|Posted: Mon Oct 24, 2011 11:33 am Post subject:
|Screwmount rangefinder lenses are designated LTM39, not M39, because they do not conform to the true metric standard. The thread is 39mm OD, but the pitch is a non-standard 26 tpi. On true M39 lenses, as used on the early Zenit SLRs, the thread pitch is 1mm. They are not interchangeable, not just because of the different register distances, but because the thread will bind after a couple of turns.
The Soviet lenses for early Zenit SLRs have a true M39 x 1mm thread, and you can use a simple double-threaded ring adapter to mount them on M42 cameras or adapters. However the register distance is virtually the same as M42, so using them on a Nikon has the same problems.
Bill, many M42 and M39 lenses can be adjusted to achieve infinity focus on a Nikon. Often it's a very simple job - just remove the name-ring round the front element and slacken 3 or 4 screws. If you want to try this with a cheap lens, I've done it with a Pentacon MC 1.8/50.
Peter - Moderator