My latest summary [ from
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/85409-m42-k-ad...tml#post884235 ] is:
I use all three types of PK-M42 adapters:
1) official Pentax infinity-focus - it stays on the K20D or ZX-M on days when I'm mounting only M42 glass, and maybe carrying another M42 body.
2) generic non-infinity focus - they stay on old telephotos that focus past infinity, and on glass for close-up or portrait use, and on an M42 bellows.
3) generic infinity-focus - they stay on my most-used M42 glass, AFTER the (sucky) locking springs are removed. I hate those (expletive deleted) keys.
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As mentioned above, you can mount virtually
any old Pentax-compatible lens on a modern Pentax dSLR. You might not have all the operational features, but you can use it. Let's see, for my K20D I have 30 K-mount (bayonet) lenses, and 65 M42 and M39 (screwmount) lenses, and only a couple of the latter are problematic.
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EDIT: Rereading the original question, there's nothing about putting M42 lenses only on modern Pentax cameras. Yes, adapters for other mounts are available -- just search for M42 on eBay etc. The limiting factor for adapters is what's called the Lens Register, the working distance from lens to sensor or film. Here is a table, sorted by register: [
http://www.graphics.cornell.edu/~westin/misc/mounts-by-register.html ]. A lens with a certain register can be adapted to a shorter distance, but not to a greater one. The Pentax-Practika mount (45.46mm) can be adapted to Exakta, Minolta-Sony, Canon, Konica, Leica, Olympus 4/3, etc -- but NOT to Petri, Ricoh, Olympus OM, Nikon, etc, at least not without losing infinity focus.