Time to update this list. Mainly because more and more commercial conversion kits and adapters are available. Below is the original one. This is how I would now rate it.
Low Level of impossibility (=easy):
Large format lenses with large registration distances that leaves plenty of room for adapters (and for many their are official or 3rd party adapters to buy prefabricated), such as Mamiya (105mm), Rolleiflex (102.8mm), Bronica (101.7mm), Pentax 67 (84.95mm), Hasselblad (74.9mm), Pentacon six (74.1mm), Pentax 645 (70.87mm), Mamya 645 (63.3mm).
Some adaptor 35mm systems with large registration distance: T/T2/TX/T4 mounts (55mm). All adapters available used, at least T2 new. No sport. Adaptamatic, adaptall-1 and adaptall-2 counts here of coure.
In this category we should include some originally medium to really difficult mounts that has become simpler due to ready commercial adapters and exchange mounts.
Exchange mounts from Leitax:
Leica R (47mm)
Olympus OM (46.5)
Exchange mounts:
Topcon (44.7mm)...it appears to me that this works only because the original Topcor mount was very thick, so by replacing it with a thinner mount, one can get the right registration distance to Pentax...Exakta is the same mount, but I don't think exakta lens mount pieces are thick enough(?)
Optical adapters:
Minolta SR (43.5mm)
Canon FD/FL (42mm)
Nikon-F (46.5mm)
All these are still more difficult if you want to do it yourself without having to use a degreading optical adapter.
Medium difficulty:
35mm lenses from systems so close to the Pentax K/m42 registration distance (45.5mm), that it is difficult to make an adapter: Contax (48mm), Yashica (45.5), Praktica (45.5mm) and more. All these will focus to infinity or a little bit beyond (but who cares). As long as you can squeze them into the K mount.
Really difficult:
35mm cameras with registration distances shorter than the K/m42 distance: Exakta, Rolleiflex (44.6mm), Minolta/Sony (44.5mm), Sigma (44mm), Canon EF (44mm), Konica AR and Konica F (40.7mm), Olympus E etc (38.67mm), Contax G (29mm), Olymous Pen (28.95mm), Leica M39 (28.8mm), Leica M bayonet (27.8mm), and more. With my Konica AR test, it appears that focus is very limited already at a 5mm flange difference. So somewhere soon below 40mm, there wont be any correct focus at all, and hacking will be necessary.
More possible lenses with great level of difficulty would be 16mm film (23.22mm), C mount (17.526mm, surveilance cameras for example), CS mount (12.52mm), D mount (12.29mm).
Then there is the added complexivity that Georg discovered in his hack: if the lens is built for something that does not have a mirror, the final lens may extend well into the mirror-box. Or the diameter of the mount may be wider on the lens than on the K mount.
Originally posted by Douglas_of_Sweden That's a cool lens. Gives sort of dreamy effect. I like the goat in your link.
I think Nikon barely count in this thread since they have about the same registration distance as Pentax and many of them actually mount.
Low Level of impossibility:
Large format lenses with large registration distances that leaves plenty of room for adapters (and for many their are official or 3rd party adapters to buy prefabricated), such as Mamiya (105mm), Rolleiflex (102.8mm), Bronica (101.7mm), Pentax 67 (84.95mm), Hasselblad (74.9mm), Pentacon six (74.1mm), Pentax 645 (70.87mm), Mamya 645 (63.3mm).
Some adaptor 35mm systems with large registration distance: T/T2/TX/T4 mounts (55mm). All adapters available used, at least T2 new. No sport. Adaptamatic, adaptall-1 and adaptall-2 counts here of coure.
Medium difficulty:
35mm lenses from systems so close to the Pentax K/m42 registration distance (45.5mm), that it is difficult to make an adapter: Contax (48mm), Leica R (47mm), Olympus OM (46.5), Yashica (45.5), Praktica (45.5mm) and more. All these will focus to infinity or a little bit beyond (but who cares). As long as you can squeze them into the K mount. Nikon-F (46.5mm) would belong to this group, would it not be for the lucky accident (is it?) that at least older F mounts will fit on K mount cameras, but not all lock (see the Nikon on Pentax thread).
Really difficult:
35mm cameras with registration distances shorter than the K/m42 distance: Exakta, Topcon (44.7mm...I've been drolling over some lenses, the common ones does not go for much), Rolleiflex (44.6mm)*, Minolta/Sony (44.5mm), Sigma (44mm), Canon EF (44mm), Minolta SR (43.5mm), Canon FD/FL (42mm), Konica AR and Konica F (40.7mm), Olympus E etc (38.67mm), Contax G (29mm), Olymous Pen (28.95mm), Leica M39 (28.8mm), Leica M bayonet (27.8mm), and more. With my Konica AR test, it appears that focus is very limited already at a 5mm flange difference. So somewhere soon below 40mm, there wont be any correct focus at all, and hacking will be necessary.
More possible lenses with great level of difficulty would be 16mm film (23.22mm), C mount (17.526mm, surveilance cameras for example), CS mount (12.52mm), D mount (12.29mm).
Then there is the added complexivity that Georg discovered in his hack: if the lens is built for something that does not have a mirror, the final lens may extend well into the mirror-box. Or the diameter of the mount may be wider on the lens than on the K mount.
*Rolleiflex: Commercial conversion available, you have to send in the lens.
Another way to express these difficulty levels are
1) You have to build an adapter
2) You have to hack the mount
3) You have to hack more than the mount (apperture ring etc)