Custom Function 4. Anyone here change it? I tested it out last weekend at the race track while practicing my panning and I don't think I'm going to change back. It allows you to change which button AF's. Right now I have AF on the thumb button. I believe it's the one with the "-" magnifying glass. That and AI Servo let me lock focus and meter seperately if I want. The half press of the shutter no longer AF's, it just meters. It allows you to focus and meter serperatly without having the camera focus everything you press the shutter half way down.