What is flash exposure lock (FEL)?
Flash exposure lock is a technology contained in all type A (E-TTL capable) EOS cameras and EX series (E-TTL capable) flash units. Think of it as the flash equivalent of auto-exposure locking (AEL). Consider this example. Let s say you re taking a photograph of someone who s standing to the side of large object – say a white building. Normally you would turn the camera to focus on the person and also set ambient light metering, then recompose the image by turning it to include the large building as well. The problem is if you re using flash then flash metering will be done on the large white building, not on your friend, since the camera s active focus point (to which flash metering is biased) is going to be over the large building. This problem with focus and recompose is one of the major reasons for flash metering turning out badly on E-TTL cameras. What you really want to do is to lock in the flash exposure settings when you ve got your friend as the main subject in the viewfinder.
