The magic lug
The magic lug
No, my problem is that I need to add it. This exists in F-mount film cameras, and may be specifically to fit the 21mm f4 lens. This is used to keep the body of the lens from rotating while you mount it with the strange breech-lock but F-compatible mount it uses. Without it, the lens will rotate and break things in the camera. I notice this lug does not exist in my Nikon DSLR's. Anyway, it should not be a big deal, but I had been trying to figure out how to do it without drilling a radial retaining hole, but now that I have given up on that nicety, it should be easy to finish.
I would like to get my hands on a manufactured one of these to see how they deal with some things. It looks like, for an adaptor that will work with the 21mm f4, that the lens mount catches for the F and S parts of the mount fall on top of each other. I have been forced to eliminate the catch on the F mount. I think I know how to deal with that, and if I make another one of these things, I will be able to retain it. I would be willing to bet that in the manufactured units the F lens is rotated counterclockwise relative to the camera body somewhat, so the focus index is not where it should be.
Cheers,
Dez