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So I bought a Petzval 85 Lomography lens second hand. It has a Canon EF mount. Before I bought it I checked that there were EF to MC/MD mount (my camera) adapters. A quick google showed they are available so I (impulse) bought it.

enter image description here

However further investigation showed that you can get MC/MD lens to EF camera adapters but not the other way around. Doh!

So looking at the lens, it seems it would be quite easy to fabricate a replacement (Aluminum part in the photo below) M42 (screw) mount for the lens. (For which there are plenty of cheap adapters to almost anything) Detail of the mount

So my question(s) is:

  1. Has anyone done this?
  2. Can I buy one?
  3. Have you got the detailed measurements so I can get one fabricated?
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    I've heard of the lens, but have never handled it or looked much into it. Why do you say that you think it looks easy to fabricate a replacement M42 mount? Can you include a photo of the lens, with maybe some annotation? – osullic Apr 16 at 21:35
  • Possible duplicate of Can I use lens brand X on interchangeable lens camera brand Y? – xiota Apr 16 at 21:46
  • @xiota This is not the same thing as asking if one can adapt a lens as is to another mount. While the information there may not even explain why an EF lens can not be easily adapted to MC/MD mount (there's not enough difference in the registration distances to account for the depth of the EF bayonet that protrudes behind the flange), it doesn't address modification of an MC/MD mount lens to M42 (or any other mount) at all. – Michael C Apr 16 at 22:35
  • Fwiw I am going to go to a local engineering shop and will get them to fabricate a m42 mount. I will report back to you the results. Although if any can find out the exact dimensions of the Petzval lens that would help greatly! – DarcyThomas Apr 21 at 6:33
  • I went to a engineering shop yesterday. They estimated $150USD to machine the part. I just need to work out the correct dimensions (notably the flange thickness, as that dictates the film to flange distance) – DarcyThomas Apr 23 at 23:23
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It is theoretically possible to reduce the barrel length by 1.46mm and replace the EF mount with an M42 threaded flange. Since the same lens is also offered in the Nikon F-mount, it's almost certain there's enough room at the back to shorten it to M42 length without getting into any internal parts of the lens. One could then use an M42 to MC/MD adapter, which are commonly available, to use the lens on a camera with the Minolta MC/MD mount.

Normally, you'd still have the problem of not being able to control the electronic aperture of an EF lens. But the lens in question does not allow the camera to control the aperture at all. It has a set of Waterhouse stops that are mechanically switched by the user.

enter image description here

It's also a manual focus only lens, so there's no issue with AF communications.

There are no electrical contacts at all on the back of this lens to worry about. So even though it is a newer lens with modern lens coatings, in terms of the mechanics of the rear of the lens it is essentially an old legacy type lens with modern bayonet mount flanges on the back.

I would not recommend trying this yourself, however. There are a few people/places that are well-experienced in modifying lenses from one mount to the other. I'd recommend getting on some old "legacy lens" forums and finding out if anyone knows someone in your area/country that you could pay to do the conversion for you.

Failing that, you'd probably be better of to just sell it to someone who wants the lens in EF mount and find another deal on a legacy lens with similar properties.

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There is no combination of pre-existing adapters that will allow you to use the lens with your camera.

  • There is no pre-existing EF-MD adapter that I am aware of. The Flange Focal Distance (FFD) of MD mount is 43.5mm, while the FFD of EF mount is 44mm. That's only 0.5mm for an adapter without optics. For a hypothetical adapter with optics, you'd likely be multiplying focal length by 1.4x, so 85mm → 128mm.

  • There is no pre-existing EF-M42 adapter that I'm aware of (in case you thought to combine EF-M42 + M42-MD adapters). The FFD of EF is 44mm. The FFD of M42 is 45.46mm. There is -1.45mm of space in which to place an adapter, and the EF mount is too large to sink into an M42 mount.

The feasibility of modifying the lens depends on its specific construction and design. Although there is no aperture or autofocus mechanism to complicate conversion, I would not bother attempting conversion myself, unless the lens was particularly cheap, like less than $10.

  • Converting to M42 is not as simple as just cutting some threads because the FFD of M42 is 45.46mm. You would need to shorten the barrel by 1.46mm. Given the apparent design of the mount portion of the adapter, just shaving the barrel down would not work. However, since the lens is available in Nikon F mount, the built-in focusing mechanism probably has enough "play" to allow infinity focus after altering or replacing the mount.

  • You can cut off the threads and drill screw holes into a thin lens reversal adapter for MD mount. (Search for "Minolta MD reverse ring". Look at the pictures to find the thinnest one available.) Since the FFD of MD and EF are similar, the lens would likely still focus to infinity.

  • If you feel the lens is worth the additional cost, you can consider hiring someone else to do the conversion for you, as Michael C recommends.

I would opt for an entirely different lens that works with your camera as-is or with a pre-existing adapter. Other lenses known for producing swirly bokeh include those in the Helios series. I have also encountered lenses that produce mild swirly bokeh that is most noticeable with foliage in the background. You may even already have one without having previously noticed the swirl.

See also:

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[Answering my own question]

I went to a engineering shop yesterday. They estimated $150USD to machine the part. I just need to work out the correct dimensions (notably the flange thickness, as that dictates the film to flange distance).

I also found a few online CNC machining sites. Which I think would end up being be cheaper.

However I found a 2nd hand Canon 500n for $30. And thought that made better sense. So I went with that.

If someone is wanting to give this a go and would advice and/or my notes, please feel free to contact me.

  • That's the right answer! Don't muck around with the lens (reducing its resale value to zero), just buy a body it will fit! – user82065 Jun 20 at 18:16

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