury,image circle,photography,frame,full frame,dimensions,radial,

Thin Meridional MTF measurement. (They should be concentric circles but I can’t draw concentric circles!).

Let’s look at that MTF curve for the 500m f4 Nikon together with a legend of ‘sharpness’ – the 300 f2.8:

MTF chart,Andy Astbury,lens resolution

Nikon MTF comparison between the 500mm f4 & 300mm f2.8

Nikon say on their website that they measure MTF at maximum aperture, that is, wide open; so the 300mm chart is for an aperture of f2.8 (though they don’t say so) and the 500mm is for an f4 aperture – which they do specify on the chart – don’t ask me why ‘cos I’ve no idea.

As we can see, the best transfer values for the two lenses (and all other lenses) is 10 lines per millimeter, and generally speaking sagittal orientation usually performs slightly better than meridional, but not always.

10 lpmm is always going to give a good transfer value because its very coarse and represents a lower frequency of detail than 30 lpmm.

Funny thing, 10 lines per millimeter is 5 line pairs per millimeter – and where have we heard that before? HERE – it’s the resolution of the human eye at 25 centimeters.

 

Another interesting thing to bare in mind is that, as the charts clearly show, better transfer values occur closer to the lens axis/sensor center, and that performance falls as you get closer to the frame corners.

This is simply down to the fact that your are getting closer to the inner edge of the image circle (the dotted line in the diagrams above).  If manufacturers made lenses that threw a larger image circle then corner MTF performance would increase – it can be done – that’s the basis upon which PCE/TS lenses work.

One way to take advantage of center MTF performance is to use a cropped sensor – I still use my trusty D2Xs for a lot of macro work; not only do I get the benefit of center MTF performance across the majority of the frame but I also have the ability to increase the lens to subject distance and get the composition I want, so my depth of field increases slightly for any given aperture.

Back to the matter at hand, here’s my first problem with the likes of Nikon, Canon etc:  they don’t specify the lens-to-target distance. A lens that gives a transfer value of 9o% plus on a target of 10 lpmm sagittal at 2 meters distance is one thing; one that did the same but at 25 meters would be something else again.

You might look at the MTF chart above and think that the 300mm f2.8 lens is poor on a target resolution of  30 lines per millimeter compared to the 500mm, but we need to temper that conclusion with a few facts:

  1. A 300mm lens is a lot wider in Field of View (FoV) than a 500mm so there is a lot more ‘scene width’ being pushed through the lens – detail is ‘less magnified’.
  2. How much ‘less magnified’ –  40% less than at 500mm, and yet the 30 lpmm transfer value is within 6% to 7% that of the 500mm – overall a seemingly much better lens in MTF terms.
  3. The lens is f2.8 – great for letting light in but rubbish for everything else!

Most conventional lenses have one thing in common – their best working aperture for overall image quality is around f8.

But we have to counter balance the above with the lack of aforementioned target distance information.  The minimum focus distances for the two comparison lenses are 2.3 meters and 4.0 meters respectively so obviously we know that the targets are imaged and measured at vastly different distances – but without factual knowledge of the testing distances we cannot really say that one lens is better than the other.

 

My next problem with most manufacturers MTF charts is that the values are supplied ‘a la white light’.

I mentioned earlier – much earlier! – that lens elements refracted light, and the importance of all wavelengths being refracted to the same degree, otherwise we end up with either lateral or longitudinal chromatic aberration – or worse still – both!

Longitudinal CA will give us different focal planes for different colours contained within white light – NOT GOOD!

Lateral CA gives us the same plane of focus but this time we get lateral shifts in the red, green and blue components of the image, as if the 3 colour channels have come out of register – again NOT GOOD!

Both CA types are most commonly seen along defined edges of colour and/or tone, and as such they both effect transferred edge definition and detail.

So why do manufacturers NOT publish this information – there is to my knowledge only one that does – Schneider (read ‘proper lens’).

They produce some very meaningful MTF data for their lenses with modulation frequencies in excess of 90 to 150 lpmm; separate R,G & B curves; spectral weighting variations for different colour temperatures of light and all sorts of other ‘geeky goodies’ – I just love it all!

 

SHAME ON YOU NIKON – and that goes for Canon and Sigma just as much.

 

So you might now be asking WHY they don’t publish the data – they must have it – are they treating us like fools that wouldn’t be able to understand it; OR – are they trying to hide something?

You guys think what you will – I’m not accusing anyone of anything here.

But if they are trying to hide something then that ‘something’ might not be what you guys are thinking.

What would you think if I told you that if you were a lens designer you could produce an MTF plot with a calculator – ‘cos you can, and they do!

So, in a nutshell, most manufacturers MTF charts as published for us to see are worse than useless.  We can’t effectively use them to compare one lens against another because of missing data; we can’t get an idea of CA performance because of missing red, green and blue MTF curves; and finally we can’t even trust that the bit of data they do impart is even bloody genuine.

Please don’t get taken in by them next time you fancy spending money on glass – take your time and ask around – better still try one; and try it on more than 1 camera body!

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.

Sensor Resolution

Posted on by
12

Sensor Resolution

In my previous two posts on this subject HERE and HERE I’ve been looking at pixel resolution as it pertains to digital display and print, and the basics of how we can manipulate it to our benefit.

You should also by aware by now that I’m not the worlds biggest fan of high sensor resolution 35mm format dSLRs – there’s nothing wrong with mega pixels; you can’t have enough of them in my book!

BUT, there’s a limit to how many you can cram into a 36 x 24 millimeter sensor area before things start getting silly and your photographic life gets harder.

So in this post I want to explain the reasoning behind my thoughts.

But before I get into that I want to address something else to do with resolution – the standard by which we judge everything we see around us – the resolution of the eye.

 

Human Eye – How Much Can We See?

In very simple terms, because I’m not an optician, the answer goes like this.

Someone with what some call 20/20/20 vision – 20/20 vision in a 20 year old – has a visual acuity of 5 line pairs per millimeter at a distance of 25 centimeters.

What’s a line pair?

5 line pairs per millimeter. Each line pair is 0.2mm and each line is 0.1mm.

5 line pairs per millimeter. Each line pair is 0.2mm and each line is 0.1mm.

Under ideal viewing conditions in terms of brightness and contrast the human eye can at best resolve 0.1mm detail at a distance of 25 centimeters.

Drop the brightness and the contrast and black will become less black and more grey, and white will become greyer; the contrast between light and dark becomes reduced and therefore that 0.1mm detail becomes less distinct.  until the point comes where the same eye can’t resolve detail any smaller than 0.2mm at 25cms, and so on.

Now if I try and focus on something at 25 cms my eyeballs start to ache,  so we are talking extreme close focus for the eye here.

An interesting side note is that 0.1mm is 100µm (microns) and microns are what we measure the size of sensor photosites in – which brings me nicely to SENSOR resolution.

 

Sensor Resolution – Too Many Megapixels?

As we saw in the post on NOISE we do not give ourselves the best chances by employing sensors with small photosite diameters.  It’s a basic fact of physics and mathematics – the more megapixels on a sensor, then the smaller each photosite has to be in order to fit them all in there;  and the smaller they are then the lower is their individual signal to noise or S/N ratio.

But there is another problem that comes with increased sensor resolution:

Increased diffraction threshold.

Andy Astbury,Wildlife in Pixels,sensor resolution,megapixels,pixel pitch,base noise,signal to noise ratio

Schematic of identical surface areas on lower and higher megapixel sensors.

In the above schematic we are looking at the same sized tiny surface area section on two sensors.

If we say that the sensor resolution on the left is that of a 12Mp Nikon D3, and the ‘area’ contains 3 x 3 photosites which are each 8.4 µm in size, then we can say we are looking at an area of about 25µm square.

On the right we are looking at that same 25µm (25 micron) square, but now it contains 5.2 x 5.2 photosites, each 4.84µm in size – a bit like the sensor resolution of a 36Mp D800.

 

What is Diffraction?

Diffraction is basically the bending or reflecting of waves by objects placed in their path (not to be confused with refraction).  As it pertains to our camera sensor, and overall image quality, it causes an general softening of every single point of sharp detail in the image that is projected onto the sensor during the exposure.

I say during the exposure because diffraction is ‘aperture driven’ and it’s effects only occur when the aperture is ‘stopped down’; which on modern cameras only occurs during the time the shutter is open.

At all other times you are viewing the image with the aperture wide open, and so you can’t see the effect unless you hit the stop down button (if you have one) and even then the image in the viewfinder is so small and dark you can’t see it.

As I said, diffraction is caused by aperture diameter – the size of the hole that lets the light in:

Andy Astbury,Wildlife in Pixels,sensor resolution,megapixels,pixel pitch,base noise,signal to noise ratio

Diffraction has a low presence in the system at wider apertures.

Light enters the lens, passes through the aperture and strikes the focal plane/sensor causing the image to be recorded.

Light waves passing through the center of the aperture and light waves passing through the periphery of the aperture all need to travel the same distance – the focal distance – in order for the image to be sharp.

The potential for the peripheral waves to be bent by the edge of the aperture diaphragm increases as the aperture becomes smaller.

Andy Astbury,Wildlife in Pixels,sensor resolution,megapixels,pixel pitch,base noise,signal to noise ratio

Diffraction has a greater presence in the system at narrower apertures.

If I apply some randomly chosen numbers to this you might understand it a little better:

Let’s say that the focal distance of the lens (not focal length) is 21.25mm.

As long as light passing through all points of the aperture travels 21.25mm and strikes the sensor then the image will be sharp; in other words, the more parallel the central and peripheral light waves are, then the sharper the image.

Making the aperture narrower by ‘stopping down’ increases the divergence between central and peripheral waves.

This means that peripheral waves have to travel further before the strike the sensor; further than 21.25mm – therefore they are no longer in focus, but those central waves still are.  This effect gives a fuzzy halo to every single sharply focused point of light striking our sensor.

Please remember, the numbers I’ve used above are meaningless and random.

The amount of fuzziness varies with aperture – wider aperture =  less fuzzy; narrower aperture = more fuzzy, and the circular image produced by a single point of sharp focus is known as an Airy Disc.

As we ‘stop down’ the aperture the edges of the Airy Disc become softer and more fuzzy.

Say for example, we stick a 24mm lens on our camera and frame up a nice landscape, and we need to use f14 to generate the amount of depth of field we need for the shot.  The particular lens we are using produces an Airy Disc of a very particular size at any given aperture.

Now here is the problem:

Andy Astbury,Wildlife in Pixels,sensor resolution,megapixels,pixel pitch,base noise,signal to noise ratio

Schematic of identical surface areas on lower and higher megapixel sensors and the same diameter Airy Disc projected on both of them.

As you can see, the camera with the lower sensor resolution and larger photosite diameter contains the Airy Disc within the footprint of ONE photosite; but the disc effects NINE photosites on the camera with the higher sensor resolution.

Individual photosites basically record one single flat tone which is the average of what they see; so the net outcome of the above scenario is:

Andy Astbury,Wildlife in Pixels,sensor resolution,megapixels,pixel pitch,base noise,signal to noise ratio

Schematic illustrating the tonal output effect of a particular size Airy Disc on higher and lower resolution sensors

On the higher resolution sensor the Airy Disc has produced what we might think of as ‘response pollution’ in the 8 surrounding photosites – these photosites need to record the values of the own ‘bits of the image jigsaw’ as well – so you end up with a situation where each photosite on the sensor ends up recording somewhat imprecise tonal values – this is diffraction in action.

If we were to stop down to f22 or f32 on the lower resolution sensor then the same thing would occur.

If we used an aperture wide enough on the higher resolution sensor – an aperture that generated an Airy Disc that was the same size or smaller than the diameter of the photosites – then only 1 single photosite would be effected and diffraction would not occur.

But that would leave of with a reduced depth of field – getting around that problem is fairly easy if you are prepared to invest in something like a Tilt-Shift lens.

Andy Astbury,Wildlife in Pixels,sensor resolution,megapixels,pixel pitch,base noise,signal to noise ratio

Both images shot with a 24mm TS lens at f3.5. Left image lens is set to zero and behaves as normal 24mm lens. Right image has 1 degree of down tilt applied.

Above we see two images shot with a 24mm Tilt-Shift lens, and both shots are at f3.5 – a wide open aperture.  In the left hand image the lens controls are set to zero and so it behaves like a standard construction lens of 24mm and gives the shallow depth of field that you’d expect.

The image on the right is again, shot wide open at f3.5, but this time the lens was tilted down by just 1 degree – now we have depth of field reaching all the way through the image.  All we would need to do now is stop the lens down to its sharpest aperture – around f8 – and take the shot;  and no worries about diffraction.

Getting back to sensor resolution in general, if your move into high megapixels counts on 35mm format then you are in a ‘Catch 22’ situation:

  • Greater sensor resolution enables you to theoretically capture greater levels of detail.

but that extra level of detail is somewhat problematic because:

  • Diffraction renders it ‘soft’.
  • Eliminating the diffraction causes you to potentially lose the newly acquired level of, say foreground detail in a landscape, due to lack of depth of field.

All digital sensors are susceptible to diffraction at some point or other – they are ‘diffraction limited’.

Over the years I’ve owned a Nikon D3 I’ve found it diffraction limited to between f16 & f18 – I can see it at f18 but can easily rescue the situation.  When I first used a 24Mp D3X I forgot what I was using and spent a whole afternoon shooting at f16 & f18 – I had to go back the next day for a re-shoot because the sensor is diffraction limited to f11 – the pictures certainly told the story!

Everything in photography is a trade-off – you can’t have more of one thing without having less of another.  Back in the days of film we could get by with one camera and use different films because they had very different performance values, but now we buy a camera and expect its sensor to perform all tasks with equal dexterity – sadly, this is not the case.  All modern consumer sensors are jacks of all trades.

If it’s sensor resolution you want then by far the best way to go about it is to jump to medium format, if you want image quality of the n’th degree – this way you get the ‘pixel resolution’ without many of the incumbent problems I’ve mentioned, simply because the sensors are twice the size; or invest in a TS/PC lens and take the Scheimpflug route to more depth of field at a wider aperture.

Become a patron from as little as $1 per month, and help me produce more free content.

Patrons gain access to a variety of FREE rewards, discounts and bonuses.