M42 (lens mount)

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M42 is a standardized, manufacturer-independent lens connection for interchangeable lenses using a screw thread in the field of photo technology , which has been used by many manufacturers. The "M" in the name stands for a metric diameter, the number for the diameter in millimeters.

M42 thread

The outer diameter of the screw thread is 42 mm with a thread pitch of 1 mm (height per revolution). The back focus is 45.46 mm. During the normal exposure measurement process on an M42 camera, the aperture is closed to the preselected value during exposure measurement. The viewfinder image darkens if the aperture is not manually preselected to be fully open. This procedure is called " aperture measurement ".

M42 screw thread with orifice plunger (silver)

M42 is the successor to the older M39 screw thread and comes from a time when camera manufacturers were not yet trying to bind customers to their own system through proprietary lens mounts. It was widely used, for example, as a lens connection for the older generations of Praktica cameras and the associated lenses (including Beroflex , Meyer-Optik ) manufactured by VEB Pentacon . Many of the M42 cameras and lenses offered by Foto Quelle (“Revue”) or Photo Porst in the 1960s and 1970s came from the same source; Furthermore, the Asahi Pentax Spotmatic and Edixa Reflex had M42 lens mounts. The M42 screw thread, which used to be widely used, now only plays a subordinate role in new business. However, numerous camera housings and an almost unmistakable selection of lenses for this thread are available cheaply on the used market.

Lenses with an M42 thread can be used on almost all cameras for which there are mechanical lens adapters . In principle, adaptation to many systems is possible, but for this purpose protruding cover transmission parts often have to be removed and / or manipulated. The rear lens may also protrude too far when the focus is at infinity and then damage the camera and / or lens. As a rule, all automatic functions are then usually lost except for the automatic timing. The camera used should therefore be able to measure the exposure through the lens even with the working aperture .

The connections of M42 lenses are by no means all the same. In addition to the normal attachment, in which the lens is fixed by tightening the thread, there are also lenses with mechanical stops or markings. These systems allow the transmission of the aperture preselected on the lens through transmission elements to the camera housing on the measuring mechanism and thus an exposure measurement with the aperture remaining open ( open aperture measurement , aperture simulation) (three electrical contacts with Praktica "electric" or mechanically on all other systems).

M42 measurement systems

These seven different open glare measurement systems are not compatible with each other and therefore each have their own SLR cameras and lens series as follows:

  • for Zeiss / Voigtländer / Rollei: Zeiss Ikon SL706, Ifbaflex (French export variant), Voigtländer VSL1. Associated lenses do not have an extra identification name, there are seven fixed focal lengths.
  • for Praktica: EE2, EE3, LLC, PLC 2, PLC 3, VLC, VLC 2, VLC 3. Lenses have the suffix “electric”, manufactured either by Carl Zeiss Jena or Pentacon, there are 10 fixed focal lengths and a zoom.
  • for Pentacon SUPER: camera bears the same name. Corresponding lenses do not have an extra identification name, there are seven fixed focal lengths (two of them with adapter Pentacon Six / Pentacon Super).
  • for Pentax: ELECTRO-Spotmatic, ES, ESII, Spotmatic F (plus Honeywell USA export variants). Associated lenses are labeled “Super-Multi-Coated” or “SMC”, there are 22 fixed focal lengths and three zooms.
  • for Fujica: ST801, ST901, ST705, ST705W, ST605II, AZ-1. Corresponding lenses have no special identification name, there are 16 fixed focal lengths and 10 zooms.
  • for Mamiya: MSX 500, MSX 1000, Sears TLS 1000 MXB (USA export variant), DSX 500, DSX 1000, DSX 1000B. Associated lenses have the suffix "SX", there are 15 fixed focal lengths and a zoom.
  • for Olympus: FTL. Associated lenses do not have an extra identification name, there are seven different fixed focal lengths.

Information on the number of the respective lens series relates to the respective brands and third-party manufacturers without adapter lenses (Sigma YS, Sun YS, Tamron Adaptall and Vivitar TX).

The only standard of aperture function that M42 lenses can partially rely on is the aperture closing function with an axially actuated pin. This pin goes down to the six o'clock position when you tighten it.

If the camera does not have an actuation lever for dimming the lens, it must either have a purely mechanical diaphragm ("preselection diaphragm") (i.e. no pin on the lens connection) or it must have an "auto / manual" switchover so that you can switch to "manual" -Position can close the aperture even without the dimming lever for exposure measurement with the working aperture. Otherwise, the aperture remains open, recording and measurement are also possible.

M42 compatibility

SLR systems

Lens adapter for Canon EF with Dandelion chip for transferring lens data from non-bayonet lenses
M42 lens with adapter for Pentax K bayonet; easily recognizable: the pressed diaphragm plunger
  • Canon: All DSLRs allow automatic exposure with aperture preselection . Measurement and AF confirmation are possible with special Dandelion adapters.
  • Nikon: As with old Nikon lenses with AI iris transfer, an iris coupling lever on the camera is required for aperture transmission, which many low-end cameras are no longer equipped with, including most DX- DSLRs, but also some more recent analog cameras . If this coupling is available, aperture preselection can be used. AF confirmation is possible, but focus at infinity requires an adapter with a corrective lens.
  • FourThirds: All DSLRs require manual settings. Image stabilization works. Measurement and AF confirmation are possible with special Dandelion adapters.
  • Pentax: All DSLRs allow aperture priority metering with AF confirmation and infinity focus. Image stabilization works. Focus trap is possible depending on the camera model. The contact surface of the lens may have to be stripped of paint for AF confirmation and focus trap to bridge camera contacts .
  • Minolta / Sony: All DSLRs require manual settings. Image stabilization works. Measurement and AF confirmation are possible with special Dandelion adapters.
  • Sigma: SD1-SD15 enables automatic exposure metering with aperture priority, focus at infinity and AF confirmation. The aperture should be set to 1.0 for correct exposure metering on the camera.
Lens connection Back focus Infinity position AF confirmation Exposure metering Image stabilization
Canon EF 44.00 mm Yes Dandelion chip Open aperture measurement No
Nikon F 46.50 mm yes , with compensating lens Yes depending on camera / settings No
FourThirds 38.58 mm Yes Dandelion chip Yes Yes
Pentax K 45.46 mm Yes Yes Yes Yes
Sigma SA 44.00 mm Yes Yes Yes No
Sony / Minolta-A 44.50 mm Yes Dandelion chip Dandelion chip Yes
M42 45.46 mm

Mirrorless systems

In digital camera systems with an even smaller flange focus and the same or smaller image circle, such as the Micro Four Thirds system or the E-bayonet from Sony, lenses with M39 or M42 lens threads can also be used with a suitable lens adapter , whereby the focus is not restricted to finite object distances and can be carried out using the software magnifying glass in the electronic viewfinder or on the monitor. The exposure is also determined with the image sensor and can be displayed and corrected in real time in a histogram with all exposure values. Image stabilization can be used if it is implemented via the image sensor by setting the current focal length of the lens used in the camera menu.

Further adaptation options

  • "Electric" lenses or most lenses with open aperture measurement also work with the standard connection for the working aperture.
  • T2 lenses can be adapted to M42 with a T-2 adapter and used with a work aperture. T2 lenses also have a thread with a 42 mm diameter at the connection, but with a different thread pitch (0.75 mm per revolution). If you try to screw the two threads together, they can be screwed together about one turn and then jam.
  • Medium format lenses: Since all medium format systems have a significantly larger flange focal length than the M42 35mm system, they can usually be adapted to the M42 system with the intermediate stage T2 adapter. The various automatic functions are then lost. Corresponding adapters are offered by special manufacturers.
  • Enlargers from Eastern Europe sometimes use the M42 thread. An adapter ring is required in order to be able to use M39 magnifying lenses.

Web links

Commons : Lenses with M42 connection  - collection of images, videos and audio files