I've been trying to figure out how to film video with the camera auto focusing constantly, without requiring me to put my finger down on the shutter button. However, I've looked at manuals and YouTube videos, and my camera doesn't give me the ability to choose AF-F mode. I only get AF-A, AF-S, and AF-C. I've tried using those while filming, but it doesn't auto focus until I put my finger down on the button. Is there anything I need to do differently?