D20 underexposing with 550EX and CF #3 set.

[Panza]

I just got my first external flash yesterday (550EX) and when I shoot in AV mode with custom function #3 enabled (shutter speed locked at 1/250sec in AV mode) I get very underexposed pictures. I don't get underexposure in AV mode with the CF #3 disabled.
Is this how it is supposed to be ?
Also with my 70-200 IS mounted the focal length doesn't show up on the flash (70mm). The flash in ETTL mode..
This is with the 20D.

[Jon]

If I understand you correctly, what you're seeing is normal. In Av mode, the flash provides a fill-in while the shutter and aperture vary to get the proper overall exposure. So when you set CF 3 to hold 1/250 sec shutter speed, you're not letting the camera set the correct overall exposure. The flash doesn't correct for this; it's still just providing a fill to your subject, which will be a stop or two less than a normal exposure. If you want the flash to do the work, you'll need to use some FEC.

[Panza]

I see, but why in the world is it this way ? I can't imagine what one would want this effect for. And the internal flash in the D20 makes correct exposures with the CF#3 set...
And what is FEC ? :confused:

[Jon]

FEC - Flash Exposure Compensation. The 20D and the 550EX both allow you to set this. Like EC, but regulates the power of the flash.
The purpose of CF3 is to allow you to decide whether you want to chance "ghosting" in the background or whether you'd prefer to let the flash do most of the work.
If the photos you take with the 20D's pop-up flash come out properly exposed while those with the 550EX are very underexposed, check to see if the 550EX has FEC dialed in. But Canon has chosen a default fill-flash level of -2/3 stop from the "correct" ambient light exposure.