PF is "power focus" -- think of it as a way to activate the electronic focus motors... but under manual control. Kind of like power-windows on a car... the electric, but they only move as long as you are holding the button on the window to go up or down... same thing.
Just in front of the focus ring on your lens there's a "playback ring" (almost looks like a big exposed gear because it has coarse teeth) That ring is spring loaded and can be twisted clockwise or counter-clockwise only a very tiny amount (and if you let go it springs back to it's neutral position). In 'PF' mode that ring can be used to electrically turn focus in or out depending on which way you twist the ring.
Normally that ring is used to return focus to the preset focus distance (when the lens is in AF mode.)
Tim Campbell
5D III, 5D IV, 60Da
