A Pioneer System

The First Standardized Interchangeable Lens Photography System

The M39 mount system invented by Leica in 1930 was the first standardized interchangeable lens photography system.

It was indeed the first system to offer a simple solution for changing lenses: Leica lenses could be directly screwed onto all of the brand’s rangefinder cameras without the need for adapters.

M39 Existed Before Leica

Leica did not invent the 39mm screw mount with a 26 tpi thread pitch. It had already been used in scientific fields since the 1920s.

Leica would gradually abandon this system during the 1950s.

This transition happened in favor of a more efficient bayonet mount system (the Leica M3 began production in 1954, and Leica stopped producing M39 mount cameras in 1960).

A Widely Copied System

As early as the 1930s, several manufacturers such as Zorki, FED, Canon, and others began copying Leica and producing their own cameras and lenses using the M39 mount.

M39: A Simple Technical Solution

The M39 mount was indeed a very simple technical solution to adopt: the standard had already existed since the 1920s, requiring no research and development, and it was much easier to manufacture than a bayonet mount.

The Soviet M39 Mount Is Different from Leica’s M39

It should be noted that there are variations between Leica M39 mounts and Soviet M39 rangefinder mounts that limit compatibility between them (the thread pitches differ, affecting focusing accuracy).

The M39 mount is therefore not a universal system like the M42, even though some Japanese lenses and cameras are compatible with the Leica M39 mount (Canon, Nicca, etc.).

A System Not Adopted by Everyone

Special Cases of Certain M39 Lenses

Not all lenses that screw onto an M39 mount are compatible with rangefinder cameras.

Flange focal distances may differ. This is obviously the case for enlarger lenses, but also for Zenit and Braun cameras, which used this mount while modifying the flange distance.

Rangefinder Cameras in Europe from the 1930s to the 1950s

While the M39 screw mount became popular among Soviet and Japanese rangefinder camera manufacturers, this was not the case in Europe, where each company followed its own path.

This was notably true for Contax and its RF mount, but also for Foca in France, which chose a screw mount with a smaller diameter.

The M39 System Today

Limited Modern Production

Cosina, through the Voigtländer brand, continued producing cameras using this mount until quite recently (the Bessa R remained in production until the 2010s).

Lenses using this mount are still being manufactured today, although production remains limited.

Easily Adaptable Lenses

However, the excellent lenses using this mount can still easily be adapted to modern cameras using adapters.

Sources:
https://fr.wikipedia.org/wiki/Monture_M39
https://en.wikipedia.org/wiki/M39_lens_mount
https://mrleica.com/canon-ltm-lenses/