The focus puller works to the director of photography and is primarily responsible for keeping the camera in focus during a shot. This will usually mean adjusting or "pulling" the lens focus ring with a follow focus unit during the take, to keep the subject in focus as the action moves. This is known as a focus pull. Because the focus puller does not look through the camera, he or she will measure focal points during rehearsal using a tape measure or laser device and mark corresponding reference points on the follow focus unit. Thus, if an actor is on his mark at 15ft from the camera and the focus ring is set to the corresponding 15ft reference mark, the focus puller knows that the actor is in focus. The focus puller is also responsible for looking after the camera throughout the filming period, and for the maintenance and cleaning of lenses and accessories. Focus pullers will often work up to camera operating and then to DOP.