What Is A Follow Focus?
Autofocus is rarely used in professional film making because the focus mechanisms are not smooth enough to be adjusted while the camera is rolling. Â Instead, the focus ring on the lens is rotated manually using a geared hand controller called a follow focus. Â Sometimes the follow focus is turned by the camera operator and sometimes there is a dedicated member of the camera crew called a focus puller.
All cine lenses are manual focus only, designed to be operated smoothly with a follow focus and they have teeth around the focus ring that mesh with the gearing on controller.  DSLR lenses can also be used with a follow focus by attaching after-market gears to the lens.  The process is quick, simple and cheap, and the lightweight nature of DSLR lenses has made the a popular option in recent years.  Not to mention the fact that DSLR lenses can be 10X cheaper than a cine lens.
A good follow focus needs to have a very precise manufacturing tolerance in the gears and bearings, so they can be quite expensive, ranging from $500 right up into the several thousands!
Additional Reading
- What Is A Focus Puller?
- What Is A Cine Lens?
A follow focus
Canon Cine lenses showing the gearing around the lens which meshes with the follow focus to ensure smooth manual focus.