follow focus
 

what does a follow focus system do?

It is essentially a knob and gear that turns the focus ring on the lens. It is mounted perpendicular to the lens, so it is more ergonomic than turning the lens itself. Instead of turning your whole wrist to focus, you just need to turn your hand and fingers which are more coordinated. Also, it usually has marking strips that allow different focus points to be indicated.

Ahh...I see.