flash exposure is controlled by aperture … but only for manual flash

Quite often the short-hand descriptions of the fundamentals of flash photography become misleading ‘facts’.  I frequently see the following statements repeated on the various photography forums and in questions that are emailed to me:

• flash is controlled by aperture
  
• shutter speed controls ambient light.
  (This is sometimes twisted around to a completely misleading version:
  ambient light is only controlled by the shutter speed.)

Even though these descriptions are half-right, they are also half-wrong and will obscure a proper understanding of how shutter speed, aperture and ISO interconnect for ambient light and flash photography.

But first, for the love of all that is good on this planet, if your camera has the ability to set 1/2 stop indents for shutter speed and aperture, please take it off that setting and change it so that your camera has  1/3rd stop indents for everything.  This is essential.

This becomes important in situations where you already have correct exposure, but you want to change any of your settings in tandem.  Then you can change either your shutter speed or aperture or ISO (or any combination of them), simply by counting clicks.  Eg: If you want a full stop more shutter speed (ie 3 clicks), then you can count 2 clicks wider on your aperture, and 1 click more on your ISO, to still give you correct exposure for your new shutter speed.

The topic of TTL flash vs manual flash has been covered here before. However, this time I want to concentrate specifically on those two statements mentioned earlier:

• flash is controlled by aperture

This depends on whether you are shooting manual flash or TTL flash. There are distinct differences.

With TTL flash, if you are within the range of what your flash can put out, then your choice of aperture and ISO in effect becomes transparent.  (Please re-read that statement until the implications become clear.)

With manual flash however, aperture does have a direct influence on flash exposure …
but so does ISO.  We should not leave ISO out of the equation with manual flash.

Shutter speed has no effect on flash exposure (below max sync speed),
BUT … with ambient light, any chosen shutter speed (at a chosen ISO), will dictate a specific aperture. Therefore your choice of shutter speed clearly does have an indirect influence on your flash exposure.

For example, the reason why we choose max flash sync speed when working in bright ambient light, is that at max sync speed we have the widest possible aperture (while remaining outside of High-Speed Flash settings), and therefore have the most range from our flash.  Which is also the reason why it makes most sense to shoot in manual exposure mode on our cameras when doing this, even when using TTL flash.

So in the case of mixing flash with available light, the shutter speed does dictate the aperture, and therefore affects our flash exposure, or how hard our flash has to work.

Now, about the other comment:

• shutter speed controls ambient light.

This is true.   But it neglects to mention the two other players here - aperture and ISO.  This somehow slips by quite often in descriptions of balancing flash with available light, and it becomes another misleading short-hand description.  And as explained earlier, choice of shutter speed actually often does have an important bearing on flash photography.

The theme of this post has been extended by a subsequent article, explaining it with some images:
when aperture does not control flash exposure.  This might help to make it even more clear.

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using faster shutter speeds for sharper photos

If your images are too soft, chances are it is because your chosen shutter speeds are too slow.

A frequent complaint I see on the photography forums, is where the photographer say they used a shutter speed of 1/60th but still have blurry images. Somewhere along the way the urban legend seem to have arisen where 1/60th is that magic shutter speed where we will be assured of sharp images. But of course things are a little more complex than that.

There is a rule of thumb which has it that the inverse of your focal length should give you a good minimum hand-holdable shutter speed.  A number of years back, the most common lens found on 35mm cameras were the 50mm lens – which then became the “standard” lens. Therefore that rule of thumb implied that 1/50th would be considered the minimum hand-holdable shutter speed for the 50mm lens.  Of course 1/50th was rounded up to 1/60th which is an actual shutter speed.

But this is the minimum hand-holdable shutter speed.  Which means you’d still have to be very careful, and be very deliberate in your technique to make sure you don’t get camera shake.  Brace yourself.  Lock your elbows in.  Steady breathing.  Gently squeeze the shutter button.  (Don’t jab at the shutter button.)  All kinds of little techniques to make sure that at that slow a shutter speed – and yes, 1/60th is slow – you don’t get camera shake.   All of this means that you still have to be very careful at that shutter speed.  1/60th won’t magically free you of the possibility of blurry images.

The usable shutter speed at which you won’t get camera shake, depends on various factors:

• focal length of the lens you’re using,
• crop factor of your digital camera (and yes, this does have an effect),
• how still you can hold,
• how still your subject is,
• and if your subject is moving,
  - how close he is to you,
  - what angle he is moving towards you,
  - what speed he is moving at,
• how much camera shake you or your clients would tolerate,

… and so on.
There just aren’t any specific settings anyone can give you.
At best, there are suggestions.  But as with everything, slavishly holding to these will often lead to problems without any understanding of why there is a problem in the first place.

Even though I do use slow shutter speeds , this is most often with my subject in lower light than the background allowing me to use flash to help freeze any movement (or make it less noticeable.) I also use stabilized / vibration reduction lenses. They are immensely useful in reducing camera shake. Of course they don’t help at all with subject movement.

Flash won’t freeze movement effectively in low light if your subject is evenly lit and you simply add some flash.  This technique works best when your subject is shaded or darker than the background.  Or alternately, your entire scene (and subject) is dark by about 2 stops or more under your ambient meter reading, and you add flash to expose correctly for your subject.

Back to the original topic:
In short, if your shutter speed is 1/60th as a default, then your shutter speed is most likely too slow.

So what is my starting point?  Obviously this is geared towards me being a wedding and portrait photographer (and not a sport photographer for example), but I most often start at 1/250th when working in bright light.  The reason for this is that this is (close to) my maximum flash sync speed.  (There are certain implications with that specific choice.)

Even when I don’t use flash, 1/250th  remains a good starting point when working in bright light, giving me a fairly high shutter speed.  In comparison to a slower shutter speed, it helps reduce camera shake.  For portraits I normally want a wide-ish aperture, and in overcast light or shade, a faster shutter speed forces a wide aperture already.

Don’t be afraid to nudge your ISO higher in order to get a faster shutter speed. Keep in mind that it is easier to fix high-ISO noise than it is to try and fix a blurry image.

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