M39 mount

About the M39 mount

M39 here means the Leica thread mount (LTM or L39): a 39mm diameter screw thread at 26 threads per inch with a 28.8mm flange distance, introduced on the Leica I (Model C) in 1930 and standardised in 1931 so lenses could be interchanged between bodies. Leica replaced it with the M bayonet in 1954, but the Soviet FED and Zorki rangefinders that copied the Leica II used it into the 1980s, as did Canon's rangefinders and lenses from Nikon, Voigtlander, and Cosina. Jupiter, Industar, and Orion rangefinder lenses from KMZ, Arsenal, and LZOS are all M39 rangefinder lenses.

Two things separate M39 from the SLR mounts in this catalog. First, the 28.8mm register is short, so rangefinder lenses are compact and sit close to the sensor; wide angles can show colour shift and corner softness on thick-filter-stack sensors, while 50mm and longer lenses are fine. Second, the same 39mm thread was used by the early Zenit SLRs at a 45.2mm register; those SLR lenses (early Helios-44, Industar-50) are not rangefinder lenses and need an M42 step ring, not an LTM adapter. On mirrorless, LTM lenses need only a plain ring, and Soviet rangefinder calibration differences that cause focus errors on Leica bodies do not matter with live-view focusing.

Aperture control

Manual aperture ring, no diaphragm automation. The ring stops the lens down directly.

Adapting it

Plain threaded adapter for mirrorless, no rangefinder coupling needed. A helicoid adapter adds close focus below the 1m rangefinder minimum. Do not confuse with the Zenit SLR M39 mount, which uses the same thread at a 45.2mm register.