M42 lens mount

Evolution and automation

The M42 lens mount originated in 1938 at the Carl Zeiss facility in Jena, created for the KW camera company’s Praktica line, which had previously used the M40 mount (40 mm by 1 mm DIN). Early lenses used a simple stop-down design, but manufacturers later added features to the mount. The first improvement was a pre-select type, which let the user choose an aperture value without closing the aperture, using a separate ring to quickly stop down to that value. This allowed bright framing and sharp focusing through the viewfinder, then closing the aperture without taking the eye away. Next came “auto” lenses, which include a pin in the mount that, when pushed, closes the aperture against a spring to the preset setting. This design became a common standard across nearly all lens makers. Cameras built for these lenses have a bar at the bottom of the mount that presses the pin when the shutter releases. Early models like the Praktica Nova range used direct finger pressure on the shutter button to move the bar and close the aperture, giving a stopped-down depth-of-field preview before the shutter fired. Later Praktica models removed this feature because some users could, during long exposures, release the shutter button and reopen the aperture before the shutter closed. In Pentax Spotmatic cameras, the bar is operated by spring pressure with timing tied to the shutter, and these cameras also have a separate switch for the light meter circuit that closes the aperture and provides depth-of-field preview. To let auto lenses work on older cameras without the bar, many lenses include a switch or button to engage stop-down mode, often called the Auto-Manual or A/M switch.

Use on modern cameras

The M42 lens mount is simple and offers a huge variety of lenses, so adapters exist for nearly every camera mount, both current and obsolete. These adapters sit between the camera body and the lens, letting you attach an M42 lens to a camera with a different mount. They work best on cameras whose flange depth is the same as or shorter than the M42’s. This includes the Canon EF, Pentax K, Minolta/Konica Minolta/Sony A, Sony E, Samsung NX, Fujifilm X, Nikon Z, and both Four Thirds and Micro Four Thirds systems. Because the lens sits at the correct distance from the film or sensor, you keep the lens’s original focus range without needing any corrective glass.

A potential problem arises when some M42 lenses protrude too far into the camera body, hitting the mirror mechanism or the lens’s own rear element or aperture pin. This is most common with certain Takumar lenses on Canon full-frame cameras.

For cameras with a longer flange focal distance than M42, like the Nikon F, you have three choices. A simple mechanical adapter lets you mount the lens, but it acts like an extension tube: you lose infinity focus while gaining a closer minimum focus distance. An adapter with an optical element can preserve the original focus range, but it will slightly reduce image quality. Alternatively, you can replace the M42 mount on the lens with the camera’s mount, or do the reverse on the camera body itself.

When you put an M42 lens on a modern body, the features you get vary. Some cameras let you shoot in aperture priority mode, while others force full manual control. Focus confirmation might not work. And if you mount an M42 lens on a digital SLR with a sensor smaller than 35mm film, you’ll get a field-of-view crop.