M-Mount Debuts at Photokina

When Leitz unveiled the Leica M3 at Photokina in Cologne in April 1954, the most consequential detail was not the camera body but the new lens mount. The M bayonet replaced the 39mm screw thread used since 1930. Where screwing a lens on took several full rotations, the bayonet locked in a single 60-degree twist. It also moved the rangefinder coupling into the mount itself, so any M lens coupled automatically with no additional adjustment.

The M mount set dimensions — flange distance, bayonet diameter, coupling-cam position — that Leica has never changed. Every M lens made since 1954 fits every M body made since 1954, including the current M11. No other camera system in continuous production can claim that degree of backward compatibility.

For a new Leica owner this matters in a practical way: a 1960s Summicron bought second-hand mounts directly onto a modern body and focuses accurately. It is one of the reasons the used M-lens market is so active, and why Leica M lenses hold their value across decades.