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I own a fair amount of vintage lenses. Some of them I use on a crop body (a6000), with or without a focal reducer. I also use them adapted to full frame (a7). I love them on their native bodies, but here lies not the question...

Considering the multiple vintage mounts and the desire to get a better reach out of some of theses lenses (the longer ones), I was wondering if adapting the lens to a modern teleconverter, rather than hunting down vintage teleconverters would be a viable option. I am not sure how it would behave optically (teleconverter directly mounted to the lens, then adapted to the body, vs lens adapted to modern teleconverter mounted to the body), and I do not seem to find any clear ressources online.

Has anyone any experience in this regard?

  • This is not an answer because I've never actually tried it, but in theory it would seem you'd be better off preserving the distance between lens and TC by using a vintage TC attached directly to the lens with the adapter between the TC and camera. Placing a spacing adapter between the lens and TC would increase the distance between the lens' elements and TC's elements which may cause additional problems. – Michael C Sep 29 '20 at 3:57
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If I'm understanding you correctly you are considering using a modern TC on your body with the adapter in front of it. I do not suggest it because the adapter establishes the proper distance to the sensor plain and needs to be on the camera. Use your TC between the lens and the adapter.

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Using a vintage teleconverter with appropriate mount adapter would work because it preserves the distance from film/sensor to TC optics to lens. However, I have a couple vintage teleconverters, and the image quality is quite poor. Cropping produces far better results.

  • The optics of many vintage teleconverters aren't too great to begin with.
  • Using a crop sensor is already like using a 1.5x teleconverter.
  • Many vintage lenses don't have sufficient resolution to handle 2-3x magnification.

I'd expect a modern teleconverter mounted directly on the camera would not work well with vintage lenses. They are also quite expensive.

I don't have one on hand to test, but did mount a vintage teleconverter close to the camera sensor so that it works with a lens with short registration distance. Lenses with longer registration distance attached with extension tube would not focus.

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I can't answer the part on optical quality. In my opinion it absolutely depends on the given combination. I wouldn't expect a general answer like "vintage TC is better" or "modern TC is better" to be valid.

The distances argument used in some other answers isn't convincing to me (see below).

Camera / Lens Communication

One aspect not yet mentioned is about the camera/lens communication:

As a decades-long owner of various Canon EOS equipment I know that this communication can be tricky. At least with analog bodies, there was a mechanical switch in the lens mount to be engaged when mounting an EF lens, which dumb-lens adapters did not activate, thus telling the SLR body not to expect a lens obeying the Canon protocol. When using dumb lenses, I was used to shooting in M or Av mode with aperture set to 1.0.

I once had my surprise when I tried to combine a dumb f/8 500mm with an EF teleconverter (for a lunar eclipse event) on a 620 body: I got an error from the camera. That was because the EF converter was designed to support EF lenses, therefore activating the lens-mount switch, making the camera trying to communicate with its lens, and of course failing. My "solution" back then was to not fully engage the teleconverter onto the body, but stop a few degrees before the locking position.

If something similar applies to your camera system as well, your only choice might be to have the adapter mounted on the body, and then use the vintage teleconverter and the lens.

So I'd recommend to test the combination with the modern converter before buying: it might fail to function.

Distances

Regarding the distances where the various components get located, both configurations are fine.

For my argumentation, we consider three values (giving example values to make it easier to follow):

  • Flange focal distance ("FFD") of the vintage system (let's say it's 55mm)
  • Flange focal distance of the modern system (let's say it's 45mm)
  • Mechanical length of the TC (let's say it's 50mm, but actually, it's irrelevant)

We can safely assume that the vintage FFD is greater than the modern FFD, otherwise adapting would fail, by not allowing you to focus up to infinity.

The adapter just allows you to mount the vintage lens in a position according to its expected FFD, so its thickness will be equal to the FFD difference, being 10mm in our example.

Vintage TC Configuration

  • vintage lens
  • vintage TC
  • adapter
  • body

Regarding the projection distances, this configuration is fine.

A vintage TC will expect a projected picture 55mm behind its front flange, enlarge it, and project it to a plane 55mm behind its rear flange. This new projection plane is behind the original plane by just the adapter's thickness, 50mm.

The lens projects its picture onto a plane 55mm behind its flange, and that's where the TC expects it: fine.

The vintage TC produces its enlarged image 55mm behind its flange, and due to the adapter with its 10mm plus the body's FFD of 45mm, the sensor is just where the picture gets projected: fine.

Modern TC Configuration

  • vintage lens
  • adapter
  • modern TC
  • body

Regarding the projection distances, this configuration is fine as well.

The modern TC will expect a projected picture 45mm behind its front flange, enlarge it, and project it to a plane 45mm behind its rear flange.

The lens projects its picture onto a plane 55mm behind its flange, and due to the 10mm adapter, that is 45mm behind the front flange of the TC: fine.

The modern TC produces its enlarged image 45mm behind its flange, and that's just the FFD of the body, so exactly where the sensor is located: fine.

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