Custom function Flash sync speed in AV

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I am trying to get my head around flash photography I only have a pop up flash to experiment at the moment.

On my camera (Cannon 400D) there is a Custom function called Flash sync speed in AV mode. It defaulted to Auto at the moment but you can change it to 1/200sec.

When In AV mode in what situation would you need to use this custom function to lock the shutter speed to 1/200sec. and why ?

Is this to place less emphasis on the ambient light? so that flash will be the more dominant light in the exposure and to prevent camera shake when hand holding ???

cheers
 
Yep, you've got it. The default when in Av mode is to use the flash only as a fill and to use a shutter speed as long as required to pull in enough ambient light for the background.

In P mode the camera will pick whatever shutter speed it thinks it needs, but will not go slower than 1/60, which might be enough to avoid shake.

If you want to shoot in Av Mode but aren't happy with either whatever the camera gives you, or 1/200 as the only other option, then you'll need to shoot in manual exposure for the ambient light, where you get to choose both the aperture and shutter speed to suit your needs. Many people will shoot in manual exposure when using flash, even if they might typically shoot in Av mode most other times.
 
In addition to what Tim has said, the reason why the shutter speed maxes out at 1/200sec with flash is because of the way focal plane shutters work (as fitted to all DSLRs - google). Some cameras are a little faster, like 1/250sec or 1/300sec, and some slighly slower. This is known as the max x-sync speed.

If you use a higher shutter speed than this, part of the frame - or even most of it - will not be illuminated by the flash. To get around this, many separate hotshoe guns have a High Speed Sync mode which increases the flash duration to cover the full focal plane shutter cycle, but this uses a great deal of power - more than you get out of the pop-up unit - which is why it's not a built-in feature.
 
Thanks for the reply guys much appreciated...As I intend to read more about flash photography Iam sure I will have more questions to ask :)
 
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