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...I was in Av and found the shutter still had a looong exposure and the flash fired twice. Why didn't the camera, 5D2, know the flash would fire and choose a shorter exp.V.?
It's the way Canons work. In P, Av, and Tv modes, the camera will try to set a good exposure for the background, and use the flash to raise the exposure on the subject, often at a "fill light" level. This is covered beginning on page 26 of the 550EX User Guide.
Many of us prefer to use Manual Exposure, where we set the aperture and shutter speed, even when using ETTL flash, so we can control DoF, motion, background level, etc., and the flash will then try to correctly expose the subject. If we set an exposure that will yield a dark scene, the flash will then automatically boost the light level, becoming the key light. If we set an exposure that will yield a lighter scene, the flash will then automatically adjust the light level to be a fill light. It'll always be our choice, not the camera's. (Of course, Manual Exposure and Manual Flash gives even more control, but that's another topic.)
...I hate to appear so ignorant of flash work but honestly never have done much due to the inability to get it off camera.
As Dave rightly says, everyone was a beginner once. Ask away; that's what we're here for.
BTW, there are two books I like to suggest for people who are just getting into flash and really want to take it to a high level. The first is The Speedliter's Handbook, which is a Canon-specific text exploring all the bells and whistles of the Canon Speedlite system, and the other is On-camera Flash Techniques for Wedding and Portrait Photographers (title self explanatory).
There's also a sequel to the latter, once you've mastered the first two:
Edited on Dec 27, 2012 at 07:57 PM · View previous versions