Originally Posted by Mellow
Also, I've found that some c-mounts just require a different adapter. Some are thinner than others and allow the lens to screw in deeper. I used to have four or five adapters from different vendors, and usually could find one that would work.
Of course there are exceptions . . . my Navitar 25mm needed to be modified in order to fit and focus to infinity. For some lenses you can just unscrew the sleeve near the base and grind it down against sandpaper. Others it's more difficult.
Are the metallic parts of lenses that soft to be easily ground down on sandpaper?
I had a alter 2 M39 adapters for a couple of Soviet lenses I bought on a whim for my m43 camera. The adapter came in 2 pieces that come together, an aluminum threaded ring (to fit the M39 lens) and a softer metallic housing (to go on the camera, made of brass maybe?).
For the Industar collapsible lens I bought, I had to file a little bit of the metallic housing off where the focusing mechanism sits, since it is flush against the surface, and you have to push down a little bit to get it out of infinity focus. That took maybe 15 minutes with a manual file.
For the Industar pancake lens, you have to bring the whole lens in towards the sensor like 2 mm or so to achieve infinity focus. So I tried to grind the aluminum threaded ring, first on sandpaper (the roughest I can buy at Homedepot) and then with a manual file. The sandpaper did very little, and even struggling with the manual file for over an hour of total filing time, I made a bunch of little jagged filing marks on the ring, but maybe only 0.3 mm of total thickness actually ground off.
So my impression is that whatever the housing is made of is SUPER soft. But then even aluminum, not exactly the hardest metal around, was a struggle to grind down with manual tools like sandpaper and file. I don't know what older lenses are made of, but I'd guess steel shell or at least some sort of aluminum alloy?