M42 Optic Cameras

External diameter42mm
Inner diameter41mm
Flange45.5 mm
ConnectorsAuto aperture

Buy and sell used M42 Pentax screw camera gear at KEH Camera. Save up to 40% off retail price and get a 180-day warranty! Financing options available.

Latest CCTV traffic image from camera #1626 on the M42 road. This image was retrieved on 9th Feb 2021 at 08:50:47 GMT Click this tab to check for new image. The M42 section of PentaxForums lens database includes information on the mount type and aperture mechanism of all Pentax screw mount lenses. Likewise, the M42 section of the camera database provides the mount information for all Pentax screw mount cameras. M42 lenses image by Brian Eager (Image rights) This page is about the 42×1.0mm thread M42 screw mount used to attach lenses to SLR cameras, most popular in the 1960s. This page is NOT about the T mount thread measuring 42×0.75mm which screws into adapters ('T-rings') which fit many other types of SLR lens mount, first developed by Tamron. Zinc Alloy Construction Make the most of your existing collection of M42-mount glass with this Vello Lens Adapter that will allow you to mount these lenses to a Sony E-mount camera. Precise construction makes it easy to attach these adapted lenses to your camera and keeps it secure during use.

The M42lens mount is a screw thread mounting standard for attaching lenses to 35 mmcameras, 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. (The M42 lens mount should not be confused with the T-mount, which shares the 42mm throat diameter, but differs by having a 0.75mm thread pitch.)

It was first used by the East German brands VEB Zeiss Ikon in the Contax S of 1949, and KW in the Praktica of the same year. VEB Zeiss Ikon and KW were merged into the Pentacon brand in 1959, along with several other East German camera makers.

M42 thread mount cameras first became well known under the Praktica brand, and thus the M42 mount is known as the Praktica thread mount.[1] 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 also used by Pentax; thus, it is also commonly known as the Pentax thread mount, despite the fact that Pentax did not originate it.

Evolution and automation[edit]

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, PLC3, EE 2 and EE 3 (1975, 1977, 1979) (EE = Electric Eye) using Pentacon Electric lenses
  • Fujica ST705, ST801, ST901
  • Olympus FTL (1970)
  • Pentax Spotmatic F, Electro Spotmatic (Japan only), ES and ESII (1975, 1971-3)
  • Yashica Electro AX (1972)
  • Zenit 18 (with Zenitar ME1 lens only)
  • GAF L-ES
  • Hanimex Praktica Nova 1B
  • Mamiya/Sekor 1000 DTL

or automated selection of aperture (shutter priority) for

  • Ricoh TLS-EE (1973)
  • Petri-designed Exakta FE 2000 (1978)
  • Sears TLS (made by Ricoh)

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 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 facility of closing the aperture with finger pressure on the release button so that the image could be framed and focused 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 range. Pentax moved to the Pentax K mount from 1975 onwards, while 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 KMZ and at Cosina (under the Voigtländer brand and the ZS line for Carl Zeiss).

Notable cameras produced for the M42 mount[edit]

  • Zenit line of SLRs from the USSR and Russia (some models)
  • Praktica SLRs from East Germany (not the B-Series)
  • Pentax SLRs from Japan (some models, all before the Pentax K2 of 1975)
  • Zeiss Ikon Icarex TM and SL-706
  • Voigtländer Bessaflex TM (2003–2007)

Use on modern cameras[edit]

M42 adaptors exist for many current lens mounts; here, a Pentacon 50mm M42 lens is mounted to a Canon EOS body.

Due to the simplicity of the M42 lens mount and the large selection of lenses, M42 adaptors exist for all current and many obsolete lens mounts. The adaptor fits between the camera and the lens, making it possible to mate an M42 lens to a body with a different lens mount. M42 adaptors work best on bodies with a flange depth less than or equal to the M42's flange depth, which includes the popular Canon EF-mount, the Pentax K-mount, the Minolta/Konica Minolta/Sony A-mount, the Sony E-mount, the Samsung NX-mount, the Fujifilm X-mount and the Four Thirds System including the Micro Four Thirds system. This allows the lens to be physically mounted the correct distance from the film or sensor, retaining the original focus range of the lens without the use of correction optics.

Some M42 lenses extend too far into the camera body, causing interference between the mirror mechanism of the camera, and rear element or aperture pin of the lens. This issue is most commonly encountered with certain Takumar lenses adapted to Canon full frame cameras.

On bodies with a flange focal distance greater than that of M42, most notably Nikon, three 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. Finally, it is possible to replace the M42 mount on some lenses with the desired camera mount, or to perform the reverse change on a camera body.

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.


Canon: All DSLRs need full manual control. With certain adaptors, metering and focus confirmation is available.
Nikon: All DSLRs need full manual control. Focus confirmation is available, focus to infinity is only possible with adapter with additional lens.
FourThirds: All DSLRs need full manual control. Image stabilisation works. With certain adaptors, metering and focus confirmation is available.
Pentax: All DSLRs allow aperture-priority with focus confirmation and infinity focus. Image stabilisation works. With grounding one contact on the camera with foil, focus-trap is also available (on Models which have it).
Minolta/Sony A-mount: All DSLRs need full manual control. Metering works and lens can focus on infinity. With chipped adaptors focus confirmation and image stabilisation are also available. Some SLTs are reported to work fine in aperture-priority mode (A65 confirmed).
Sigma: SD9 allows auto exposure with aperture-priority mode, infinity focus and focus confirmation. User needs to calibrate camera for each change of aperture on the lens by changing aperture value on the camera.

Lens mountFFDInfinity focusFocus confirmationMeteringImage stabilization
Canon EF44,00 mmyesadaptorStop-down Meteringno
Canon EF-M18 mmyesmanualStop-down Meteringno
Nikon F46,50 mmadaptoryesno/yes (depending on the model and mode settings)no
Olympus FourThirds38,58 mmyesadaptoryesyes
Pentax45,46 mmyesyesyesyes
Minolta/Sony A-mount44,50 mmyesadaptoryesadaptor
M4245,46 mm


  • Thread: M42 × 1
  • Flange focal distance: 45.5 mm (The figure 45.46 mm is also commonly seen, particularly in Pentax literature.)


  1. ^The M42 mount is sometimes referred to as a 'P' thread. See, e.g., 'Questar Corporation: Photographic Camera Adapters 'P' Thread'. Retrieved 2017-03-01.

External links[edit]

Wikimedia Commons has media related to Lenses with M42 mount.
Retrieved from 'https://en.wikipedia.org/w/index.php?title=M42_lens_mount&oldid=997996257'

The Evolution of the Pentax M42 Mount

History, Variations, and Techniques for Pentax Digital

By stevebrot in Articles and Tips on Mar 20, 2019

The original Asahi Pentax (informally called the 'AP')

With the introduction in 1957 of their first SLR with a pentaprism dubbed the Asahi Pentax (colloquially known as the 'AP'), Asahi Optical Co. opted for the M42 screw mount which we shall take a closer look at in this article.

Since the 1936 debut of the Ihagee Kine Exakta, the first 35mm SLR interchangeable lens camera, there have been a multitude of SLR camera models sporting a large variety of different mount designs. While all satisfied the basic task of mating to compatible lenses, most are no longer being used and only a very few have been used for more that 30 years with only three (M42, Nikon F, and Pentax K) surviving to the present day. Of those three, the M42 screw mount has been in use the longest and is of particular interest to Pentax camera owners in that it is one of three mounts historically used by Pentax SLR cameras and continues to be popular, in adapted form, with Pentax owners today.

One of the unique characteristics of the M42 mount has been its broad acceptance by many makers of lenses and cameras as a simple and practically universal basis for attachment of lens to camera. That acceptance has been based on the relative simplicity of the mount as well as non-proprietary character of its common forms. Dozens of makers have produced M42 mount cameras with Praktica and Pentax being the most prominent associated names. An even larger number of lens and accessory makers have supported the mount over the years such that there remains an impressive catalog of surviving vintage lenses and accessories compatible with current and legacy Pentax bodies. It is highly probable that more lenses and cameras have been made in M42 mount that any other. It is because of that ubiquity that M42 has been referred to as 'Universal Screw Mount' along with the other common monikers 'Praktica Screw Mount', 'Praktica/Pentax Mount', and 'Pentax Screw Mount'. That fact has also been much appreciated by photographers able to adapt this rich source of quality optics to their camera system of choice.

Requirements: What Must an SLR Lens Mount Support?

The history and evolution of the M42 mount follows that of SLR cameras in general with features being added as user needs became evident. The basic requirements, however, remain simple:

  • Provide consistent and secure attachment while allowing relative ease of attachment
  • Allow for through-the-lens focus and framing with the aperture wide open with lens stop-down to shoot
  • Support the above points without undue interference with the focus-frame-shoot flow expected from a hand-held 'miniature' camera

Original M42 Mount

The original M42 mount was designed in 1938 by Carl Zeiss at their plant in Jena in collaboration with KW camera. The design parameters were simple and consisted of a circular flange with 42mm threaded inside diameter (1mm pitch) set to 45.46mm flange focal distance. The Second World War delayed implementation such that the design debuted in 1949 on both the Contax S and KW Praktica model cameras. Note that the original design provided no mechanism for aperture actuation by the body:

The original M42 mount

As with the contemporary Exakta models, lenses were offered with either fully manual iris diaphragms or a so-called preset aperture mechanism. Manual aperture lenses required the operator to open the aperture fully with the aperture ring to focus and frame and then turn the aperture ring to stop down to the desired F-stop to actually shoot. Providing click stops allowed the user to count clicks, but the process was cumbersome at best.

Takumar lens with the original M42 mount and a manual aperture ring

The availability of lenses with preset aperture mechanism was a huge boon to SLR photographers. These lenses allowed the user to focus and frame with the lens wide-open and allowed the lens to be stopped down to a preselected aperture just before exposure. This was done by use of a lever or an additional ring on the lens barrel. With practice, the flow of meter the light, pre-set the aperture, focus, frame, stop-down, and shoot came quite naturally and was the standard for several years. Because of mechanical simplicity, easy implementation even on long lenses, and portability across mount types, pre-set lenses were the norm for much of the 1950s and remained popular well into the early 1970s with some lens types (e.g. tilt-shift) using this approach to this day. Preset lenses are easily identified by the lack of coupling pins or levers on the lens rear and by presence of a lever or 'extra' aperture ring on the lens barrel. The original M42 Takumar lenses featured preset aperture mechanisms and were matched for use with the Asahi Pentax (AP) (1957) and Pentax S (1958) cameras.

Preset ring set to F11 with the lens still wide open ('real' aperture ring is set to wide open (F5.6))

Aperture now closed down hitting the mechanical stop at the preset value (F11)

Semi-Automatic Diaphragm

The Pentax K camera of 1958 was innovative in that it added a top shutter speed of 1/1000s, but its most significant new feature was that it, along with the 1956 Zeiss Contax F, used an M42 mount capable of supporting automatic aperture actuation. This was accomplished by use of a bar ('flipper') in the mirror box near the bottom of the mount that moved forward just prior to the shutter opening to depress a small pin on the rear of the lens to stop the lens down for exposure. The bar actuator design was common to both makers and allowed backward compatibility to the growing number of German preset M42 lenses as well as Auto-Takumar lenses made for the Pentax K. The addition of the actuator bar to the mount was seminal, as shall be seen below and all Asahi screw mount cameras from the Pentax K forward featured a mount so equipped.

The hallmark feature of semi-automatic lenses is the presence of a lever on the side to open the aperture ready for release. The user flow went like this:

  1. Meter using hand-held or camera-mounted non-TTL meter
  2. Set shutter speed and aperture
  3. Open diaphragm using lever
  4. Focus, frame, and release shutter to take the picture
  5. Advance film and use lever to re-open the diaphragm
  6. Repeat steps 4-5 until either the subject or light changes

Auto-Takumar lens with semi-automatic diaphragm

Fully-Automatic Diaphragm

The next innovation was development of a fully-automatic aperture mechanism capable of stopping down for exposure and reopening without user intervention. This improvement along with the earlier innovation of an instant return mirror allowed the photographer to easily address rapidly changing events and active subjects without removing their eye from the viewfinder. Fully-automatic aperture first made its appearance with Pentax in 1960 on some Auto Takumar lenses, but it was the Super-Takumar series that fully exploited this feature. As support for this feature was taken up by other makers of M42 cameras and lenses, it became commonplace to include 'Auto' in the name for lenses having this feature, hence names like Auto-Rikenon, Auto-Yashinon, Auto Mamiya/Sekor, and so on. To allow backward compatibility with older-model bodies, most auto-aperture M42 lenses include a small switch or slider to allow either automatic or manual aperture operation. A happy side-effect of the Auto/Manual switch is that it effectively allowed operation equivalent to using a preset lens which comes in handy today when using these lenses on a digital SLR.

Super-Takumar with M42 mount with fully automatic aperture

TTL Metering

While not an M42 mount feature per se, adding the aperture actuator to the camera allowed auto-aperture lenses to provide seamless support for stop-down TTL metering as originally featured in the Pentax Spotmatic and numerous other competing brands through the 1960s. Turning on the Spotmatic's meter triggered the stop-down mechanism for metering. This allowed for further streamlining of the process of taking pictures:

  1. Focus and frame the scene
  2. Turn on meter (this stops the lens down) and set aperture and shutter speed
  3. Shoot (this turns off meter and opens up the aperture)
  4. Focus, frame, and shoot again
  5. Repeat #4 as needed until either light or subject changes.

If haste is required, the above might be streamlined to an abbreviated form on the Spotmatic:

  1. Meter frame while focusing/framing, stopped-down
  2. Shoot (diaphragm will reopen with meter automatically switched off immediately afterward)
  3. Focus, frame, shoot again
  4. Repeat #3 as needed until a need to re-meter

Spotmatic light meter/stop-down switch

Open-Aperture Metering Pentax M42 F/ES-mount

The early 1970s brought major changes to allow TTL metering with the lens aperture fully open. While Asahi Pentax was not alone in offering this feature with a variation of the M42 mount, its implementation is notable in that both supporting cameras and lenses were backward compatible to previous offerings from Pentax and other makers. Requirements for the implementation consisted of two main goals:

  • Provision for constraining user-initiated lens stop-down with supporting lenses while retaining seamless support for legacy stop-down TTL metering with other lenses
  • Coupling to communicate the aperture ring position relative to its full-open position

Changes to accomplish those goals included (the numbers in (..) refer to the illustrations below):

  • Carving out a narrow ledge (1) at the inner margin of the body mount flange to accommodate a small interlock pin (2) on supporting lenses
  • Providing couplings in the mirror box to engage matching pieces on the lens to detect lens orientation (3) and relative aperture ring position (4)
  • Adding a lens position boss (5) and aperture ring 'follower' tab (6) to the lens rear
  • Adding a small pin (2) as part of an interlock to prevent use of the auto/manual switch on supporting lenses

Pentax M42 F/ES mount

Variations and Compatibility

M42 Optic Cameras Lens

While the vast majority of M42 cameras and lenses are cross-compatible with each other with few concerns, physical considerations and proprietary features on some lens series may create compatibility concerns for use with some cameras and/or adapters. Concerns fall into a few broad categories:

  • Physical interference with mirror, aperture actuator, or other host body parts
  • Light leaks from narrow lens flange when using an M42 mount adapter
  • Poor engagement with adapter

If you have encountered compatibility issues please contact a PentaxForums administrator so that we can augment the table of known issues:

Auto-Takumar 55mm F1.8Do not use lenses with a serial number smaller than 462500 with the SV and S1a since its automatic diaphragm will not work correctly (source: H3v/H1a operating manual)
Super-Takumar 50mm F1.4 lens (8-element version)

This note refers to the early 8-element version of the Super-Takumar 50/1.4 (LINK) only. The protruding rear element on this lens may not clear the mirrors on pre-Spotmatic Pentax bodies. These include:
(equivalent Honeywell models in parentheses)

  • S1(H1)
  • S2(H2)
  • Super S2
  • S3(H3)
  • SV(H3v)
  • S1a(H1a)

Later production bodies are free of this issue.

Techniques on Pentax Digital

First and foremost a M42 to K-mount adapter must be fitted to the digital camera body. The adapter must sit flush with the camera body. Certain third parties offer adapters with a flange - these are not recommended since they prevent focusing far out and at infinity. The adapter offered by Pentax is the best:

Genuine Pentax M42 to K-mount Adapter

Installing the adapter in the mount can be fiddly. The easiest way to do it is to first screw the adapter onto a lens, then mount the lens (with adapter), and finally screw the lens out leaving the adapter in place on the camera. You now effectively have a digital SLR with an M42 mount!

The genuine Pentax adapter mounted on lens

M42 Optic Cameras

The genuine Pentax adapter sits flush with the bayonet mount


There is no aperture linkage between the camera and lens, so the lens must be stopped down manually to the working aperture for metering and before shooting. This can conveniently be done with the A/M switch if present.

Shooting with M42 lenses on a Pentax DSLR goes like this:

  1. Dial in the focal length when prompted. This ensures that the shake reduction works correctly (if the camera doesn't prompt you for some reason, insert the value in the Recording Mode menu)
  2. Set the camera to Av exposure mode. Optionally set Auto ISO
  3. Open the lens aperture up fully, focus and frame
  4. Stop the lens down to the desired F-stop. Half-press the shutter button, the camera then adjusts shutter speed accordingly. Unless you use Auto ISO, adjust the ISO to the desired value (the camera will readjust the shutter speed to match the new ISO value)
  5. Shoot
  6. Repeat from step 3

You nay have to set some exposure compensation to achieve proper exposure. The compensation value required may vary by lens and F-stop.

This method works in live view as well as in viewfinder view.

In a Nutshell

There are three variants of the M42 lens mount when talking Pentax and four types of aperture mechanism:

M42 Lens MountAperture MechanismPentax Lens Name *
- Manual aperture
- Preset aperture
With Stop-down Pin
- Semi-automatic aperture
- Automatic aperture
Super Takumar, Super-Multi-Coated Takumar
With Open-aperture Metering- Automatic apertureSuper-Multi-Coated Takumar, SMC Takumar

M42 Optic Cameras Wireless

* There are some exceptions from this naming convention

M42 Optic Cameras Camera

The corresponding three camera mounts and the camera names are:

M42 Camera MountCamera Models
OriginalAP# (Tower 26), S (Tower 26), cameras fitted with the M42 to K-mount adapter
With flipperK (Tower 29), S2 (H2), S3 (H3), S1 (H1), Super S2, SV (H3v), S1a (H1a), Spotmatic, SL, Spotmatic II, Spotmatic IIa, SP500, SP1000
With flipper and open aperture meteringSpotmatic F, Electro Spotmatic, ES, ESII

# The first Asahi Pentax model had no engraved model name. It is colloquially called 'AP'

M42 Optic Cameras Nikon

Any M42 lens can be used with any M42 camera. The features supported will be the least of what the lens and camera provide. There are a (very) few exceptions, though: refer the section 'Variations and Compatibility' above.

The M42 section of PentaxForums lens database includes information on the mount type and aperture mechanism of all Pentax screw mount lenses. Likewise, the M42 section of the camera database provides the mount information for all Pentax screw mount cameras.

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