[jsjohnst]

I agree the mounts have been mostly reverse engineered, but that doesn’t detract from my point (which I might not have done a good job of articulating). The point I was trying to make is that each mount has different focal characteristics which would make it non-trivial to support on a single body (with a swappable collar adapter). Doable, yes, but not trivial. Even ignoring the focal differences, electrically (aka lens to camera communication) wouldn’t be what I would call trivial either.

(For background, a friend and I worked on designs for making an open camera platform a few years ago)

[sudosysgen]

Oh, I didn't mean that the physical mounts have been mostly reverse engineered. I meant that the camera→lens communications have been fully to almost fully reverse engineered.

Mount optical characteristics are very simple. Just make sure your base mount has a larger diameter and a shorter flange distance, and you're golden. If you are designing your camera around that it is almost trivially done.

The code for implementing a basic E mount is on github, for the EF mount you just have to go looking around on some forums or buy it from the guy behind Metabones, same for Nikon G.