The M42 Universal Camera Lens Mount
When I first got interested in photography as a kid, there were very few choices of SLR cameras available to the amateur photographer on a tight budget. While moderately affordable brands like Chinon, Fujica, Mamiya, Petri, Ricoh and Yashica existed – they were uncommon. Smaller retailers, tended to stock better established safe bets, and if the local camera shop didn’t sell something, then it was pretty much unobtainable. Folks just didn’t go out of town to shop in the early 70s.
Nikon and Olympus cameras were out of reach for most buyers. Minolta and Canon cameras were expensive and a bit new, and tamron lens cheap that made them something of any unknown quantity. The common choice was very much limited to the Japanese Pentax, East German Praktica, or Russian Zenith.
A Pentax camera was an object of desire, but too costly for small boys. Zenith was the cheapest option, and therefore my first camera. Praktica was a make bought by people who couldn’t quite stretch to a Pentax. I’m sure there are plenty of Praktica collectors today, who would disagree, but I don’t believe anyone actual aspired to Praktica ownership back in the early 1970s, and the brand’s main attraction was – it was better than a Zenith.
The factor that united all the affordable brands was the lens mount. Pentax, Praktica, Zenith (plus Chinon, Fujica, Mamiya, Petri, Ricoh and Yashica) all shared a common M42 screw mount, while the more expensive cameras employed different systems.
That was the key to the attractiveness of the M42 family; you could mix and match different makes, and buy other manufactures’ lenses (such as Sigma, Soligor, Tamron, and Vivitar) according to budget and quality requirements. You could even change/upgrade your camera body without needing to buy a whole new kit.
The M42 lens mount comprised a screw thread of 42 mm diameter and 1 mm thread pitch. The system dates back to about 1949 and was first used in by the East German branch of Zeiss on cameras sold under the Pentacon name. Pentacon became Praktica, and accordingly the M42 thread mount became known as the Praktica thread mount.
Many other manufacturers adopted the system because there was no ownership of the standard, which resulted in a new name – the Universal screw mount. Pentax did the most to popularise the M42, causing it to also become known as the Pentax screw mount.
Back in those early days, product development didn’t often go back to the drawing board, and designs evolved by addition of new features as and when required. The first M42 lenses were a simple stop-down design (an aperture setting ring), but this was enhanced to produce a “semi-automatic” diaphragm type of lens. These allowed an aperture value to be pre-selected without actually closing the diaphragm, and a separate – manually operated – ring was added to quickly close the aperture just before exposure. The benefit of this was comfortable framing and focusing with a bright viewfinder, followed by the ability to stop-down without taking your eye from the eyepiece. This is how my first Zenith’s lenses worked.
The next development was the “auto” lens. This had a pin in the mount, which closed the aperture to the chosen setting when it was depressed. Cameras were redesigned to include a bar in the bottom of the mount, which pushed the pin when the shutter was released. To allow auto lenses to be used on earlier cameras, many had an “Auto/Manual” switch to put the lens into stop-down mode.
The next development of M42 lenses was to accommodate open aperture metering, via the introduction of a link to pass the lens aperture setting information to the camera. This advance proved to be the demise of the universal screw mount, since the methodology of operation varied between manufacturers.