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Originally Posted by wizard
Hi Giuseppe, and welcome to this forum! What you say about the different size of the brightline frames in older versus newer Leica-M cameras is true, however, I believe the reason for this is not a different viewfinder magnification, but simply a different size of the respective frames in the various models.
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Andy is correct. There never was a .77 version. Only .58/.72/.85/.91
In the M2/M3/M4/M5/M4-2 and some M4-P cameras the framelines show a larger area of coverage, than in the newer bodies. As an example, the 50mm framelines in any of these bodies shows the coverage at 1 meter.
Starting with the M6 Leica shrank the amount of coverage that all markings show to reflect the new close up distances the modern lenses were capable of focusing at. An example: In the newer cameras the 50mm markings show the coverage at 70 cm, instead of 1 meter. Older 50mm lenses only focused as close as 1 meter. The newer versions focus as close as 70cm.
Basically the idea is that Leica is indicating the absolute minimum amount of the image you will get on film. The reason why the amount of coverage changes at different distances, is that the actual focal length of the lens changes ever so slightly as you focus from close-up to infinity. So, when focused at 70cm your Summicron may truly be a 50, but when set at infinity it turns into perhaps a 47mm. The framelines do not compensate for the shift in focal length. They only compensate for parallax error in x and y, not z (depth).
The only problem is that in doing so Leica made all markings on the newer cameras from 50mm up very inaccurate at normal working distances (3-10 meters and infinity), because you are starting out with a much smaller area of coverage (70cm vs 1 meter).
I do not expect 100% accurate framing from a rangefinder camera (that is what an SLR is for), but we've gone from the markings being reasonable accurate, to being wildly off the mark. With the new style markings you will end up with as much as 20% more on your negative than you expect.
I for one will not shoot anything longer than a 35 on my modern bodies, because the 50 markings are so inaccurate. The 50 ends up on my M2 or M4.
(another way to look at it is like this. The new markings show what you get on a framed slide. The old markings show what you get on the whole negative.)