M42 lens mount
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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 far further from the film plane.
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[edit] Evolution and automation
Many manufacturers extended the M42 lens mount to provide extra features. The first change was the inclusion of an automatic stop-down pin. This pin sticks out from the lens and is pushed in to close the aperture to the desired setting for exposure by a lever in the body. This allowed comfortable framing and focusing with the lens wide open and hence a bright viewfinder image with clear focus separation. Many M42 lenses have an A/M switch to select between automatic and manual aperture stop down. "Manual" needs to be selected for use on older M42 cameras and some adapters. Some of the later M42 lenses did not include such a switch, they are Auto-only, which can create problems on certain adapters.
Some later lenses could transmit the selected aperture to the camera body while the lens was still wide open, by means of an additional pin or lug that engaged the camera body. This allowed true open aperture metering with dedicated proprietary lenses.
These mechanisms also created the possibility for automated shutter speed selection by the camera (aperture priority) for
- Fujica ST901 (1975)
- Olympus FTL (1970)
- Pentax Electro Spotmatic (Japan only), ES, Spotmatic F and ESII (1971, 1971, 1973, 1973)
- Praktica EE 2 and EE 3 (1977, 1979, EE = Electric Eye)
- Yashica Electro AX (1972)
or automated selection of aperture (shutter priority) for
- Ricoh TLS-EE (1973)
- Petri-designed Exakta FE 2000 (1978)
These special lenses could be used on standard M42 cameras as well and standard M42 lenses could be used on those special cameras, but of course without the advanced automation.
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. The image would be framed and focussed normally with the lens wide open. As the shutter button was depressed the lens would be stopped down to the selected aperture, a quick stop-down meter reading taken, and the shutter fired for correct exposure. Chinon used a then-modern Silicon (Si) metering cell. Its quick reaction time compared to the then-standard CdS cells made it usable for this application. While this method did not offer the same sensitivity advantage of true open-aperture metering, its versatility was much better, as it did not depend on 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 reported when mounting lenses with aperture transmission lugs (e.g., SMC Takumars) on older bodies, such as Zenit-E or Mamiya DTL. The aperture lug interlocks with the screws that affix the mount effectively impeding lens removal.
[edit] Use on modern cameras
Current bodies with M42 lens mount include the (2007 discontinued) Cosina-made Voigtländer Bessaflex TM, as well as Russian-made Zenit SLRs. M42 lenses are also still in production at KMZ and at Cosina (under Voigtländer brand and the ZS line for Carl Zeiss).
Furthermore, because of the simplicity of the M42 lens mount and the large selection of existing lenses, M42 adaptors exist for all current and many obsolete lens mounts. This allows the use of M42 lenses on modern film and digital SLRs.
M42 adaptors work best on bodies with a flange depth less than or equal to M42's flange depth of 45.46 mm. This allows the lens to be mounted the correct distance from the film or sensor, retaining the original focus range of the lens.
On bodies with a flange focal distance greater than that of M42, most notably Nikon, two options are available. A simple mechanical adaptor allows the lens to be mounted, but the effect is similar to the introduction of an extension tube, reducing the minimum focus distance at the expense of losing infinity focus. Alternatively, an adaptor with an optical element can be used to retain the original focus range of the lens, at the expense of some image quality.
The level of functionality available from a modern body when an M42 lens is mounted may vary. Some bodies may be operated in aperture-priority mode, others will only allow full manual control in this circumstance. Focus confirmation may not be available.
Mounting an M42 lens on a digital SLR with a sensor smaller than 35 mm film results in FOV crop.
[edit] Specifications
- Thread: M42 × 1
- Flange to film: 45.46 mm (front of mount to film plane)
