E-TTL/iTTL also has the ability to make subtle changes in output for flash fill techniques. To add even more ability to flash evolution, Canon added a great feature called AIM (Advanced Integrated Multi Point Control System,) which allows the camera to favor flash exposure where the focus point is set for more accurate exposure with off-center subjects. This type of technology is also seen in the Nikon equipment. (Please read your manual for specifics). What this means is that whichever auto focus point you have set in the cameras viewfinder in where the flash will measure for proper flash output.The content you are trying to access is only available to members. Sorry.