system useless. The Arri D-20 and D-21 system, however, does have removable lenses and a rotating optical shutter, which means that the CMOS sensor can be exposed in much the same way as a film gate and thus needs to be kept assiduously clean. Methods of checking the gate There are three different commonly accepted ways to check a film gate for hairs. Pulling the lens In this method, the lens is taken off of the camera and the shutter is moved either with an automatic button (usually labeled "phase" or "dust check") or by manually inching it to an exposed position. The focus puller examines the gate with a flashlight, sometimes attached to a magnifying glass. Through the lens This method tends to be most useful on long lenses and zoom lenses, especially when larger lenses may be difficult to take on and off again quickly. The aperture is fully opened, with the shutter opened, focus set to infinity, and if a zoom, zoomed in as tight as possible. The lens itself acts as a large magnifying glass for the focus puller, who only needs to position the flashlight and their line of sight properly to scan the gate. Pulling the gate Many cameras, though not all, allow for the gate itself to be removed from the camera and examined under a light by the focus puller. See also List of film formats Pin registration Wet-transfer film gate References Film and video technology Cinematography