Distance between lens mount flange and sensor plane — must be exact, or the image won't be sharp.
The flange focal distance is the precise distance between the mounting surface of the lens mount and the sensor plane. PL has 52mm, LPL 62mm, EF 44mm, E-Mount 18mm. If the measurement is incorrect, light will hit the sensor at an angle — one side of the image will be sharp, the other not.
Back Focus / Shimming
Cinema cameras are regularly "shimmed" — thin metal shims between the mount and the camera body adjust the flange focal distance to the hundredth of a millimeter. An experienced camera assistant can do this on a test chart within 30 minutes.
News
The discussion about flange focal distances highlights their central role in lens adapters. RED cameras deliberately use very short flange focal distances to allow the use of various lens mounts (PL, EF, Leica) through adapters. For large format cameras, the flange focal distance varies between 99.8 and 116.9mm, while the PL mount, despite its standardized mechanics, exhibits minimal tolerances.
Related terms
Quiz
1. Was beschreibt „Auflagemaß" am besten?
2. Zu welchem Department gehört „Auflagemaß"?