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Good thread Greg. Would be good to hear from Bert and Marc on their exploits.
I've had very limited experience with a few Pentax k-mount lenses and found these have worked very well in general on both my QHY9 (with Peter Tan adaptor) and Canon 450D. In fact I've had a lot of un-anticipated fun using them on my kids as portrait lenses. 28mm f/2.8 for widefield and 200 f/4 for closer in. 100 f/2.8 and 50mm f/1.8 still untried, but have Barnard's loop in mind. As nice as Pentax 67 lenses are, the reality is they're getting up in price towards what you might pay for an ED80, and the extra illuminated field un-necessary for 8300 sensors (perfectly understandable if you have a larger sensor or are buying for future redundancy though). Marc put me on to Pentax lenses, and I believe he buys M42 threaded lenses.
It sounds like a similar story to Nikon - they've managed to keep the mounting systems essentially unchanged over the years, such that older prime lenses with excellent glass but no electronics are now available at a substantial discount (and as I mentioned earlier they have the spin-off benefit of being fun but totally manual on your DSLR for daytime use - often fast enough to allow high speed indoor portraits of fasting moving children). Canon "nifty 50" has been good on my DSLR but one corner shows some coma. I haven't had this issue with the Pentax lenses.
Still need to experiment with different f stops for the pentax lenses. I believe Bert used to leave his wide open and use coffee tins or some such thing to stop down and avoid diffraction spikes? I'm a little surprised we don't see more lens work posted here. Imaging at shorter FL's is often quite rewarding, less demanding, and even possible piggy-backed beside/on top of your main imaging rig.
Getting the right adaptors for lenses can be fiddley. Oh to have a lathe and know what you're doing!
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