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Which mount do I have? Compatibility with my camera Full compatibility list

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Pentax K body + M39 lens

NOT COMPATIBLE

Unfortunately the flange focal depth doesn't allow conversion.

There was an item titled M/L39 which was actually an alternate name for the L39 with a rangefinder. Just to make life more complicated, the Soviets used the M39 designation not only for a rangefinder, but an SLR system as well, which is a 39mm wide threaded connector with a flange focal depth of 45.2mm. This is very similar to the M42 and several early Helios optics (for example the Helios 44 2/58) were available both in M42 and M39 (SLR) version.

Nowadays most misunderstandings besides the names stem from the fact that there are M39-M42 adapters sold on eBay, which are actually rings made of very thin material and threaded on their in- and outside. Screwing them on a 39mm thread they convert it to 42 mm wide allowing to fix an 39 lens on an M42 mount. However, you might have noticed that the flange focal depth of the M39 is less than of the M42 (45.2mm vs. 45.46mm, 0.26mm difference).

The difference might seem to be marginal, but anyone who is familiar with manual lenses and had faced the issue with focusing on distant objects will know this minusucule difference will be decisive when very trying to sharpen a distant subject's image.

The usual solution - to buy a simple M42 adapter and an M39-M42 ring, very carefully sand off 0.26mm won't work here due to the similar flange focal depth

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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.

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