In the normal Nikon F-Mount, there is a spring arrangement which pulls the lens to flange contact but this can be defeated by the hanging weight of a heavy lens.
There is another rough and ready solution for an alternative design Nikon F-Mount. The original Letus35 groundglass adaptor for 1/3" videocameras had a mount machined from aluminium. The lugs inside were slightly tapered to provide a wedging action as the lens was rotated in the mount to pull it firmly to flange contact.
It works well with no rocking and the mount itself will pull from the Delrin housing before letting the lens droop. There is no pin latch mechanism which is where the unwanted rotational movement comes from when focusing. Various third-party lenses have their own flavours of locating hole for the pin to engage in. Some are quite loose.
A lens in the wedge-styled early Letus F-Mount would unwind out of the mount if the focus helicoid on the Nikon lens was stiff. I added a small setscrew on radial axis to bear against the inner diameter rim of the bayonet of the lens tail to bind it from winding loose in the mount.
It is crude and if too much Tarzan force instead of common sense is applied to the screw, crush deformation of the lens tail and chipping the rear element might occur. The screwhead is also a fiddly sod to get at with fingers and the screw being long, risks being bent or damaging the mount. But it works.
The camlike inner bayonet surfaces of the soft aluminium Letus mount soon wore and the lens would turn furthur in the mount before wedging tight to flange. A stainless steel mount would solve this issue.
Like the PL-Mount style of locking mount, two distinct two-handed operations are required versus the single-handed offer and twist task of fixing a Nikon or Canon lens to a camera. Offer and hold the lens to the mount and twist to security, then secure the locking screw with the other hand.
In my example, the small screw position was about fourty-five degrees higher around the mount on left side of the camera. If a really draggy lens focus did twist the lens tail under the screw-end, a bayonet barb would already almost be engaged against the side of the screw-end before the lens could fall out of the mount.
I guess I should have registered the design but I was never much of a patent troll and the original was Quyen Le's design slightly modified.
- LETUS F-MOUNT MOD..jpg (277.67 KiB) Viewed 2671 times
If anyone is curious about the offset holes for rails, this was a means of securing this early generation Letus35 adaptor securely to the the camera via the camera's own base without overstressing the relay lens-to-camera fitment.
This image illustrates an original short screw. A much longer screw with a tube shoulder ended up being re-used on another project.