NIKON: I have some Nikon-mount glass. I'm not at all familiar with all the permutations of the F-mount, but one lens is AF and DEFINITELY doesn't fit. Many of the manual lenses force-fit with no great problem. I've done basic surgery on a couple for better fit -- with a Dremel Moto-Tool and metal-cutting disc, I removed protrusions (like a flag) from the lens base, and thinner protrusions (carefully!) from the bottom of the aperture ring. They lock into a Pentax mount with about 1/8 turn, nice and solid. And they work with Catch-In-Focus, nicely shorting the appropriate pin.
Other F-mounts, third-party, have a lip around the aperture ring, rather than just two protruding sections. I haven't tried cutting away that much; I just force the lens into the Pentax mount, and it seems to lock-on solidly. But as was warned above: these are not rock-solid connections. I take great care not to slam the lenses, not to strain the mounts. Anyway, these old Nikon mounts work fine on my K20D, and I'm looking for more.
CANON: I have some Canon FD lenses that I'd surely love to use on my K20D. None are so unique that they're worth wasting money on an adapter. Dare I try a bit of surgery on them? Maybe I'll miscegenate one with the Nikon-F base from a Vivitar teleconverter. (I don't have any extra PK bases, ratz...) Will that gain me the 3.46mm I need for infinity focus? I'll have to try that the next time I feel brave -- haul out to my machine shop (the Dremel on the back-porch railing) and see what happens. Stay tuned for developments...
ANOTHER APPROACH: In
this recent thread at MFLenses.Com a Kiwi converted a Pentax P30t to a Nikon mount. Terrible waste of a P30t, eh? If I only had a Canon TC, I could try something like that with one of my excess M42 SLRs -- oops, they all have metal mounts. What to do, what to do?? Dang, these Canon lenses are like an albatross around my neck.