From my blog: http://oldlenses.blogspot.com/2011/02/taylor-hobson-ortal-3-inch-75mm-f2-tv.html

This lens, without modification, can not be used on DSLRs and achieve infinity focus, even on the Sony NEX, it's tough to get it to work, because the flange is so short. It was designed as a TV lens with fixed mount. I have been dying to to try it, but without much success.

I was tinkering with adding aperture control to the projection lens, and looking for some adapters, and found that I had a Series 8 to Series 7 step down ring. This ring fits almost perfectly to the mount of the Ortal 75mm f2 lens. Eventually, the ring was fixed to the mount, and with a Series 7 to 55mm stop-down ring, I was able to add four 55mm filter rings to make an extension tube that allows me to focus the lens to about five feet, just long enough for some portraiture samples. I could add/remove rings to make it focus further/closer. This is the best solution so far until I get my hands on a very thin focusing helicoid.

A word about the C-Mount to NEX adapter I got. Whoever designed this put a lot of though into it. The center of the adapter has a 42mm thread size. This has many uses. You can attach an M42 lens directly to the adapter, or, I guess the original purpose, is to allow one to attach a focusing helicoid with M42 mount on it. The surprise, is that the outer ring of the adapter. It has a 55mm thread size. I don't know what the reason for this was, but it was perfect for me, as I can attach the Ortal 75mm f2 lens with the filter rings directly on it.

All this means that I can take pictures with the lens with a fixed focus range, and I have to move the camera back and forth to focus. Pain in the neck, but pictures can be taken.