How accurate is the M's frame-line display?
Generally speaking you can forget about exact framing accuracy when using an M rangefinder. If you really, really need to know where the exact edge of the image frame is, then stow your M and get a 100% finder-area Nikon F!
As "Masatoshi Yamamoto" <firstname.lastname@example.org> succinctly put it in July 2002:
"...how can we obtain precise composition with the provided frame lines in the viewfinder? "
You can't. A Japanese photo magazine did an article recently showing the difference between what the framelines show and what appears on film. Not only do the framelines show much less than the film, the percentage varies with distance, and the parallax correction wanders around within the frame, so it doesn't do much good to try to remember some way to compensate. They best you can do is use the framlines, know that at least this much will be on the film, and crop.
For those who crave more detail, John Collier provides the following guide:
There are no dumb questions and this is an important one to know about when using a M6/M4-P camera (earlier M cameras show slightly more than the M6/M4-P).
For a 50mm lens at closest focus - 0.7m - the area bounded by the inside edge of the framelines will approximate what is shown in a mounted slide (23mm x 35mm), which is equates to 93% of the actual film area.
With the 50mm lens focused at 2 metres, this same 23mm x 35mm film area is now delineated by the outside of the frameline box. Finally, with the 50mm focused at infinity, you need to imagine an area three frameline width's thicker all around to get the same 93% coverage.
BTW, the change in coverage also varies with the focal length of the lens being used. A 90mm lens would give an even greater amount of change such that at infinity, the 75mm frameline set is pretty close to what you will get on the film. With a wide angle lens such as a 21mm the change is negligible and can be ignored.
In Dec 2001, "Jay" added the following:
A lens that is focused closer by moving the optical cell farther from the film plane gains a bit of focal length and therefore loses some angle of coverage the farther it is extended. So at closer distances the field of view becomes smaller. The Leica framelines are sized at this smallest dimension, less "the amount masked by a typical slide mount", so that one will always be sure at least everything in the frames will be on film.
[…] In reality, at infinity with a 50mm lens you will get a view on film equal to an area amout midway between the 50 and 35mm framelines. With the 90mm lens at infinity the on-film coverage is almost exactly equal to the view seen through the 75mm framelines. With the 135mm you can just compose just outside the framelines at infinity and be ok. With 35 and 28mm framelines, you can all but forget the issue altogether.
A camera like the Contax G2 adjusts frame size as it is focused, the Leica mechanism does not allow for this. Do not confuse the movement of the frames down and right: that is parallax correction, to compensate for the fact the viewfinder is offset from the lens' axis and this becomes more critical at closer distances.
FYI, with internal-focusing lenses (not found on the Leica M), the focal length remains constant as the lens is focused (since there is no extension); however the maximum aperture changes slightly, eg. with the 280/2.8 at its 2.5m closest range, the effective aperture is actually about ƒ2.2.