How do Leica rangefinder cameras work?

[Malcolm Stewart]

I bought my first proper interchangeable lens rangefinder body, a Leica M6, a few weeks ago, and I've realised that I don't know how the range-finding part works.
I'm OK with non-interchangeable lens bodies, but I can't get my head round how the camera knows the difference between a 50mm lens, and say a 28mm or a 90mm lens. For a subject at say, 10 feet (~3.3m) away, the lens movement for a 50mm lens will be different to that for a 90mm lens, and yet the rangefinder cam follower has to move the same distance, I assume, as its base stays constant. Does the lens incorporate some form of "focus rate" changer? Puzzled...

 

[locutus]

It does, the cam on Leica lenses is standardised to follow the movement curve matching a 52mm lens.

This means that the cam has some gearing on any kind of lens.

 

[Malcolm Stewart]

Many thanks for your clarification.

 

[mdarnton]

You can see this yourself by looking at the back of the lens while focusing from close to infinity. Long lenses will move very far, wide angles very little, but that ring or finger on the back of the lens that contacts the little rangefinder wheel at the top, inside the camera behind the lens, will move the same distance for all lenses. For your entertainment you can press on that wheel inwards gently while looking through the camera, and watch the focus and framelines move.

The other thing is if you look inside the lens box on the camera, at 3:00 on the right you will see a slim metal slider behind the lens mount retaining spring. Flip the frame preview lever and you can see this tiny slider moving. When you mount a lens, the bayonet finger at that position is cut longer or shorter to move this slip of metal to the right position for the camera to show the proper frame. As you move the lens home the last bit in mounting it, you can see the frame lever on the outside move to the position for that lens because the finger inside is being pushed to its proper position