The M42 Mount - History, Cameras, and Adapting the Lens Mount

The M42 mount is a screw mount with a 42mm diameter and 1mm thread pitch. It dates back to the late 1940’s.

It’s an open mount, meaning anyone could make lenses or cameras that used it. Pentax is the most well know manufacturer to use it, which is why it is often referred to as the “Pentax screw mount.”

The T-mount looks similar because it also has a 42mm diameter, but uses a 0.75mm thread pitch. The flange focal distance is also longer, which allows T-mount lenses to easily be adapted to any SLR mount.

That’s different from modern lenses that use a bayonet mounts that lock into place with a partial turn. Examples would be the Pentax K, Canon EF, Canon RF, Nikon F, Nikon Z, or Sony E mounts.

History of the M42 Mount

The M42 lens mount was first used by two East German brands in 1949. VEB Zeiss Ikon with their Contax S and Praktica with the KW. In 1959 they would merge and switched to the Pentacon brand.

The Cold War are cost of the cameras prevented them from being popular in the West.

If you read older books or magazines the M42 mount can be referred to as the “Praktica thread mount” or “universal thread mount.”

The Asahi Pentax Spotmatic was Pentax’s first camera to use the M42 mount. The Spotmatic series was very popular and sold well. So despite not being the creator, the M42 mount became synonymous with Pentax.

Additional Features

The original M42 lenses used stop-down metering. That meant manually stopping the lens down in order to meter. Then you could focus with the lens wide open, which allowed as much light as possible to illuminate the focusing screen.

One of the early major innovations was the pre-select type, a mechanism that allowed users to pre-select an aperture value without actually closing the aperture.

The next major change was the introduction of “auto lenses.” These are what you’ll most commonly come across. They have a pin sticking out the back of the mount. When pressed, the aperture closes down to the chosen f-stop.

This is what the M/A switch on lenses controls. The ‘A’ is “auto” and the aperture stopping down is controlled by the pin. Selecting the ‘M’ means the aperture will be manually controlled by the aperture ring. This is helpful for backwards compatibility with older cameras.

The End?

Pentax switched to the bayonet style K-mount in 1975. The most well known models are the K1000 and LX.

Praktica switched to the electronic B-mount in 1979.

With no manufacturers actively using the M42 mount, it fell out of favor in the 1980’s.

Cosina made a homage to the mount with Voigtländer Bessaflex TM in 2003. This was possible because of the sustained popularity of some M42 lenses. The radioactive lenses doped with thorium still have some of the best color rending of any lenses ever made.

Disadvantages of M42 Mount

Two major problems have become more common with age.

The first is that it’s possible to cross thread a lens, getting it stuck on camera. The lens won’t be usable either because it will not be perpendicular to the film plane.

Lenses can also get stuck due to corrosion. Many cameras have sat unused for decades. It can be a challenge to unmount lenses.

The second problem is helicoid grease drying up. This can make turning a focus ring more difficult than it should be. So instead of focusing, you end up unscrewing the lens. Very annoying.

Universal Problems

Mixing camera bodies and lenses from different manufacturers can cause indexing problems. The aperture ring might not point up, which makes seeing what aperture is selected difficult.

I have most commonly experienced this problem when using lens adapters.

Adapting M42 Lenses

M42 lenses have a flange focal distance of 45.46mm. They need to be that far away from the film (or sensor) plane in order to focus correctly.

If the flange focal distance is too large, a lens will not be able to focus to infinity. Too close, and it will act as a macro lens.

Pentax kept backwards compatibility in mind with the K-mount. It has the same 45.46mm flange focal distance, but has a slightly larger diameter. Pentax made adapters that allow M42 lenses to be used on K-mount cameras.

It’s possible to find adapters for the Konica AR, Minolta SR (MD), Canon EF, FD, and FL mounts because they all have shorter flange distances.

There are also lens adapters for every mirrorless system. They have short flange distances because they don’t need the space for a mirror.

Olympus OM and Nikon F mounts aren’t good choices as they have longer flange distances. Adapters exist, but you won’t be able to focus to infinity. There are adapters with optics to allow infinity focus, but they are low quality and destroy image quality.