Friday, June 20, 2008

Lens Mount

A lens mount is an interface — mechanical and often also electrical — between a photographic camera body and a lens. It is confined to cameras where the body allows interchangeable lenses, most usually the single lens reflex type or any movie camera of 16 mm or higher gauge. Lens mounts are also used to connect optical components in instrumentation that may not involve a camera, such as the modular components used in optical laboratory prototyping which join via C-mount or T-mount elements.

A Leica R series teleconverter, with
the female side of the Leica R
bayonet mount. This side
is also used on the
camera body.

A lens mount may be a screw-threaded type, a bayonet-type, or a friction lock type. Modern still camera lens mounts are of the bayonet type, because the bayonet mechanism precisely aligns mechanical and electrical features between lens and body. Screw-threaded mounts are fragile and do not align the lens in a reliable rotational position, yet types such as the C-mount interface are still widely in use for other applications like video cameras and optical instrumentation.

…and the male side.

Bayonet mounts generally have a number of tabs (often three) around the base of the lens, which fit into appropriately sized recesses in the lens mounting plate on the front of the camera. The tabs are often "keyed" in some way to ensure that the lens is only inserted in one orientation, often by making one tab a different size. Once inserted the lens is fastened by turning it a small amount. It is then locked in place by a spring-loaded pin, which can be operated to remove the lens.

Lens mounts of competing manufacturers (Nikon, Canon, Contax/Yashika, Pentax, etc.) are almost always incompatible. Many allege that this is due to the desire of manufacturers to "lock in" consumers to their brand. However, since there are other differences between manufacturers — specifically the flange focal distance from the lens mount to the film or sensor — one would not want to mount a lens which wasn't specifically designed for their type of camera, at least not without an adapter to correct the spacing.

In movie cameras, the two most popular mounts in current usage on professional 35 mm cameras are Arri's PL mount and Panavision's PV mount. The Panavision mounts are exclusively used with Panavision lenses, and thus are only available on Panaflex cameras or third-party cameras "Panavised" by a Panavision rental house, whereas the PL mount style is favored with most other cameras and cine lens manufacturers. Both of these mounts are held in place with locating pins and friction locking rings. Other mounts which are now largely historical or a minority in relation to current practices are listed below.

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