face detect

Face detect is useful when someone's face is the intended subject in the frame. It serves two benefits: 1) It focuses on the face(s), which is especially handy when you let someone else who doesn't know how to focus and recompose use the camera; and 2) It prioritizes the proper exposure of detected faces when metering for automatic exposure, which can be a very handy substitute for dialing in exposure compensation or switching to spot metering. If you're taking photos of people and using autofocus and autoexposure, face detection is generally the way to go. I used to think it was a gimmick, but I now find it genuinely useful.