The M42 lens mount is a screw thread mounting standard for attaching lenses to 35 mm cameras, primarily single-lens reflex models. It is more accurately known as the M42 × 1 mm standard, which means that it is a metric screw thread of 42 mm diameter and 1 mm thread pitch. It was first used in Zeiss' Contax S of 1949; this East German branch of Zeiss also sold cameras under the Pentacon name; after merger with other East German photographic manufacturers, the name Praktica was used. M42 thread mount cameras first became well known in Japan under the Praktica brand, and thus the M42 mount is known as the Praktica thread mount there. Since there were no proprietary elements to the M42 mount, many other manufacturers used it; this has led to it being called the Universal thread mount or Universal screw mount by many. The M42 mount was popularized in the United States by Pentax; thus, it is also known as the Pentax thread mount, despite the fact that Pentax did not originate it.
The Tamron T2 mount also uses a 42mm thread, but the pitch is finer at 0.75 mm, and the T-mount is further from the film plane. The two lens mounts are not interchangeable, and attempting to mount an M42 lens on a T2 mount — and vice-versa — will damage the thread of both the lens and the mount.
== Evolution and automation==
The M42 mount was first developed by Carl Zeiss at their Jena plant in 1938 at the request of the KW camera company for their Praktica line, which had previously used M40 (40 mm by 1 mm DIN).
The first lenses were plain stop-down design but many manufacturers extended the M42 lens mount to provide extra features. The first innovation was the pre-select type, which allowed an aperture value to be pre-selected without actually closing the aperture, with a separate ring to close down the aperture quickly to the chosen value. This gave the user the benefit of comfortable framing and focusing with a bright viewfinder and clear focus separation, and then closing the aperture without the need to remove the eye from the eyepiece. A further development followed with "auto" lenses, which have a pin in the mount which closes the aperture against a spring to the chosen setting when it is pushed. This was adopted as a common standard by virtually all lens manufacturers. Cameras designed for these lenses have a bar in the bottom of the mount which depresses the pin when the shutter is released. The first cameras, such as the Praktica Nova range, used physical finger pressure on the shutter button to operate the bar and close the aperture, allowing a stopped-down preview of the depth of field before the shutter fired. However, this function was removed in later Praktica models because some users found it was possible, with longer exposure times, to release the shutter button and open the aperture before the shutter had closed. The bar on [[Pentax Spotmatic]] cameras is operated by spring pressure with timing linked to the shutter, but these cameras also had a separate switch for the light meter circuit which closed the aperture and gave the depth of field preview in this way. To allow auto lenses to be used on earlier cameras without the bar, many lenses were provided with a switch or button to put the lens into stop-down mode, commonly referred to as the Auto-Manual or A/M switch.
The last development of M42 lenses was the introduction of a link between camera and lens to transmit the lens aperture setting, which allowed light metering with the aperture remaining wide open. The means of doing this was not standardised, Praktica's method was to use an electrical connection which transmitted a variable resistance value to the camera's metering circuit. Asahi Pentax developed an additional lever on the lens which operated a variable resistor in the camera mount. These mechanisms spurred the use of electronic shutters linked to the metering circuit, allowing for automated shutter speed selection by the camera ([[aperture priority]]). M42 cameras with wide open metering facility include:
* Praktica PLC2, EE 2 and EE 3 (1975, 1977, 1979) (EE = Electric Eye)
* Fujica ST901
* Olympus [[Olympus FTL|FTL]] (1970)
* Pentax Spotmatic F, Electro Spotmatic (Japan only), ES and ESII (1975, 1971-3)
* Yashica Electro AX (1972)
or automated selection of aperture ([[shutter priority]]) for
* Ricoh TLS-EE (1973)
* Petri-designed Exakta FE 2000 (1978)
Forward and backward compatibility was maintained in most cases so that the newer lenses could be used on older cameras, and old standard lenses could be used on the newer cameras, but of course without the advanced automation. However, Olympus FTL lenses and Fujica screw mount lenses had a projecting cam which means that they cannot be fully screwed down on a regular screw mount body.
[[Chinon Industries|Chinon]] used a different system to provide aperture priority mode with standard Auto-M42 lenses with the CE Memotron (1974), CE-II Memotron and CE-3 Memotron bodies. A similar system was used in the [[Cosina]] Hi-Lite EC. These cameras retained the facitlty of closing the aperture with finger pressure on the release button so that the image could be framed and focussed normally with the lens wide open. As the shutter button was depressed the lens would firstly be stopped down to the selected aperture, then the meter would be switched on and a reading taken. Chinon used a then-modern Silicon (Si) metering cell with fast reaction time compared to the then-standard CdS cells, which made it possible to close the aperture, meter the light and set the shutter speed automatically in one full press of the button. While this method did not offer the same sensitivity advantage of true open-aperture metering, it was much more versatile as it did not require specially-equipped proprietary lenses. Both Cosina and Chinon sold their cameras to various other companies for [[rebranding]], which increased the number of almost identical cameras considerably.
Compatibility problems have been experienced when mounting lenses with aperture transmission levers (e.g., SMC Takumars) on older bodies such as Zenit-E or Mamiya DTL. The aperture lug or a pin can interlock with the screwheads affixing the mount, which then impedes lens removal.
M42 mount cameras fell out of general production during the late 1970s and early 1980s, with the exception of the Russian [[Zenit (camera)|Zenit]] range. Pentax moved to the [[Pentax K mount]] from 1975 onwards, whilst Praktica adopted the electronic B-Mount in 1979. It was briefly revived with the [[Cosina]]-made Voigtländer Bessaflex TM launched in 2003 but this was discontinued in 2007. M42 lenses are still in production at [[Krasnogorskiy Zavod|KMZ]] and at Cosina (under the Voigtländer brand and the ZS line for Carl Zeiss).