In the past I've done a bunch of flash-related posts for photographers sharing some of my tricks and techniques. If you're a newer blog reader and want to catch up on some of them, I'll provide the links here:
I realized recently after talking with my second shooter about her tendency to underexpose reception shots, that I have never shared one of the simplest, most basic things you can do to more accurately expose using flash. Many of you may already be proficient at this but since I've talked with a number of photographers lately who have never heard of it, I thought I would slap it on the blog.
Do you find that most of your images shot with flash (when it is set to ETTL mode) -- whether you use just on-camera flash, or a combination of on and off-camera flash -- tend to be underexposed? The reason for this is that most images taken at a reception have some type of ambient lighting - whether it's candles, wall or ceiling lights or twinkle lights - in the background. Note the lights in the background of both of these shots:
When your on-camera flash is set to ETTL mode, your camera evaluates how much flash power should be put out and includes these lights in the equation. So in trying to accurately expose for the ambient lights in the shot, your flash under-exposes your subjects. This scenario is much like shooting a subject in a room with window light behind them. You need to over-expose (according to your in-camera meter) in order to properly expose your subject. In the case of our reception scenario, you need to ask your flash to put out more power than your camera is telling it is needed.
Flash compensation is the setting you use to ask your flash to over-expose. As each camera is different, look for this symbol on the top of your camera:
On the 5D mark II the button used to access the flash compensation settings is shared by the ISO setting:
Once you find that button, push it and then try both of your wheels to determine which is used to control your flash compensation. Look for a meter like this on the top of your camera:
This meter is just like the one in your viewfinder expect that it is used to over or under-expose your flash when it is on your camera and in ETTL mode. It's really a brilliant thing.
Because I am always dealing with ambient lights in my reception shots, I tend to leave my flash compensation setting on +2/3 or +1 full stop for the entire reception. But I check my LCD screen to see if I need to make adjustments.