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 without looking through the camera (the camera operator is doing that), to compensate for camera or subject movement. The depth of field may sometimes be very small, as little as 1/4 or 1/8th of an inch (3–6 mm) in extreme circumstances. Most people on the set will agree that the focus puller's job is among the most technically difficult during production.
To accurately focus on the subject, it is common to measure the distance (usually with a tape measure or, more recently, with electronic tape measures using lasers) between the camera and the subject being focused before the take. Specifically the measurement is from subject to film plane or image sensor, most movie cameras have a small stud even with the film plane where the focus puller will attach his tape measure. Professional 1st ACs 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 in front of the lens and for being aware of lens flares.