As a fan of old lenses, I like this idea, but I'm wondering about the benefits apart from the fun factor? Sure, we can use a 40 or 50 Pentax lens on a M body, but unless you have a real attachment to the way that lens renders and also an attachment to how M bodies handle, I'm struggling to see how much benefit the user gets from this outside of novelty. I have Dad's lovely Pentax M SMC 50mm f1.4, and it's great on a film body like the ME, but less than stellar adapted to a digital body like the Panasonic S5, particularly wide open.
Just brainstorming here, but assuming one does not want to wait for the C/Y adapter, here are some 50mm M42 lenses that would be fun to use on an M, especially if you already have them.
- Any of the 50mm f/1.4s by the major 1970s manufacturers -- like Pentax, Fujinon, Mamiya-Sekor, Yashica, Olympus, etc.
- German gems like the CZJ 50mm f/1.8 Pancolar, the Pentacon 50mm f/1.8, Domiplan 50mm f/2.8, the 50mm f/1.8 Icarex Ultron, CZJ 50mm f/2.8 Tessar
- Modern Zeiss/Cosina M42 ZS 50mm f/1.4.
- Exotic M42 50mm lenses like the Iscos, Alpas, etc.
- Any 50mm lens that you can convert to M42 with full focusing (like all those old Cine lenses that one guy in Europe does).
- All of those basically-free-today M42 50mm f/1.7 lenses made by the major Japanese manufacturers.
- All those decent Soviet 50mm standard lenses that came with the later Zenits.
- The new Meyer Optik 50mm f/2.8 Trioplan M42 lens, whose Leica M version does not RF couple.
- It would probably block the RF, but you could easily stack adapters to use a Pentacon Six CZJ 50mm f/4 Flektogon.
I think ultimately that a 35mm FL adapter may be more popular. However, if one is a fan of M42 lenses, this adapter is worth it in my opinion but it could be a little cheaper.
