There are two types of M42-to-K-mount adaptor:

• The Flange type
• The Recessed type

Don't be like me and get the wrong type, which is the Flange type. While the flange type is cheap, is commonly available and the flange appears thin, it is still thick enough to prevent focus at infinity.

Get the recessed type. I've got a "Roxsen" 3rd-party version. This is very easy to fit on a K100D Super body, but I find it is difficult to fit or remove this quickly on a K20D. The genuine Pentax M42 recessed adaptor is reported to be easier to fit compared to 3rd-party adaptors.


Here's some pictures I took showing the difference between the two M42-to-K-mount adaptors.

Recessed-type adapter plus insertion/removal key on left – Flange-type on right. Both showing the side that fits to the camera:



Flange-type fitted:






Recessed-type fitted:


Dan.

[photogem]

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With the genuine ones, you really need to leave in the spring/lock because they are much more loose in the mount than the knockoffs.

I didn't experience this with my genuine M42 adapters at all. They fit very well and aren't more loose than the Hama nor the chinese ones. Maybe your's got loose due to a lot of use?

[vonBaloney]

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I didn't experience this with my genuine M42 adapters at all. They fit very well and aren't more loose than the Hama nor the chinese ones. Maybe your's got loose due to a lot of use?

It is easy to see -- if you attach them to the body with no lens there is a lot of play in the genuine ones. (I've had several, old and new.) Without the spring lock they'd probably fall out fairly easily. They are known for that, and they are designed that way on purpose -- they snug up when you screw a lens on. Whereas the third-party ones (universally it seems even though many different makers over the years) are always quite tight even with no lens mounted.