Dan Tamarkin here from Tamarkin Camera in downtown Chicago to talk a little bit about Leica screw mount lenses on the Leica M camera.
Originally back in the day, with screwmount Leica cameras, the lens is threaded on and off the camera, which is one of the reasons that Leica lens, early Leica lens for screw mount or M have an infinity lock. That way, when you were ready to take the lens off the camera, you didn't have to rack it all the way to its near focus, but you could simply grab and twist.
When Leica came out with the M3 camera, it became obvious that there was no reason to not use screw mount lenses on the Leica M camera, even though differences between the cameras lenses were pretty big. A thread mount lens and a bayonet lens, fortunately, both require the same distance from the lens flange to the film plane. And so, Leica devised little adapter rings, and each one is milled in such a way to bring up the correct frame line.
This is a 50mm Summicron lens, and I'm putting a 50mm adapter on it, and it threads in nice and neatly, and then checks right into the camera without a hitch. Absolutely collapsible on the M camera and a rangefinder coupled just like an M lens. Many Leica M lenses are actually screw mount lenses with an adapter. There's no difference in the capabilities of the thread mount lens to an adapted lens. It's absolutely identical, and it gave screw mount lenses longevity in the new Leica M system.
These adapting rings can be used on any of the Leica M cameras without exception. One can always adapt from screw mount to M bayonet, but not the other way around. Here are a few examples of lenses made for the screw mount system and a little trick for disengaging your screw mount lens: hit the button, twist it off, and keep twisting a little bit. That will loosen the ring on the back of your lens.
Screw mount Leica lenses on the M camera. No reason not to use a vintage lens on your Leica M or digital M camera.
They make beautiful images.