Fun fact for the day: ThorLabs’ SM2 lens tube standard screws right onto the front end of Nikon’s SLR lenses. Other manufacturers probably use the same threading, I just haven’t tried them.

I don’t know if this is by design or not, but it makes coupling 35mm SLR lenses into optical setups fairly straightforward. I’m using it for a tandem lens macroscope. In the picture above I used ThorLabs part SM3A2. BTW, they also sell some F-mount adapters for connecting to the other side of the lens.