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I'm confused by the choice of adapters offered for Leica M cameras. Now obviously Leica M mount lenses can be mounted, but I have also seen the M39 screw mount being mentioned a lot. I guess they are not compatible but can be adapted? Does this work with the rangefinder? Typically an M mount lens must have a lever to communicate the focussing distance to the rangefinder prism, correct? I also saw that the are adapters for M42, but that is definitly a mount used for SLRs. Does that mean that you could only focus that using the distance scale on the lens or live view on newer Leicas?

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    Seems there are two questions buried in there: a list of lenses (possibly off topic) and how an adapter works especially with non rangefinder lenses (more interesting and definitely on topic). Can you clarify? – Hueco Jun 25 '18 at 20:25
  • I changed the question a little to make it more clear. I'm not asking for a list of lenses, rather for a list of mounts that can be adapted and whether they work properly with the rangefinder. – Andreas Hartmann Jun 25 '18 at 20:44
  • Any mount can be mechanically adapted. If you want to focus to infinity, the flange distance of the adapted mount must be equal or larger than of the Leica M mount. The list of possibly adaptable mounts is very long, the list of available adapters likely quite a bit shorter and you can find them with the internet search engine of your choice. The rangefinder in the camera will work if it is mechanically supported by the adapter. – jarnbjo Jun 26 '18 at 15:17
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    Possible duplicate of Can I use lens brand X on interchangeable lens camera brand Y? – xiota Dec 8 '18 at 16:58
  • Just me, but I'd change the title to something more like "What are Leica M-specific issues when adapting lenses?" The X/Y question xiota mentions can answer the "which lens mounts" portion, but doesn't go into rangefinder coupling issues. – inkista Dec 8 '18 at 19:58
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When it comes to adapting lenses with simple ring adapters so that they'll still focus to infinity, the issue is merely one of image-plane to lens mount depth. Because a ring takes up physical space, you can really only adapt from a thicker mount to a thinner one without requiring a glass element to act as a teleconverter (and possibly diminishing image quality). The only other caveat is that the image circle of the lens needs to cover your sensor/frame of film, so adapting from a smaller format (half-frame, 4/3"-format, 16mm motion picture, etc.) will result in vignetting, so you want to start with a 135-format (35mm film/full frame) lens.

The Leica M mount is roughly the same size as most mirrorless mounts, so is thinner than SLR mounts, and can adapt all SLR and medium format lenses. That's why M42 will work: that's the old Pentax screwmount. M39 will work because it's the old Leica screwmount and they designed the M bayonet mount to still work with the older Leica lenses in the older mount (much as Pentax did with M42 and K mount).

Rangefinder coupling is a different issue. The lens itself must have some form of physical focus mechanism that can be translated via the adapter to how the Leica M mount does rangefinder coupling. So, this will depend not just on the mount, the adapter, but also on the lens. Chances are good though, that most SLR lenses won't rangefinder couple and you will have to use scale focusing instead. And you'll want to stick to film-era SLR lenses, because the newer dSLR lenses typically lack an aperture ring (so you won't have aperture control, either), and as autofocusing lenses will have focus scales that are more or less useless for manual focusing with any accuracy.

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