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Leica M + M42 lens

COMPATIBLE MF MA MAC

M42 lenses can be easily fitted on Leica M cameras.

Some basic math shows that a 17.66 mm thick ring will be necessary with a Leica M and a M42 receptacle on one end.

One thing worth mentioning: some M42 lenses have a small lever which is responsible for operating the diaphragm. On the simpler, older M42 lenses the shutter will react according to the values set on the calibration dial only when this bit is pressed in. Not all adapter rings are fit for these, since there are two versions manufactured: one with aperture ring and one without it (this issue usally comes out with older Helios optics, the newer M42 lenses have an A-M switch, which allows using full manual mode, thus the aperture will be on the set value without having to use a separate bit).

The aperture ring can cause problems in one more case: if the lens has an aperture value feedback mechanic (a small sticking out red lever). This lever may touch the bezel and get the whole aperture control stuck. The SMC Takumar 1.4/50 is also has this feature, which means it needs an adapter without a bezel or you'd need to sand off a few milimeters from it. On the figure below you can see an adapter without a bezel, it's clearly visible why you'd have troubles with the other type.

As for the adapter ring brands, there is a huge assortment of them from noname ones to major manufacturers' solutions.

In most converters, the M42's threaded receptacle is a separate ring within the whole unit, which is fixed by three small bolts. After loosening the bolts, the thread can be turned within the converter, allowing to set the lens precisely, with the numbering on top.

If you're interested in small things, macro adapters are also available. This will move the lens a bit away from the sensor (shortening the near focus point) without interfering with the imaging capabilities of the lens. Some can be found here:


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Terminology

In terms of compatibility, we distinguish four separate categories:

COMPATIBLE
PARTIALLY COMPATIBLE
COMPATIBLE IN THEORY
NOT COMPATIBLE
This means that you'll be able to apply the lens on the camera in question with a proper adapter, and full focus range will be available with it (often with aperture control too). If AF is supported, you'll see it in the description.
This means that due to the flange focal depth or other factor this conversion would be normally unfeasible, but with some compromises (e.g. using a speedbooster adapter) can be made possible.
This means that the flange focal depth should make mounting and using inifinty focus possible, but for some reason there are no adapter rings available or other factors make it unviable.
This means that you won't be able to mount your lens due to mechanical incompatibility or the difference in flange focal depth doesn't make inifinity focus possible. Although numerous lenses can be mounted on cameras without infinity focus, but this cannot be considered actual compatibility.

Additional information for compatible variations:

MF
AF
MA
AA
SB
MAC
TS
manual focus (on the lens, with its own control ring)
autofocus available (AF controlled by the camera body)
manual aperture control (on the lens, with its own control ring)
automatic aperture control (controlled by the camera body)
speedbooster (focal reducer) adapters are available
macro (variable length) adapters are available
tilt-shift (or other flexible) adapters can be used

If you'd like to read more in-depth about compatiblity, you'll need to get familiar with flange focal depth, see this article.