Why does my Canon 60D sometimes lengthen exposure when using the flash?

Question

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?

Answer

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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