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.

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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).

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Canon Lens Mount Converters – from Canon’s 1969 System Equipment catalog (courtesy: Pacificrim.com)

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.

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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).

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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
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The screw mount to FD adapter.
  • Mount the lens on the Canon T90
  • Set the lens to “M”
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Asahi Pentax Super Takumar lens – it has to be set to “manual”
  • push the stopped down metering lever
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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)
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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).

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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 .

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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.

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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.

 

 

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