May I add "fit"? I don't have any issues with M42 lens to Canon EF adapters - simple to mill, even the cheapest ones fit reasonably. I have about 4 of them from different vendors - 3 of 4 are great, the 4th is tight, but at USD6.00, I expect variation and pitch the unsatisfactory one. Nikon F lens to Canon EF adapters: adapter #1 turned out to fit the EF end securely, but there was *wobble* in the F end; adapter #2 fit well at both ends. Adapter #2 had an extra leaf spring in it for the F incoming bayonet. (#1 Fotodiox less expensive; #2 Fotodiox more expensive).
I guess I've been lucky. I've got about 18 adapters of various description, and I've never encountered a fit issue, although I've encountered it with some lenses, including a Zeiss Loxia. But thanks to this thread, I've updated my blog to deal with that issue.
Jim
I purchased the better Fotodiox adapter for my A7, but discovered that it was absolutely useless. Any suggestions?
For what adapted mount did you buy it and why is it useless?
Canon EF. It would not AF anything. Now I have the Metabones IV which focuses everything.
For Canon EF and the AF lenses, the Metabones latest adapters and the Sigma MC-11 seems the best in that regards.
There are two things to keep separate, the physical (mechanical) quality of the adapter which is all that matters on a dumb adapter without any electronics for use with fully manual lenses.
Where on the other side the electronics in the adapter and how well it translates from the native side to the adapted side with AF commands and general communication protocol is also a very important point for a smart adapter.
And an adapter that is good on one thing doesn't have to be good at the other things of what it does.
I guess Fotodiox makes some good dumb adapters, I have two of those (SR+M42 mount) that I find good enough. But I have not read anyone that really raves about their smart adapter skills.
The only thing I can think of to do if it doesn't work well at the smart side for you is to see if it is upgradable in firmware or in hardware (meaning having to send it in to them for fix, check their homepage for support if they have anything like that), or use it as an dumb adapter with manual focus lenses if the dumb side is good on it (meaning the mechanical side), or sell it etc.