M39 lens mount

M39

Leitz Summicron 50 mm in M39 lens mount
Type	Screw
External diameter	39 mm
Inner diameter	N/A
Tabs	N/A
Flange	28.8 mm
Connectors	Telemeter coupling

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.

The Soviets later adopted the LTM mount for their Zenit single-lens-reflex (SLR) cameras, though with the longer optical registration of 45.2 mm, required to allow the mirror room to flip out of the focal path when a picture was taken.

The mount was developed by Oskar Barnack at Leica to provide a system that would allow for the exchange of lenses on their new small film cameras (Leica Type 1 and Leica Type 2), as Zeiss Ikon had indicated that their forthcoming Contax rangefinder cameras would have interchangeable lenses. The LTM system was tested at the request of Leitz on lenses manufactured in small batches by Hugo Meyer in Germany and marketed with a run of Leitz cameras by A. O. Roth in London. The test marketing program was a success, so regular production with Leitz camera bodies and lenses was introduced on the Leica II, and featured on the Leica Standard and Leica III.