In film production the focus puller, sometimes known as the first assistant camera (1AC), is responsible for maintaining correct focus throughout each camera shot.
The act of altering focus is called pulling focus or racking focus. The focus puller sits to the side of the camera lens and adjusts the focus either by rotating the focus ring or (more commonly) using a follow focus device. A follow focus is simply an attachment to the lens which makes it easier to pull focus.
The focus puller will usually calculate the correct focus setting by measuring the distance from the camera to the subject. Specifically, they will measure the distance from the film or image sensor inside the camera.
The reason that focus pulling requires a dedicated staff member is that it can be very technically challenging. Focus is critical in cinematic film production — this is something that must be done accurately.
Focus pullers may also be given additional responsibilities such as keeping the lens clean and changing filters.