Hello to those interested in shooting some of the classic (and sometimes great) M42 lenses in the modern world. I discovered the world of M42 while shooting a 60D body and of course crop bodies have no problem with mirror tolerance on almost any M42 lens. I knew when I switched to FF that some of my favorites would not work as well or at all. There are lists out there regarding the 5DC and to a lesser extend, the 5DII.
One of the things I was eager to test when my 6D arrived was the mirror tolerance on my M42 collection. Photos of the build show that it is narrower than the 5DII, and I wondered if that would create less tolerance for mirror fouling. Here is a list of my current M42 kit and my findings:
1) Anything 85mm+ is almost never a problem. There tends to be a fairly deep well in them and nothing fouls the mirror. I have an Image/Vivitar 135mm f/2.8 that is no problem, nor is my Super Takumar 150mm f/4
2) SMC Takumar 55mm f/1.8. Works fine at all focal lengths.
3) SMC Takumar 28mm f/3.5. Works fine at all focal lengths (and is a killer lens, BTW).
4) Super Takumar 35mm f/3.5 - still hangs close to infinity. Has been my worse offender on the 5DII and so have only used it on my 60D. Is marginally better on the 6D but still not really an option here.
5) Helios 44-2. I love this old lens, particularly for portraiture, and I was very happy to find that it now works almost to infinity focus without hanging the mirror, which is a big improvement over the 5DII. It is far more of an option through normal viewfinder focus than before.
All in all, I was pleasantly surprised to find that there seems to be a little more mirror clearance in the 6D, so those lenses that were on the edge with the 5DC or 5DII may be usable with the 6D. I have found that some M42 lenses just work better on a FF body. Some get shown up as being inferior, of course, but for some the narrower depth of field inherit in FF gives them more character. Happy shooting!