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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Noise and the Camera Sensor

Posted on December 9, 2013 by Andrew Astbury

Camera sensors all suffer with two major afflictions; diffraction and noise; and between them these two afflictions cause more consternation amongst photographers than anything else.

In this post I’m going to concentrate on NOISE, that most feared of sensor afflictions, and its biggest influencer – LIGHT, and its properties.

What Is Light?

As humans we perceive light as being a constant continuous stream or flow of electromagnetic energy, but it isn’t! Instead of flowing like water it behaves more like rain, or indeed, bullets from a machine gun! Here’s a very basic physics lesson:

Below is a diagram showing the Bohr atomic model.

We have a single positively charged proton (black) forming the nucleus, and a single negatively charged electron (green) orbiting the nucleus.

The orbit distance n1 is defined by the electrostatic balance of the two opposing charges.

Andy Astbury,noise,light,Bohr atomic model

The Bohr Atomic Model

If we apply energy to the system then a ‘tipping point’ is reached and the electron is forced to move away from the nucleus – n2.

Apply even more energy and the system tips again and the electron is forced to move to an even higher energy level – n3.

Now here’s the fun bit – stop applying energy to the system.

As the system is no longer needing to cope with the excess energy it returns to its natural ‘ground’ state and the electron falls back to n1.

In the process the electron sheds the energy it has absorbed – the red squiggly bit – as a quantum, or packet, of electromagnetic energy.

This is basically how a flash gun works.

This ‘packet’ has a start and an end; the start happens as the electron begins its fall back to its ground state; and the end occurs once the electron arrives at n1 – therefore it can perhaps be tentatively thought of as being particulate in nature.

So now you know what Prof. Brian Cox knows – CERN here we come!

Right, so what’s this got to do with photography and camera sensor noise

Camera Sensor Noise

All camera sensors are effected by noise, and this noise comes in various guises:

Firstly, the ‘noise control’ sections of most processing software we use tend to break it down into two components; luminosity, or luminance noise; and colour noise. Below is a rather crappy image that I’m using to illustrate what we might assume is the reality of noise:

This shot shows both Colour & Luminance noise.
The insert shows the shot and the small white rectangle is the area we’re concentrating on.

Now let’s look at the two basic components: Firstly the LUMINANCE component

Here we see the LUMINANCE noise component – colour & colour noise components have been removed for clarity.

Next, the COLOUR NOISE bit:

The COLOUR NOISE component of the area we’re looking at. All luminance noise has been removed.

I must stress that the majority of colour noise you see in your files inside LR,ACR,CapOne,PS etc: is ‘demosaicing colour noise’, which occurs during the demosaic processes.

But the truth is, it’s not that simple.

Localised random colour errors are generated ‘on sensor’ due to the individual sensor characteristics as we’ll see in a moment, because noise, in truth, comes in various guises that collectively effect luminosity and colour:

Shot Noise

This first type of noise is Shot Noise – called so because it’s basically an intrinsic part of the exposure, and is caused by photon flux in the light reflected by the subject/scene.

Remember – we see light in a different way to that of our camera. What we don’t notice is the fact that photon streams rise and fall in intensity – they ‘flux’ – these variations happen far too fast for our eyes to notice, but they do effect the sensor output.

On top of this ‘fluxing’ problem we have something more obvious to consider.

Lighter subjects reflect more light (more photons), darker subjects reflect less light (less photons).

Your exposure is always going to some sort of ‘average’, and so is only going to be ‘accurate’ for certain areas of the scene.

Lighter areas will be leaning towards over exposure; darker areas towards under exposure – your exposure can’t be perfect for all tones contained in the scene.

Tonal areas outside of the ‘average exposure perfection’ – especially the darker ones – may well contain more shot noise.

Shot noise is therefore quite regular in its distribution, but in certain areas it becomes irregular – so its often described as ‘pseudo random’ .

Read Noise

Read Noise – now we come to a different category of noise completely.

The image is somewhat exaggerated so that you can see it, but basically this is a ‘zero light’ exposure; take a shot with the lens cap on and this is what happens!

What you can see here is the background sensor noise when you take any shot.

Certain photosites on the sensor are actually generating electrons even in the complete absence of light – seeing as they’re photo-voltaic they shouldn’t be doing this – but they do.

Added to this are AD Converter errors and general ‘system noise’ generated by the camera – so we can regard Read Noise as being like the background hiss, hum and rumble we can hear on a record deck when we turn the Dolby off.

Thermal & Pattern Noise

In the same category as Read Noise are two other types of noise – thermal and pattern.

Both again have nothing to do with light falling on the sensor, as this too was shot under a duvet with the lens cap on – a 30 minute exposure at ISO 100 – not beyond stupid when you think of astro photography and star trail shots in particular.

You can see in the example that there are lighter and darker areas especially over towards the right side and top right corner – this is Thermal Noise.

During long exposures the sensor actually heats up, which in turn increases the response of photosites in those areas and causes them to release more electrons.

You can also see distinct vertical and some horizontal banding in the example image – this is pattern noise, yet another sensor noise signature.

Under Exposure Noise – pretty much what most photographers think of when they hear the word “noise”.

Read Noise, Pattern Noise, Thermal Noise and to a degree Shot Noise all go together to form a ‘base line noise signature’ for your particular sensor, so when we put them all together and take a shot where we need to tweak the exposure in the shadow areas a little we get an overall Under Exposure Noise characteristic for our camera – which let’s not forget, contains other elements of both luminance noise and colour noise components derived from the ISO settings we use.

All sensors have a base ISO – this can be thought of as the speed rating which yields the highest Dynamic Range (Dynamic Range falls with increasing ISO values, which is basically under exposure).

At this base ISO the levels of background noise generated by the sensor just being active (Pattern,Read & Thermal) will be at their lowest, and can be thought of as the ‘base noise’ of the sensor.

How visually apparent this base noise level is depends on what is called the Signal to Noise Ratio – the higher the S/N ratio the less you see the noise.

And what is it that gives us a high signal?

MORE Photons – that’s what..!

The more photons each photosite on the sensor can gather during the exposure then the more ‘masked’ will be any internal noise.

And how do we catch more photons?

By using a sensor with BIGGER photosites, a larger pixel pitch – that’s how. And bigger photosites means LESS MEGAPIXELS – allow me to explain.

Here we see a representation of various sized photosites from different sensors.

On the right is the photosite of a Nikon D3s – a massive ‘bucket’ for catching photons in – and 12Mp resolution.

Moving left we have another FX sensor photosite – the D3X at 24Mp, and then the crackpot D800 and it’s mental 36Mp tiny photosite – can you tell I dislike the D800 yet?

One the extreme left is the photosite from the 1.5x APS-C D7100 just for comparison.

Now cast your mind back to the start of this post where I said we could tentatively regard photons as particles – well, let’s imagine them as rain drops, and the photosites in the diagram above as different sized buckets.

Let’s put the buckets out in the back yard and let’s make the weather turn to rain:

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

Various sizes of photosites catching photon rain.

Here it comes…

It’s raining

OK – we’ve had 2 inches of rain in 10 seconds! Make it stop!

All buckets have 2 inches of water in them, but which has caught the biggest volume of rain?

Thank God for that..

If we now get back to reality, we can liken the duration of the rain downpour as shutter speed, the rain drops themselves as photons falling on the sensor, and the consistency of water depth in each ‘bucket’ as a correct level of exposure.

Which bucket has the largest volume of water, or which photosite has captured the most photons – in other words which sensor has the highest S/N Ratio? That’s right – the 12Mp D3s.

To put this into practical terms let’s consider the next diagram:

Increased pixel pitch = Increased Signal to Noise Ratio

The importance of S/N ratio and its relevance to camera sensor noise can be seen clearly in the diagram above – but we are talking about base noise at native or base ISO.

If we now look at increasing the ISO speed we have a potential problem.

As I mentioned before, increasing ISO is basically UNDER EXPOSURE followed by in-camera “push processing” – now I’m showing my age..

The effect of increased ISO – in camera “push processing” automatically lift the exposure value to where the camera thinks it is supposed to be.

By under exposing the image we reduce the overall Signal to Noise Ratio, then the camera internals lift all the levels by a process of amplification – and this includes amplifying the original level of base noise.

So now you know WHY and HOW your images look noisy at higher ISO’s – or so you’d think – again, it’s not that simple; take the next two image crops for instance:

Kingfisher – ISO 3200 Nikon D4 – POOR LIGHT – Click for bigger view

Kingfisher – ISO 3200 Nikon D4 – GOOD LIGHT – CLICK for bigger view

If you click on the images (they’ll open up in new browser tabs) you’ll see that the noise from 3200 ISO on the D4 is a lot more apparent on the image taken in poor light than it is on the image taken in full sun.

You’ll also notice that in both cases the noise is less apparent in the high frequency detail (sharp high detail areas) and more apparent in areas of low frequency detail (blurred background).

So here’s “The Andy Approach” to noise and high ISO.

1. It’s not a good idea to use higher ISO settings just to combat poor light – in poor light everything looks like crap, and if it looks crap then the image will look even crappier.When I get in a poor light situation and I’m not faced with a “shot in a million” then I don’t take the shot.

2. There’s a big difference between poor light and low light that looks good – if that’s the case shoot as close to base ISO as you can get away with in terms of shutter speed.

3. I you shoot landscapes then shoot at base ISO at all times and use a tripod and remote release – make full use of your sensors dynamic range.

4. The Important One – don’t get hooked on megapixels and so-called sensor resolution – I’ve made thousands of landscape sales shot on a 12Mp D3 at 100 ISO. If you are compelled to have more megapixels buy a medium format camera which will generate a higher S/N Ratio because the photosites are larger.

5. If you shoot wildlife you’ll find that the necessity for full dynamic range decreases with angle of view/increasing focal length – using a 500mm lens you are looking at a very small section of what your eye can see, and tones contained within that small window will rarely occupy anywhere near the full camera dynamic range.

Under good light this will allow you to use a higher ISO in order to gain that crucial bit of extra shutter speed – remember, wildlife images tend to be at least 30 to 35% high frequency detail – noise will not be as apparent in these areas as it is in the background; hence to ubiquitous saying of wildlife photographers “Watch your background at all times”.

Well, I think that’s enough to be going on with – but there’s oh so much more!

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