Is an M42-to-Nikon F adapter with focus to infinity really mechanically impossible or just dangerous?

Question

By conservative opinions, such a device is considered impossible.

However, given the facts that:

• Some, especially preset style, M42 lenses have a body that is, on the rearmost few centimetres, no thicker (or hardly thicker than, with only a minimal flange) than the threaded end – you could shove that section of the lens body through the mount.

• The same style of lens often has nothing protruding past the threaded end.

• There are several millimetres of space between the mirrorbox and the mount flange, at least on some F-mount APS-C DSLRs.

Could an adapter providing a (probably interrupted/sectioned) thread and minimal flange sunk a few mm into the camera throat be made (similar to some C-mount to mirrorless adapters), even if it would be unsafe to use with the wrong combination of lens and camera?

Answer 1

Ordinarily, it is impossible to use an unmodified M42 lens on a body with a Nikon F mount with an adapter that maintains infinity focus without the use of additional optics. Exceptions will be described later. The problem is similar to using FD lenses on EOS bodies with EF/EF-S mounts.

• The M42 mount has a flange-focal distance of 45.46mm, while Nikon F mount has a flange-focal distance of 46.50mm, so you would have to sink the lens about 1mm into the body.

  To sink the lens into the body, the lens diameter needs to be smaller than the mount throat diameter. C-mount lenses work because their diameters are typically much smaller than the throat diameter of nearly any other mount. However, with M42 (42mm) and F-mount (44mm), there are only 2mm to work with, which are more than taken up by the flange width (typically > 5mm).

  • If you own a lens with a flange width < 1mm, you can see if it will work by free lensing. If you put the camera into live view mode, you won't have to worry about the mirror. When you're ready for the adapter, you'll probably have to design and 3D print it yourself.

• The distances are close enough that it is possible to modify an M42 lens to work with the Nikon F-mount.

  • If you remove the mount from the lens, you may be able to fit a new mount on it (as described by Tetsujin). Depending on the lens, you might have to shave a bit from the barrel.

  • Focus can be adjusted and calibrated on many manual-focus lenses by loosening a set screw. Depending on the lens, it may be possible to "correct" focus for a thin-enough adapter.

    The focus ring may turn past infinity on some lenses without the need to loosen a set screw. In this case, you may be able to use the lens as-is with a thin adapter. Be aware that the focus scale will likely be inaccurate.