Short for Autofocus, AF is an a sensor, a control system, and a motor that focuses automatically or on a manually selected point or area. The AF speed is highly dependent on the maximum aperture offered by the lens. F-stops of around f/2 to f/2.8 are usually considered optimal in terms of accuracy and focusing speed. Faster lenses than this (e.g.: f/1.4 or f/1.8) normally have very low depth of field, meaning that it takes longer to achieve correct focus, despite the increased amount of light.