It's hard to give you any help without further info, as kiwi is asking for. I have a quick question though. Have you noticed that the flash seems loose on the hot-shoe?
Hi Camerasnoop, No the flash doesn't seem loose on the hot shoe. I'm thinking maybe it's when I focus then recompose as the settings may change when I recompose the shot. Maybe I need to lock the exposure in AV mode in order for the camera to keep the correct exposure settings to match the flash settings. Does that make sense?????
Post some examples with exif attached.
Yep. post some of the photos, we can assist more when we can see what the problem is and what the settings were that you used.
I think I know what's happening, if it occurs mainly when you pre-focus & recompose. I'm guessing, for example, you are trying to take a portrait of a friend - half pressing shutter to focus on them, then moving camera to one side to get classic "Rule of Third's" composition? I assume you have the flash in ETTL mode (not manual)? Well the half press on the shutter is locking focus but NOT exposure. When you recompose, the central part of the lens (where ETTL mainly measures distance/exposure) is now pointed off into the distance so when fully press shutter the flash tries to light up the entire background and in the process overexposes your subject in the foreground!
I'm not familiar with Canon but I assume that when you enable the flash in Av mode that the shutter speed is set to your camera's sync speed - probably 1/200 or 1/250. If you have a wide aperture set at the time then there is a good chance that the scene will be over-exposed. There is possibly a warning in the viewfinder but it can be easy to miss when you are concentrating on the scene in front of you. (From memory Nikons set the shutter speed display to "HI" when this occurs).
For example, on a bright day you set the aperture to f/5.6 - the camera sets the shutter speed to 1/800. You decide to use the flash to add some fill light. The shutter speed is now set to 1/200 but the aperture is still at f/5.6 so the scene will be over-exposed by 2 stops. You would need to stop down your aperture until 1/200 was the correct shutter speed for the ambient light conditions (f/11 in this example).
If you want to use flash with wide apertures in daylight check out whether you gear supports "high-speed flash sync" (not sure exactly what Canon call it). You trade off a reduction in flash power for the ability to be able to shoot at higher than sync shutter speed.
I assume that when you enable the flash in Av mode that the shutter speed is set to your camera's sync speed - probably 1/200 or 1/250.Just to add, if the shutter speed is already slower than the sync speed before activating the flash it probably won't change (although this can depend on settings), so you should get correct exposure of the ambient conditions in that case.