Mounting a Pentax 42mm screw mount lens or a Nikon F lens on a Canon T90

Mirrorless cameras have made us familiar with the concept of mounting old manual focus lenses manufactured many decades ago on a modern camera. A little known fact is that Canon’s T90 (their top of the line manual focus SLR in the eighties) can work in a full featured semi-automatic mode with Pentax screw mount AND Nikon F lenses, thanks to adapters which were at some point sold by Canon themselves.

Canon T90 with Asahi Pentax Super Takumar (35mm f/2)

How is it even possible?

The Canon FD mount has one of the shortest flange to film distances of all 35mm SLRs at 42mm. On the other hand, the Nikon F flange distance is one of the longest, at 46.5mm (source:  Wikipedia – Flange focal distance). The “universal” 42mm screw mount (used by Asahi Pentax and the East German offspring of Zeiss until the mid seventies) is close to the Nikon’s flange distance at 45.6mm. Therefore, if a lens mount adapter can be made less than 4.5 mm thick, it will be possible to mount a Nikon lens on a Canon camera without losing the ability to focus to the infinite (and 3.6mm is the right thickness for a 42mm screw mount adapter).

The difficult part of course is to transmit aperture information to and from the lens – but if the camera is designed to work – at least in one specific mode –  without having to exchange information with the lens (semi-automatic exposure with stopped down metering and no aperture pre-selection, for instance), a very simple lens mount converter will be able to do the job.

Such adapters can be found on eBay for less than $10.00 (recent Chinese manufacturing). More surprisingly, it appears that Canon used to sell Canon branded, made in Japan adapters in the sixties (source: Cameraquest, Pacificrim).

Canon Lens Mount Converters – from Canon’s 1969 System Equipment catalog (courtesy:

42mm screw mount lenses

I recently found one of those 42mm screw mount to FD adapters, (it does not look like the genuine Canon item shown in the picture below, but it’s made in Japan) and decided to test it with a Pentax Super Takumar 35mm F/2 on a Canon T90.

The T90 is an interesting camera – while it does not offer a true semi-automatic metering mode at full aperture with Canon’s native FD lenses, it simply has to be set to stopped down metering to gain a fully functional semi-automatic exposure mode, non only with Canon FD and FL lenses, but also with “adapted” screw mount lenses.

Canon Lens Mounter Converter P (Credit: origin of photography unknown)


The main mission of a lens mount adapter is to position the guest lens (the Pentax 35 mm f/2 in our case) so that its flange will sit at precisely 45.6mm from the film plane – as if it was mounted on an Asahi Pentax camera.

The converter does not provides any mechanical linkage between the adapted lens and the camera, and it has no mechanism to force the lens to stop down to the pre-selected aperture when the photographer presses the shutter release. Therefore, it can only work with lenses with no automatic aperture pre-selection, or lenses where the aperture pre-selection can be switched off to force the lens to always keep the iris at the value shown on the aperture ring.

Not all 42mm screw mount lenses are created equal

Lenses deprived of such a switch can only be operated at their maximum aperture – which makes them mostly unusable. Lenses (such as the Fujinon screw mount lenses) designed to support full aperture metering add another constraint – they typically use a non-standard derivative of the 42mm lens mount (with a protruding pin in the case of the Fujinon) and can not be physically mounted on this adapter (I tried).

Nikon lenses

Nikon has been using the same F bayonet layout for 60 years, but had to go through many iterations of its lens mount to stay current (support of through the lens metering (TTL), introduction of program modes, of matrix metering, and many variants of autofocus).

Pixco Nikon AI to FD adapter (bought on eBay)

Genuine and Canon-branded Nikon AI to FD adapters are rare and very expensive (I saw one selling for $150.00 on eBay under the name “MC-N Lens Mount Converter”). I bought  a Chinese one, for a fraction of the cost.

Being devoid of any aperture transmission mechanism, the converter is compatible with any Nikon lens AI, AIS, AF, AF-D lens, and I don’t see why it could not also accept pre-AI lenses.

Does it work? 

Yes. With the right adapter, a 42mm Screw Mount lens set in “manual” (no aperture pre-selection) will work on the T90 the same way a Canon FL lens (set in “manual”) would.

  • screw the adapter on the lens
The screw mount to FD adapter.
  • Mount the lens on the Canon T90
  • Set the lens to “M”
Asahi Pentax Super Takumar lens – it has to be set to “manual”
  • push the stopped down metering lever
The stopped down metering / depth of field preview lever has to be pushed towards the lens.
  • turn the camera ON
  • set the Exposure Mode to “T” (for shutter priority exposure)
Canon T90 – the settings for shooting stopped down in semi-auto exposure mode
  • turn the aperture ring or the control wheel (controlling the shutter speed) to adjust the exposure as if it was a Canon FL lens used stopped down (the “OP” message on the viewfinder’s LED panel means “Open the iris”, “CL” stands for “close the iris” and “oo”  for “you nailed it”.
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T90 – semi-auto mode – stopped down. Correct exposure – (cursor and triangle aligned on bar graph, “oo” message)
  • Of course, you operate stopped down – but it’s not so much of an issue:
    • the viewfinder of the T90 is very bright and the matt screen very fine, you can focus accurately up to f/8 if you shoot outside on a sunny day,
    • photographers are unlikely to mount slow lenses on the camera, or to shoot at F/16. They will most probably use the converters to mount old and ultra-luminous lenses on the T90, for the bokeh, and for the way the pictures shot with old lenses look.

With screw mount lenses, the T90 is as easy to use as any other semi auto camera, and exposure seems accurate (I obtained the same recommended aperture with the Pentax lens, the FL and the FD lenses, and on a Nikon camera I used as a benchmark).

Canon T90 with a Nikon 50mm AF lens. It can be physically mounted but the exposure is off by at least 1 stop (compared to FL or FD lenses)

With Nikon lenses, I observed multiple issues: with some lenses, the aperture ring of the lens does not seem to control the aperture, and with some lenses, the exposure is off (1 to 1 1/2 stop) compared with FD, FL or Pentax screw mount lenses. I suspect it’s because the lever controlling the aperture on a Nikon lens is normally pushed to the preselected aperture by a spring loaded lever on a Nikon camera’s body. With this adapter, the spring loaded lever is missing.

Does it make sense?

Owners of 42 mm screw mount lens with manual preselection don’t have many options if they want to use their lens “natively” on modern cameras: Pentax stopped selling screw mount cameras in 1975, Fujica at the end of the seventies, and Cosina briefly sold a Voigtlander Bessaflex SLR in small volumes at the beginning of this century. Nothing recent or widely available. The best they can do is use adapters, to mount their lens on Pentax K SLRs and dSLRs, or of course on many mirrorless cameras. In that perspective, if you’re a T90 enthusiast and still own a few very good 42mm lenses it could  make sense to look for a 42mm to FD adapter.

I’m less convinced it makes sense for owners of old Nikon lenses to mount them on a T90.  Nikon lenses don’t like to be mounted on an adapter that does not control their aperture lever. And if you have old Nikkor lenses that you love, there is no shortage of good film and digital Nikon cameras which still accept them, and will offer full aperture metering and more auto exposure options than an adapted lens on the T90 .

Canon FT with Nikkor AI lens – it’s not because it’s possible that you should do it.

Other Canon bodies

Any Canon body which can operate stopped down with Canon FL lenses can in theory work with the 42mm screw mount or the Nikon F adapter.

  • Canon AV-1: being an “aperture priority auto exposure camera,  it works stopped down with Canon FL lenses and adapted screw mount lenses.
  • Canon FT: a semi-automatic camera operating natively with FL lenses, it also works with adapted screw mount lenses.

Plancy l’Abbaye – France – Canon T90 – Canon FD 24mm lens – Kodak Ektar 100. I was surprised by the way the Ektar film rendered the colors – pretty different from the reality.




Old lenses on new gear – manual focus lenses on mirrorless cameras

One of the most remarkable changes brought by the advent of mirrorless camera systems (micro 4/3rds, Fujifilm X and to an even larger extent Sony E and FE) is the ability to mount and effectively use almost any old lens designed originally for a 35mm camera system.

With SLR and dSLR camera systems, it was pointless to try and mount lenses designed for another system, and very often, lenses from a

Two mount adapters: Canon FD to Fuji X, and Nikon F to Fuji X. Those Fotasy adapters are not fancy but they're cheap and they do the job.
Two mount adapters: Canon FD to Fuji X, and Nikon F to Fuji X. Those Fotasy adapters are not fancy but they’re cheap and they do the job.

previous generation of the same camera system:

  • SLRs and dSLRs have optical viewfinders – the photographer needs all the light he/she can get for focusing and composing the picture, and the cameras are therefore designed to work at full aperture with aperture pre-selection – which used to require rods and springs and cams, and since the Canon EOS mount opened the way, now requires electronics. There is no simple way to emulate the pre-selection mechanism of one SLR system  with a lens designed for another one.
  • There are also physical limitations:
    • The adaptor designed as the interface between a lens of System A and a camera body of System B is more or less a cylinder with the female part of the mount of System A at one end, and the male part of System B at the other end. Such an adapter would necessarily have a depth of 5 to 15mm, which adds to the flange distance. Unfortunately, all cameras derived from 35mm SLR systems have a very similar flange distance (from 42mm for the Canon FL/FD mount up to 46.5mm for the Nikon F), and there is not enough room for an adapter (the adapted lens would sit too far from the camera’s film plane, and would not focus to infinite).

Mirrorless camera systems don’t have such limitations:

Canon FD to Fuji X (left) and Nikon F to Fuji X (right).
Canon FD to Fuji X (left) and Nikon F to Fuji X (right). The Nikon mount range distance is a bit higher than the FD’s. Therefore the adapter is thicker.
  • They have electronic viewfinders – and offer a clear and bright view of the subject even stopped down at f/16. If fact, most of the mirrorless cameras operate at stopped down aperture even with their native lenses.
  • The flange distance of mirrorless systems is much shorter (17 to 20mm for the most common systems), which leaves plenty of room (almost 30mm ) for the adaptor if you want to mount a lens designed for a SLR or DSLR system.
  • Thanks to their electronic viewfinders, mirrorless systems have multiple ways to assist the operator trying to focus manually (magnifier, zebra, focus peaking).

The use of CAD and CNC is now widespread and it’s easy and cheap to manufacture mechanical mount adapters: users of each of the big mirrorless camera systems have access to adapters for :

  • Most pre-AF era mounts for 35mm systems: (39mm and 42mm, Canon FL/FD, Konica, Nikon F, Minolta MD, Olympus OM, Leica M and R, Topcon, …)
  • Stranger or more exotic mounts (C mount, Holga, medium format cameras)
Canon FD to Fuji X adapter, and Canon FL 55mm
Fuji film X-T1, Canon FD to Fuji X adapter (Fotasy), and Canon FL 55mm. The X-T1 is a pleasure to use even with old lenses.

Even if it’s physically possible, mounting recent AF/all electronics lenses is generally pointless – not only you can’t set the aperture for lack of an aperture ring, but you can’t focus the lens because modern lenses are devoid of any mechanical connection between the focusing ring and the focusing mechanism of the lens. Unless a third party vendor develops an adapter which embarks the complex software required to translate the communication protocols of a lens of Brand A into something the body of Brand B will understand.

As far as I know, it has only been attempted with some level of success between a few lenses with a Canon or Sigma mount and a few Sony bodies (the A7R II or the A6300).

Therefore, the best candidates are lenses from the manual focus era (up to 1985), and the Nikon and Pentax autofocus lenses designed before Year 2000 – they all still have aperture rings.

Even if it is possible, mounting an old manual focus lens on a mirrorless body is not necessarily the best thing to do:

  • in spite of all the focus assistance mechanisms, it’s much slower to get the focus with an adapted vintage lens than with the native autofocus lens – adapted lenses are not a good fit for mobile subjects, unless you adopt old school focusing techniques (pre-focus, wait for subject to be at right distance, and shoot)
  • Older lenses were designed for 35mm film cameras, and are unnecessarily large and heavy when mounted on M4/3rd and APS-C cameras
  • Lots of older lenses were not that good in their heyday, and become really bad if mounted on a camera with a high resolution sensor. It’s true in particular for zooms and to a lesser extent for wide angle lenses.

As a conclusion, why mount old lens on a modern mirrorless body?

  • Because you can (of course)

    Leica Summicron C (40mm f/2) mounted on Sony NEX 3 with Metabones adapter. It worked pretty well.
    Leica Summicron C (40mm f/2) mounted on a Sony NEX 3 with Metabones adapter. The Nex 3 was surprisingly easy to use with a manual lens. The Metabones adapter is really stiff, and it’s one step above the Fotasy in terms of quality. Not sure it’s worth the price, though.
  • If you already have the lens… Considering adapters sell for $20.00, it’s tempting to buy one to use your old lenses, as a stop gap until you buy a modern equivalent in the mirrorless system, or even permanently
    • All macro lenses are a very good fit because macro photography does not require to focus fast, and old macro lenses are still up to the task, when compared to their modern equivalents
  • If you want to experience really exceptional glass
    • Canon FD Aspherical or “L” lenses (50mm f/1.2, 85mm f/1.2) for instance, or some of the gems that Leica, Minolta, Nikon, Pentax and others have produced in the past…
  • If the modern equivalent does not exist…
    • a 55mm f/1.2 lens – it doesn’t exist for the Sony E/FE mount
    • a teleobjective with Defocus Control – only Nikon has them
    • a tilt and shift lens (only Canon and Nikon have them)
  • or exists but is crazy expensive
    • can an amateur afford the new Sony 85 f/1.4 FE GM?

As a result, old lenses of good reputation hold their value extremely well. Some of the  Canon and Nikon lenses I mentioned above sell for more than $700.00 on eBay.

Jules - Fujifilm X-T1 - Canon FL 55mm f/1.2
Jules – Fujifilm X-T1 – Canon FL 55mm f/1.2