The M was introduced around 1954, I believe, with a slight overlap of lens availability between the two mounting systems. Generally, however, the thread mount lenses are of older designs, some of which have a strong aesthetic following, even if they were less sharp or contrasty than newer designs. There are, however, LTM-M adapters available, allowing the use of the older lenses on Ms.Originally Posted by kwmullet
Yep. There are, indeed, compatibility issues between some of the Leica clones and earlier wide-angle lenses. A couple of the Leica "reference" sites provide a run-down on which lens won't work with which camera. Also, some of the non-Leica M-mount cameras have a slightly different mount-to-film measurement that needs some caution.Originally Posted by kwmullet
True. There is also an issue of slight variation in the triangulation of the rangefinder when re-composing in critical focus situations when shooting at f/1.4 or f/0. Essentially, if you rotate or turn the camera on its center (the lens axis) when re-composing, you change the distance to the subject slightly, thus changing what's in focus. So, you either have to "slide" the camera to re-compose, or make a slight adjustment in the focus through experience.Originally Posted by kwmullet
There is also an issue of rangefinder accuracy between Leica Ms and the clones. The clones mostly have shorter rangefinder base lengths (the distance between the two rangefinder windows), making the rangefinder slightly less accurate.
I've only used the M6TTL, which has lens-linked finder frames, but its finder is plenty bright for even low-light shooting. Older cameras often need a good CLA, including a cleaning of the viewfinder and rangefinder elements, as over time, these get grunged by atmospheric contamination.Originally Posted by kwmullet
Note, too, that there are variations between earlier M models as to which frameline sets were included for which lens focal lengths. The popular M3, for example, included frame lines for 50, 90 and 135 lenses, but not 35 or 28. Thus, auxiliary finders (hot-shoe mounted) are necessary for those lenses. Later Ms offer different viewfinder magnification ratios, which also affects the frameline set included. The choice essentially pivots around whether you want a 135 frame or the 28 frame.
In many cases, I think the differences can be rather subjective. Later M lenses included design improvements that increased resolution and contrast, and some of the latest designs include aspheric elements that provide even better correction for certain optical abberations. Wear within the focusing mount can certainly affect sharpness, but the old 50mm DR Summicron is still one of the sharpest 50s around.Originally Posted by kwmullet
Yep. Leica IIIGs and earlier are knob winds, and early M3s used a double throw advance, as there was concern early on about potential film damage resulting from the thumb lever being too fast.Originally Posted by kwmullet
The auxiliary finders have no lens linkage, so they can't be parallax corrected. Each has a set of instructional guidelines for estimating the framing through experience, however. Even with the framelines in the viewfinder, there is an inherent difference in magnification based on distance, so there are guidelines as to whether to use the inside edge of the frameline or the outside edge, depending on subject distance.Originally Posted by kwmullet
In general, I think you'll find the more recent lens designs to be sharper, particularly those lenses with aspheric elements.
Some of the M4s (or perhaps M3s) had both M and X sync connectors, I believe. Later models are X-only, or perhaps triple-X, depending on your subject matter.
I'm not sure about lens compatibility issues with the Russian clones, but my impression is that most of the problems related to specific wide lenses, particularly the 28mm Super Angulon that was offered at one point. If you get a modern body and use modern lenses, I'm not aware of any problems. With earlier models, particularly clones, you have to pay attention to the specific model/lens issues.