An autofocus (or AF) optical system uses a sensor, a control system and a motor or tunable optical element to focus automatically or on a manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication. Autofocus methods are distinguished by their type as being either active, passive or hybrid variants. Wikipedia
The autofocus should be parked if full automatic is avaiable, as otherwise problems occure at night or in fog. Once the automatic no longer finds the subject again to focus, the auto focus function starts up and down what results an ugly video. Try your hand at a preset with autofocus and then set the autofocus off and save your preset.