This Polaroid follow focus is a must for videographers who want to maintain perfect focus on a subject moving closer to or farther from the camera. Most auto-focus lenses are unable to react instantly to subject movement. Moving one step closer to the camera and the subject will be momentarily out of focus until the lens motor catches up to the auto-focus sensors request to refocus. The control moves quietly and accurately; the gears have no play in them. The follow focus can be mounted on either side of the lens and is mounted to an optional mount or stabilizer system.
Because most consumer level video cameras, DSLRs with video capability, and even some professional video camera lenses are not designed with geared barrels, a flexible, geared strap is included which allows you to give those lenses a temporary external gear. The Polaroid follow focus provides the camera operator with a metal pointer and a removable ring for annotating start and stop focus points using a nonpermanent marker."