Trap Focus

Posted on March 18, 2014 by Andrew Astbury

Trap Focus on the Nikon D4

Trap focus comes to my D4 – Yay!!!!!!!!

What was it Nikon said – “we left it off the D4 because no one wanted it”….or words to that effect.

Well, with today’s (March 18th 2014) update version 1.10 trap focus is back – in a fashion.

What is trap focus some may ask. Well it’s basically pre-focusing on a particular distance or spot where you expect the subject to be or to pass through.

As the subject enters the frame and gets closer to the camera it’s also getting closer to the pre-focused distance, and when it reaches the set focus distance the camera actually detects the subject/image is sharp and so takes the shot.

Basically you sit there with the shutter button fully depressed, but no shots get taken until the camera AF system deems the subject is now in focus.

It’s a technique that a lot of sports photographers find very useful, but I find it has very limited use for my wildlife & natural history work. Having said that, it’s got me out of a bind more than once over the years, but ever since the D4 came out you’ve not been able to use it.

The failing lay in the flawed D4 focus priority – even if you told it to only trip the shutter when the image was deemed ‘in focus’ by setting CS a1 & a2 to FOCUS, it would still fire as if a1 and a2 were set to release priority.

But the new firmware update v1.10 has given trap focus back to the D4, but before you start jumping up and down and getting all excited you need to know how to set it up, and bare in mind that “as a technique” trap focus might not suit what you had in mind.

Setup for D4 Trap Focus

Update firmware to v1.10 – read the instructions FULLY before you attempt this, otherwise you may need another camera!
Go to Custom Settings a2 AF-S priority selection and set to FOCUS.
Go to Custom Settings a4 AF activation and set to AF-ON only – this takes to AF activation away from the shutter release button.
Put a wide angle lens on the camera.
Set the lens focus switch to M/A
Set the D4 focus mode selector (the lever on left side of the body front) to AF
Press the AF mode button and rotate the Command Dial (back one) to select AFS and NOT AFC.
Rotate the Sub Command Dial (front one) to select S (single) and NOT Auto.
Focus on your computers monitor screen using either the manual focus ring of the lens or the rear AF-ON button next to the Command Dial.
If you’ve pressed the latter TAKE your thumb OFF!
Move the camera directly away from the computer monitor screen so the image in the viewfinder goes soft.
Jam your finger down on the shutter release. Nothing happens (if it does then start again!).
Keeping that shutter button depressed and NOT touching the lens or AF button, move back towards the computers monitor screen – the shutter will fire when the monitor screen is sharp.

Got that? Good! Oh, and by the way, the award-winning shot you just missed – it would have been epic!

Now you’ve got a D4 that does trap focus.

Now for the trap focus caveats:

Trap Focus only works in AFS – not in AFC.

Trap Focus only works with a single AF sensor, AFS-S – so correct prediction of that one AF sensor/subject alignment to get the required ‘bits” in sharp focus and DoF is going to be difficult.

wildlife photography, common Kestrel, photography technique,manual focus trap,trap focus

Common Kestrel Landing
©Andy Astbury/Wildlife in Pixels

Do NOT think you can pull this wildlife shot off using TRAP FOCUS.

By the time the camera has detected the sharp focus and got over the system lock time and triggered the shutter, the bird will be way closer to the camera – and sharp focus in the resulting image will be behind the tail!

This shot is done with a manual focus trap – a completely different technique, as described HERE

The subject is too small and so to close to the camera and 500mm lens for trap focus to work effectively.

However, if you are doing sports photography for instance, you are imaging subjects that are much bigger and a lot further away.

A 500mm f4 on an FX body has over 2 meters depth of field at f5.6 when focused at 40 meters. Take a baseball match for instance – not that I’ve ever covered one mind!

Set the single AF sensor focus distance at home plate.

Then tilt the camera up slightly, or move the sensor with the Dpad so it can’t see/is not overlaying what you just focused on. Hold the shutter button down and wait for a player to make a dive for home plate. As he enters the area of the AF sensor the camera will fire continually if you’re in continuous shooting mode, and will only stop when the camera detects focus has been lost.

Works like a charm!

The key thing is that the depth of field generated by the focus distance makes trap focus work for you – at much shorter distances where depth of field is down to an inch or so if you’re lucky, then couple that with a fast subject approach speed, and trap focus will fall down as a reliable method.

If I’m doing studio flash work like this:

which is never often enough any more! – I sometimes find it useful to use trap focus because it can help doing hand held work under the lowish flash unit modelling lights when you want to make sure eyes are sharp.

Using Trap Focus in a sort of ‘bastardised’ manner can help you maintain sharp focus on models eyes whilst giving you freedom to move around, change composition, zoom etc. by controlling the sharpness of the image with the lens focus ring.

Like I said earlier, it’s a technique that can get you out of trouble every now and again, but up until today you hadn’t got recourse to it on the D4.

But you seriously need to understand the limitations of trap focus deployment before you rush out and use it – you could be very disappointed with the results, and it’ll be all your own fault for trying to bang a square peg through a round hole.

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What Shutter Speed?

Posted on March 17, 2014 by Andrew Astbury

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.

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:

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:

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:

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:

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

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

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

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.

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.

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In Conclusion

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!

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Metering Modes Explained

Posted on February 27, 2014 by Andrew Astbury

Camera Metering Modes

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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

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Nikon D4S

Posted on February 25, 2014 by Andrew Astbury

The new Nikon D4S announced today

Nikon D4S left & D4 right

Well, that’s about right, my sexy Nikon D4 is officially out of date, and thanks to the Nikon D4S I’ve just lost a grand off the resale value of my camera – cheers chaps…..

Is Uncle Andy stressed at all about being kitted out with yesterdays gear?

Nope, not really.

So what’s new on the Nikon D4S ?

Well there’s been a few ergonomic tweaks which basically mean nothing for starters.
Seemingly dispelled are the rumours that it would have a higher Mp count – apparently this stays the same at 16.2Mp.
I was expecting some major change in AF but no, they’ve kept the venerable Multi-Cam 3500FX system.
New sensor design.
BUT – they’ve changed the image processor to Expeed 4 from Expeed 3.
AND – they’ve changed the battery from EN-EL18 to an EN-EL18a.

Bare in mind all I’m going on is the web – perish the thought that Nikon would ever think my opinion worthy of note and ACTUALLY SEND ME ONE.

Other changes:

A new Group Area AF mode – which from my own photography PoV is fairly meaningless, seeing as we already have 9 point dynamic AF – I can’t see it’ll make much difference. Plus, the Group AF mode always focusses on the nearest point – something you rarely want the camera to do!
6 possible white balance presets as opposed to 3 on the D4 – I jam all my cameras into Cloudy B1 custom WB and leave them there – so this improvement isn’t worth jumping up and down about either.
Fairly gimmicky S Raw
Spot White Balance

On the storage front most reports say that the D4S carries over the D4 crazy arrangement of 1x CF plus 1x XQD.

My Basic Thoughts:

New Sensor – well the benefits can’t been seen by yours truly until I see a few RAW files from it – preferably taken by myself.

I’m glad they’ve kept it to 16.2Mp – if you crunch the numbers this is the optimum Mp count for an FX sensor – as Canon worked out aeons ago with the 1DsMk2; but then joined the stupid Mp race.

Image Processor changes – well, it’s reportedly 30% faster than the Expeed 3, which basically means that the D4S fires off images to storage 30% faster.

Now I can go out with the D4 and shoot getting on for 100 uncompressed 14bit RAW files in one continuous burst at 8 or 9 fps – do I want to chew through my storage any faster? NO!

The Expeed 4 gives better high ISO performance?

Well perhaps it does, but I look at it this way. If light is so damn low that you need to shoot at crackpot ISO numbers then you can say one thing – the light is crap.

If the light is crap then the image will look like crap – it’s just that with the Expeed 4 it’ll be slightly less noisy crap.

If I can pull 1/8000th sec at f7 or f8 at 3200ISO in half descent looking light using a D4 – which I do regularly – then why do I need a higher ISO capability?

The Red Squidger images you’ve seen in the previous blog articles are all 2000ISO and there is ZERO noise degradation – so again, why do I need more ISO capability.

Now if I was a ‘jobbing’ photo-jounalist, or I was embedded with the troops in Afghanistan or something of that ilk then I’d perhaps have a much different attitude.

But I’m not, and from my own perspective of wildlife & natural history photography these changes are of little interest to me – especially when they have a £5k price tag.

Battery Changes

There was always a persistent gripe about the battery life of the D4 EN-EL18 power cell – well, I’ve got two of them and have had no problems AT ALL with batteries running low.

I was REALLY annoyed that they switched from EN-EL4A D2/D3 style batteries – I’d got a handful of those already, and now when I go to Norway in June I’ve got to take 2 bloody chargers with me: yes the venerable D3 will be getting a summer holiday this year as second camera.

So, for me at least, the increased battery life of the new Nikon D4S 18a batteries is somewhat inconsequential – why do I want a battery that lasts longer than ‘for ever’ ??

Other Changes/Additions

I can’t see anything that excites me: spot white balance? Go and buy a Colour Checker Passport and do the job right – and that doesn’t cost £5k either (though they are a bit pricey).

Group Area AF – do me a favour (see above).

6 White Balance presets – what’s the point?

All of the above could be given away by Nikon as a firmware update for the D4 if they fancied being generous!

What I Would Have Got Excited About.

Twin UDMA 7 CF card slots and an XQD slot for dedicated video recording.

An improved AF module.

The ability to select ‘matched pairs’ of sensors – Canon offered this years ago and it was brilliant.

Internally recorded FX video of EXACTLY the same quality as that of a Canon 5D3, or at least the same quality as internal 1080p CROP.

AF mode selector back WHERE IT SHOULD BE!

Me being put in charge at Nikon!

In Conclusion

Do I want to buy one (even if I had the dough) – NO!

Do I wish I could afford one – NO!

Would I swap my D4 for a D4s – well of course I would.

Seriously though, I can just see an awful lot of people getting “hot under the collar” and stressing over this latest incarnation of this pro body from Nikon; but seriously, if you are then you need to just take a quiet step back and think about things calmly.

There is nothing – IMHO of course – on the D4S that warrants upgrading from the D4 – unless you have a penchant for spending your money that is.

But if you are still on a D3 or something older, and were thinking about buying a D4 – then hold off a while until the D4S in available; it’s makes better fiscal sense.

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Auto Focus & Shooting Speed

Posted on February 24, 2014 by Andrew Astbury

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:

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:

Mirror lifts
Front shutter curtain falls
Rear shutter curtain falls
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!

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Flash Photography

Posted on February 21, 2014 by Andrew Astbury

Flash Photography

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, 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 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 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.

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!

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Paper White – Desktop Printing 101

Posted on February 17, 2014 by Andrew Astbury

Paper White video

A while back I posted an article called How White is Paper White

As a follow-up to my last post on the basic properties of printing paper media I thought I’d post this video to refresh the idea of “white”.

In this video we basically look at a range of 10 Permajet papers and simply compare their tints and brightness – it’s an illustration I give at my print workshops which never fails to amaze all the attendees.

I know I keep ‘banging on’ about this but you must understand:

Very few paper whites are even close to being neutral.
No paper is WHITE in terms of luminosity – RGB 255 in 8 bit colour terms.
No paper can hold a true black – RGB 0 in 8 bit colour terms.

In real-world terms ALL printing paper is a TINTED GREY – some cool, some warm.

If we attempted to print the image above on a cool tinted paper then we would REDUCE or even CANCEL OUT the warm tonal effects and general ‘atmosphere’ of the image.

Conversely, print it to a warmer tinted ‘paper white’ and the atmosphere would be enhanced.

Would this enhancement be a good thing? Well, er NO – not if we were happy with our original ‘on screen’ processing.

You need to look upon ‘paper white’ as another TOOL to help you achieve your goal of great looking photographs, with a minimum of fuss and effort on your part.

We have to ‘soft proof’ our images if we want to get a print off the printer that matches what we see on our monitor.

But we can’t soft proof until we have made a decision about what paper we are going to soft-proof to.

Choosing a paper who’s characteristics match our finished ‘on screen’ image in terms of TINT especially, will make the job of soft proofing much easier.

How, why?

Proper soft proofing requires us to make a copy of our original image (there’s most peoples first mistake – not making a copy) and then making adjustments to said copy, in a soft proof environment, so that it it renders correctly on the print – in other words it matches our original processed image.

Printing from Photoshop requires a hard copy, printing from Lightroom is different – it relies on VIRTUAL copies.

Either way, this copy and its proof adjustments are what get sent to the printer along what we call the PRINT PIPELINE.

The print pipeline has to do a lot of work:

It has to transpose our adjusted/soft proofed image colour values from additive RGB to print CMYK
It has to up sample or interpolate the image dpi instructions to the print head, depending on print output size.
It has to apply the correct droplet size instructions to each nozzle in the print head hundreds of times per second.
And it has to do a lot of other ‘stuff’ besides!!

The key component is the Printer Driver – and printer drivers are basically CRAP at carrying out all but the simplest of instructions.

In other words they don’t like hard work.

Printing to a paper white that matches our image:

Warm image to warm tint paper white

Cool image to cool paper white

will reduce to the amount of adjustments we have to make under soft proofing and therefore REDUCE the printer driver workload.

The less work the print driver has to do, the lower is the risk of things ‘getting lost in translation‘ and if nothing gets lost then the print matches the on screen image – assuming of course that your eyes haven’t let you down at the soft proofing stage!

IMPORTANT – Click Image to Enlarge in new window

If we try to print this squirrel on the left to Permajet Gloss 271 (warmish image to very cool tint paper white) we can see what will happen.

We have got to make a couple of tweaks in terms on luminosity BUT we’ve also got to make a global change to the overall colour temperature of the image – this will most likely present us with a need for further opposing colour channel adjustments between light and dark tones.

IMPORTANT – Click Image to Enlarge in new window

Whereas the same image sent to Permajet Fibre Base Gloss Warmtone all we’ll have to do is tweak the luminosity up a tiny bit and saturation down a couple of points and basically we’ll be sorted.

So less work, and less work means less room for error in our hardware drivers; this leads to more efficient printing and reduced print production costs.

And reduced cost leads to a happy photographer!

Printing images is EASY – as long as you get all your ducks in a row – and you’ve only got a handful of ducks to control.

Understanding print media and grasping the implications of paper white is one of those ducks………

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Desktop Printing 101

Posted on February 10, 2014 by Andrew Astbury

Understanding Desktop Printing – part 1

Desktop printing is what all photographers should be doing.

Holding a finished print of your epic image is the final part of the photographic process, and should be enjoyed by everyone who owns a camera and loves their photography.

But desktop printing has a “bad rap” amongst the general hobby photography community – a process full of cost, danger, confusion and disappointment.

Yet there is no need for it to be this way.

Desktop printing is not a black art full of ‘ju-ju men’ and bear-traps – indeed it’s exactly the opposite.

But if you refuse to take on board a few simple basics then you’ll be swinging in the wind and burning money for ever.

Now I’ve already spoken at length on the importance of monitor calibration & monitor profiling on this blog HERE and HERE so we’ll take that as a given.

But in this post I want to look at the basic material we use for printing – paper media.

Print Media

A while back I wrote a piece entitled “How White is Paper White” – it might be worth you looking at this if you’ve not already done so.

Over the course of most of my blog posts you’ll have noticed a recurring undertone of contrast needs controlling.

Contrast is all about the relationship between blacks and whites in our images, and the tonal separation between them.

This is where we, as digital photographers, can begin to run into problems.

We work on our images via a calibrated monitor, normally calibrated to a gamma of 2.2 and a D65 white point. Modern monitors can readily display true black and true white (Lab 0 to Lab 100/RGB 0 to 255 in 8 bit terms).

Our big problem lies in the fact that you can print NEITHER of these luminosity values in any of the printer channels – the paper just will not allow it.

A papers ability to reproduce white is obviously limited to the brightness and background colour tint of the paper itself – there is no such think as ‘white’ paper.

But a papers ability to render ‘black’ is the other vitally important consideration – and it comes as a major shock to a lot of photographers.

Let’s take 3 commonly used Permajet papers as examples:

Permajet Gloss 271
Permajet Oyster 271
Permajet Portrait White 285

The following measurements have been made with a ColorMunki Photo & Colour Picker software.

L* values are the luminosity values in the L*ab colour space where 0 = pure black (0RGB) and 100 = pure white (255RGB)

Gloss paper:

Black/Dmax = 4.4 L* or 14,16,15 in 8 bit RGB terms
White/Dmin = 94.4 L* or 235,241,241 (paper white)

From these measurements we can see that the deepest black we can reproduce has an average 8bit RGB value of 15 – not zero.

We can also see that “paper white” has a leaning towards cyan due to the higher 241 green & blue RGB values, and this carries over to the blacks which are 6 points deficient in red.

Oyster paper:

Black/Dmax = 4.7 L* or 15,17,16 in 8 bit RGB terms
White/Dmin = 94.9 L* or 237,242,241 (paper white)

We can see that the Oyster maximum black value is slightly lighter than the Gloss paper (L* values reflect are far better accuracy than 8 bit RGB values).

We can also see that the paper has a slightly brighter white value.

Portrait White Matte paper:

Black/Dmax = 25.8 L* or 59,62,61 in 8 bit RGB terms
White/Dmin = 97.1 L* or 247,247,244 (paper white)

You can see that paper white is brighter than either Gloss or Oyster.

The paper white is also deficient in blue, but the Dmax black is deficient in red.

It’s quite common to find this skewed cool/warm split between dark tones and light tones when printing, and sometimes it can be the other way around.

And if you don’t think there’s much of a difference between 247,247,244 & 247,247,247 you’d be wrong!

The image below (though exaggerated slightly due to jpeg compression) effectively shows the difference – 247 neutral being at the bottom.

247,247,244 (top) and 247,247,247 (below) – slightly exaggerated by jpeg compression.

See how much ‘warmer’ the top of the square is?

But the real shocker is the black or Dmax value:

Portrait White matte finish paper plotted against wireframe sRGB on L*ab axes.

The wireframe above is the sRGB colour space plotted on the L*ab axes; the shaded volume is the profile for Portrait White. The sRGB profile has a maximum black density of 0RGB and so reaches the bottom of vertical L axis.

However, that 25.8 L* value of the matte finish paper has a huge ‘gap’ underneath it.

The higher the black L* value the larger is the gap.

What does this gap mean for our desktop printing output?

It’s simple – any tones in our image that are DARKER, or have a lower L* value than the Dmax of the destination media will be crushed into “paper black” – so any shadow detail will be lost.

Equally the same can be said for gaps at the top of the L* axis where “paper white” or Dmin is lower than the L* value of the brightest tones in our image – they too will get homogenized into the all-encompassing paper white!

Imagine we’ve just processed an image that makes maximum use of our monitors display gamut in terms of luminosity – it looks magnificent, and will no doubt look equally as such for any form of electronic/digital distribution.

But if we send this image straight to a printer it’ll look really disappointing, if only for the reasons mentioned above – because basically the image will NOT fit on the paper in terms of contrast and tonal distribution, let alone colour fidelity.
It’s at this point where everyone gives up the idea of desktop printing:

It looks like crap
It’s a waste of time
I don’t know what’s happened.
I don’t understand what’s gone wrong

Well, in response to the latter, now you do!

But do we have to worry about all this tech stuff ?

No, we don’t have to WORRY about it – that’s what a colour managed work flow & soft proofing is for.

But it never hurts to UNDERSTAND things, otherwise you just end up in a “monkey see monkey do” situation.

And that’s as dangerous as it can get – change just one thing and you’re in trouble!

But if you can ‘get the point’ of this post then believe me you are well on your way to understanding desktop printing and the simple processes we need to go through to ensure accurate and realistic prints every time we hit the PRINT button.

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Gamma Encoding – Under the Hood

Posted on January 29, 2014 by Andrew Astbury

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?

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

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:

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:

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:

V_O = V_I^γwhere V_O = output value, V_I = input value and γ = gamma

Any input value (V_I) 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

V_I 0 = V_O 0
V_I 0.25 = V_O 0.532
V_I 0.50 = V_O 0.729
V_I 0.75 = V_O 0.878
V_I 1.0 = V_O 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 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:

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?

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:

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 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!

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Photoshop CC Update

Posted on January 16, 2014 by Andrew Astbury

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:

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:

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 “about screen” showing version number.

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

Preferences Interface tab.

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

Next, click File Handling:

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 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 PROPER COLOUR SETTINGS!

That’s it – you’re done!

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