A follow focus is a focus control mechanism used in film and video cameras. It is ergonomic rather than strictly necessary; in other words it does not contribute to the basic functionality of a camera but instead allows the operator to be more efficient and precise. It is usually operated by a focus puller but some camera operators prefer to pull their own focus .
The mechanism works through a set of gears on the follow focus that are attached to teeth on the focus ring of the lens. These gears feed to a wheel which, when turned by a focus puller, will spin the teeth and thus the ring on the lens.