What is a follow focus?
A follow focus is a mechanical device that uses gears to make the rotation of a lens’ focus ring easier and more precise for the camera operator or focus puller. It consists of a knob (or two) attached to a body that mounts on rods beneath the lens. A cog on the follow focus meshes with the teeth on the focus ring of the lens. Rotation of the knobs is transferred to the lens ring through a gear mechanism.