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In low light conditions, my first shot will often be handheld with no flash. If there's not enough light however, the shutter speed can be too slow and I'm not steady enough for a crisp image - so I'll pop up the flash.

However, I've found that the camera often decreases the shutter speed (i.e. a longer exposure) even though the flash is firing! I mainly find it when shooting in Aperture Priority mode (with ISO on auto).

Why is this? Surely the additional light from the flash would mean a shorter exposure would suffice?

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4 Answers 4

up vote 4 down vote accepted

There are two reasons why a camera might choose to use a slower speed with flash than without:

  • it tries to fit exposure into sync speed (1/250s on Canon 60D).
  • and/or it chose lower auto ISO thanks to flash usage

Usually, flash is only used to illuminate subject. Your camera still tries to make sure the background will be correctly exposed too.

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Ahh thank you - your link also helped me understand some other terms I hadn't got my head around (front and rear curtains). Given we're talking about shutter speeds of a second or more, it's likely to be choosing a lower ISO and trying to expose for the background as well. –  Damovisa Sep 26 '11 at 14:45

When in Av mode the shutter is set for correct background exposure with fill flash. In order to get short exposures, use Manual mode and set the shutter to 1/200-1/250, using the aperture value you would in Av mode.

This Av functionality allows you to capture better balanced photos where the background is not dark as you'd get in most automatic modes. It lets the flash just freeze the motion of the subject, if there is some.

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Is there any way I can get it to meter on just the subject? Would spot-metering do it? –  Damovisa Sep 26 '11 at 14:49
    
Consider the situation as if you meter and shoot w/o the flash. Whatever metering mode you use will determine how the background looks. The flash really just function s as a fill flash. There probably is some difference in the exposure compared to a true non-flash mode, but I have no information what it is. Did you try reading the section at the manual? It may be more elaborated. –  ysap Sep 26 '11 at 15:02
    
I'll have a look at the manual, thanks - generally though, the manual just describes the functions, not what they're useful for :) –  Damovisa Sep 26 '11 at 15:22

The rule of thumb for flashes is "aperture controls flash, shutter speed controls ambient". What this means in practice is that by making aperture smaller camera can reduce the relative power of the flash. Because the flash of light from small (ie. non-studio) flashes is very short in duration, exposure times above sync speed (1/125s is typical) do not affect how much light from flash gets to sensor, they only affect ambient.

Thus, by making aperture smaller and exposure time longer the camera tries to get some of the background registered, too, and not just the harsh light of the integrated flash.

Technically the camera could just decrease the length of the flash cycle, but auto-exposure works against pre-programmed profiles, ie. camera tries to guess what kind of situation you are in and in what relation should different areas be exposed, and sometimes it just guesses wrong.

If short exposure time is crucial, then using Tv profile might help.

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That's a really good bit of information there. And like everything I've found while learning photography it seems obvious now I've heard it! –  Damovisa Sep 27 '11 at 1:18

You mention that you are shooting in Av with auto ISO, when using the flash the 60D changes the way it calculates shutter speed to synchronise with the flash and may use the extra light to lower ISO. So you may want to switch to a fixed ISO to give you more control over exposure. Each setting within the camera has a weighting associated with it in the calculations performed by the camera and these will change when you turn on the flash. If ISO is high it may take priority and lead to the issue you are experiencing where the camera does not up the shutter speed and lowers ISO instead because priorities change when a flash is added into the mix.

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