Flash "problem"?????

HI I am new to photography and have bought a 7D to grow into. I have recently bought a EX580 II flash. I have been at a party today and have been messing around with both. I am not sure if the camera has a fault. If I put the flash onto ETTL mode and the camera in fully automatic I get a perfectly exposed shot. However if I replicate the ISO and aperture and put the camera in Aperture priority the shutter speed slows right down so that I get a very blurry picture. Am I doing something wrong or is the camera playing up?

JOHN

No.. you are not doing anything wrong. When you set your camera in Av you are telling the camera to pick the appropriate shutter speed for the amount of light you have as read by your light meter. If that light requires say 1/5 second... that is how long your shutter will stay open. The flash at that point is not very bright and your image suffers from motion blur. The camera does what it is told.

If you set your camera in M (manual) and set your iso to say 100, aperture to 8 and shutter to 1/60, now your camera says... hmmm.. we don't have enough light. I'll let the flash determine how much light is needed and voila... the flash doles out the proper amount of light. Now.. at that point let's say you think the exposure is over or under exposed, you can adjust the flash with flash compensation: tell the flash to give you say 1/3 stop more exposure...

I've never used a fully automatic setting but what is happening there is it is picking a shutter speed that is fast enough to avoid motion blur.

Richard

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+1 to that.

For the OP, see;
http://photonotes.org/articles/eos-flash/index2.html#confusion
and maybe also;
http://cpn.canon-europe.com/content/education/masterclass/canon_flash.do
especially sections 2 & 3.

Have you tried searching or browsing these forums?

Another current discussion here on dpreview at the moment just a few posts down from yours... http://forums.dpreview.com/forums/read.asp?forum=1019&message=42115196

Thanks guys.