I have a ragtag collection of Nikon F-mount to Leica M-mount, Nikon F-mount to Sony E-mount, and M-mount to E-mount adapters from Novoflex, Metabones, and MTF. None of these are inexpensive; all cost over $200, and one cost more than twice that. As has been discussed before, none are the correct length, almost certainly on purpose.
There’s something else that’s wonky about them all; when bayoneting the lens into position, none of them require the same torque as bayoneting that same lens into the camera body. I never appreciated it before that virtually all Nikon lenses mount on virtually all Nikon bodies with pretty much the same twisting force. It’s one of those things, like breathing, that I usually take completely for granted.
Not so with adapters. It seems the usual practice is to make it much harder to twist the lens home onto the adapter than it is to mount the same lens on a camera body – the MTF F-to-E adapter I have makes me worry about damaging the lens when I use it. However, that’s not a given; some adapters are looser than the standard.
Nikon has figured out how to make this consistent. Zeiss and Sigma, the third-party lens suppliers with which I have the most experience, have as well. Granted, they are larger companies, but why is this so hard? It can’t be a manufacturing cost issue: Nikon makes lenses that cost less than these adapters!
Like finding the seat belt improperly installed on an airplane makes you wonder about the maintenance practices on the engines, this seemingly random variation makes me wonder if the front and back flanges are really parallel.