GAS? Me? Not for the rest of this month, anyway!
You lost me! Just when I felt I was starting to get a grip on how this all works. I thought the reference to a focal length as "normal" (ie giving a normal perspective in relation to what the human eye sees) is related to the diagonal measurement of the negative, about 43mm for a 35mm negative. Any lens about that focal length can be thought of as a "normal" lens and most makers have interpreted that as about 50mm. Does the 52.3mm Contax standard and the 51.6mm Leica standard refer to this? I don't see how those numbers relate to the critical dimensions that determine whether the image will be in sharp focus at the film plane.
Tell me if I've got this right, please. This is long-winded but it’s the only way I know to check my understanding, so forgive me. Sorry if I sound like a PITA.
The LTM lens/body system relies on consistent dimensional relationships for focussing accurancy, between infinity and minimum focusing distance, across different lenses and different bodies.
The critical bits are that the lens elements have to be held a certain distance from the film plane, and the scale markings on the lens focusing sleeve should accurately reflect the distance to the object in focus, at all points between minimum and infinity.
For convenience, the rangefinder mechanism, with its cam and mirrors, relays a graphic representation of that information to me. So there's two indicators that the lens should be/ought to be in focus - the distance scale markings on the lens itself, and the overlapping images in the camera viewfinder. When all is working properly these two agree, and also coincide with sharp focus at the film plane, for that distance.
Now lets mate the lens up to the body. The lens flange is the flat surface where the lens meets the body. Leica set the lens flange-to-film plane distance (lens register) at 28.8mm, and all LTM lenses and bodies should follow this standard.
Lens designers, knowing that the mounting flange for the lens body will be 28.8mm from the film, know where to put the lens elements to achieve sharp focus at both ends of the focusing distance range. And this 28.8mm dimension also determines where they have to position the sleeve that moves the rangefinder cam in the camera body that relays the focus distance to me as overlapping images. That is the other part of the LTM standard – the correct distance of the rangefinder cam from the film plane. Because that’s relative to the mounting flange we can also measure it from the film plane.
Now I’ve just measured a modern CV LTM lens: at infinity on the lens focusing scale, the sleeve that rides the rangefinder cam is 12.5mm proud of the lens mounting flange (ie it sits deeper inside the camera, where it contacts the cam). I get the same measurement for the Canon 50/1.8. So the sleeve will be (28.8 minus 12.5 =) 16.3mm from the film plane when the lens is set at infinity. I hope that’s where my cam lies, and that the rest of the rangefinder mechanism lines up the overlapping images at that point. (I can’t measure it at the moment because both my LTM cameras have film in them.) With the lens focusing scale set at 1 meter, the sleeve is 4.6mm proud of the mounting flange, so the sleeve and the cam where it touches, should be (28.8mm – 4.6mm =) 24.2mm from the film plane. The full travel of the rangefinder cam, and the sleeve in the lens that pushes the cam, should be 7.9mm. When all these dimensions are held accurately and consistently by the makers of both the camera bodies and the lenses, and the rangefinder mechanism that moves the prisms are correctly adjusted, it should all work together.
How did I do?
If I’ve got that straight, the issue for momosgarage is that he wants a camera that will let him use his Canon 50/1.2. His Canon lens is built accurately for the Leica 28.8mm flange-to-film plane standard, so that’s what he needs. The problem is that some FSU LTM bodies are not built accurately to the 28.8mm lens register standard. He also needs enough clearance of the viewfinder and rangefinder windows to be able to see around that big chunk of lens.
Hey momosgarage, just buy a Canon and be done with it!
Chris
