In cinematography, a focus puller or first assistant camera (1 AC) is the member of a film crew responsible for keeping the camera's focus right during a shoot. Often this requires pulling the focus with a follow focus device during the take, to compensate for camera or subject movement.
To accurately focus on the subject, it's common to measure the distance (usually with a tape measure) between the camera and the subject being focused before the take. Specifically the measurement is from subject to film plane or CCD, most movie cameras have a little stud even with the film plane where the focus puller will attach his tape measure. Profesional 1st AC's have many tricks for pulling focus in difficult situations, such as when accurate measurement is impossible.
The 1st AC is also responsible for maintaining the camera including keeping the lens and film gate clean during filming. They are also responsible for placing filters in the matte box infront of the lense and for being aware of lens flares.
