First Assistant Camera
1. A focus puller, or 1st assistant cameraman, is a member of a film crew’s camera department whose primary responsibility is to maintain image sharpness on whatever subject or action is being filmed.
“Pulling focus” refers to the act of changing the lens’ focus distance setting in correspondence to a moving subject’s physical distance from the focal plane. Additionally, the focus puller may shift focus from one subject to another within the frame, as dictated by the specific requirements of the shot (cinematic techniques).
A good focus puller will have an intimate knowledge of cinematographic and optical theory.
Source: Wikipedia