I've been having trouble using fill flash with my 60D in Av mode. Whenever I use flash in Av mode, the background gets blown out. That is not a flash exposure problem because this happens also when shooting outside when the background can be far away - beyond the reach of the flash. So, I understand that, when using flash, somehow the camera sets a wrong exposure in Av mode. The only solution I found so far is to shoot in manual. I know that many people prefer manual mode but this is not a good solution for me. Manual adjustments always take some time to make and I am loosing precious moments then.
...it is shutter speed that controls your ambient light - so if AV sets your shutter to ... say, 1/30th sec then perhaps the ambient is too bright for that reason.
My guess is that if you shoot in AV, you will run that risk since you are not controlling ambient with the shutter speed.
but obviously more qualified folks should chime in
With a flash on the camera, the shutter speed will be limited to flash sync speed in Av mode. It often leads to blown out backgrounds. A better choice is Tv or manual. P also works if you don't want to think about exposure. Or if you need to use Av, set the flash to HSS (high speed sync).
Thanks to the OP for asking the same question that I hadn't figured out how to ask yet. I've noticed the same symptoms he has but hadn't yet narrowed it down to only being an issue in AV mode. Good diagnosis job there!
And thanks for all the great replies too! Your responses all make perfect sense and I'm anxious to try some more fill-flash shots in TV mode. I might even try Program mode and see how well that sorts things out.