M39 lens mount

The M39 lens mount is a screw thread mounting system for attaching lenses to 35 mm cameras, primarily rangefinder (RF) Leicas. It is also the most common mount for Photographic enlarger lenses.

True Leica Thread-Mount (LTM) is 39 mm in diameter and has a thread of 26 turns-per-inch or threads-per-inch (tpi) (approximately 0.977 mm pitch) of Whitworth thread form. Whitworth threads were then the norm in microscope manufacture. The Royal Microscopical Society (RMS) thread, also known as society thread, is a special 0.8" diameter x 36 tpi Whitworth thread used for microscope objective lenses and Leitz was a major manufacturer of microscopes, so the tooling at the plant was already set up to produce the Whitworth thread form. The Soviets in the 1930s produced their early FED cameras in M39×1 (39 mm by 1 mm DIN thread). Early Canon cameras also used a different M39 × 24 tpi thread mount, called "J-mount".

True LTM lenses have a flange focal distance of 28.8 mm, though this is of little importance for lenses used on bellows enlargers.

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 rangefinder camera, 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 lens mount may be a screw-threaded type, a bayonet-type, or a breech-lock (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.