The M rangefinder changed a few times since 1954. These changes fall into three categories. Actual mechanical revisions to the RF unit, changes to the area of coverage that the frame lines indicate, amount of focal lengths shown and magnification.
Revisions to the rangefinder unit:
The M3 has a RF unit that is unique among all M bodies. The alignment of the frameline mask etc is different, making this incarnation very resistant to flare.
Leica made changes with the introduction of the M2 and this unit carried over into the M4/M5. This is a very good design. Very flare proof and very bright framelines. The frameline masks are frosted glass sandwiched between etched sheets of metal. Truly an amazing piece of engineering. Starting with the M4 Leica started to bond the prism blocks with a UV cured glue, instead of Canadian Balsam as they had done in the M3/M2. This is a much stronger construction and will not degrade as severely with age.
Erwin Puts has a good article illustrating the differences here.
http://www.imx.nl/photo/leica/rangefinder_engineering_fro.html
After the M5 meltdown, Leica reverted to the M4-2 and once again the RF unit was modified. A condenser was eliminated, which led to the flare problem that plagued M cameras until the introduction of the M7/MP. Also the frameline mask is all metal.
Starting with the M7/MP Leica redesigned the RF unit once again. The condenser that went missing with the M4-2 was reintroduced and other changes were made. The very earliest M7 bodies do not have the flare fix, but can be upgraded. This may only be the first and second batch. I have one of these cameras and it has been upgraded.
You can have a M6/M6ttl upgraded with the 'flare fix' kit from Leica or a repair shop. I had this done to my M6ttl and oddly enough it now has the brightest RF of all of my bodies.
Overall the M3 is the least prone to flare.
The M2/M4/M5 are very close. I rarely have a problem.
The M6/ttl was a pain in the neck. My .85 flared like crazy, but then again I used to live in the desert. The .58 flares the least and the .72 is somewhere in between. If you center your eye perfectly over the viewfinder the flare goes away.
The M7/MP are actually quite good. I would say on par with the M2/M4/M5 as far as flare resistance goes. Both my M7 and upgraded M6ttl work like a charm.
Revisions to the area of coverage as indicated by the framelines:
All lenses are prone to a change in magnification, as they are focused from their closest focus distance to infinity. In other words the lens will 'zoom' ever so slightly as you change focus.
On an SLR this isn't a problem, because you are looking through the lens and see what is happening, but in the case of the M camera the area of coverage indicated by the framelines remains constant, as you focus from close up to infinity.
Leica decided that the framelines should indicate the
minimum area of coverage you will get on your neg, regardless of where you are focused.
That would be the area of coverage at the closest focusing distance.
When you are focused at infinity you will get a little more on your negative than the area of coverage indicated by the framelines, but you are guaranteed to capture what is within the area indicated by the markings.
Leica made this change in coverage to all focal lengths, but it is most noticeable with the 50/75/90/135 markings.
As an example.
The 50mm markings on an M3/M2/M4/M5/M4-2/M4-P indicate what a 50mm lens will capture, when focused as close as 1 meter. During this period
most 50mm lenses made by Leica would not focus closer than 1 meter.
Minimum area of coverage: 50mm
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M3 - 1 meter
M2 - 1 meter
M4 - 1 meter
M5 - 1 meter
M4-2 - 1meter
M4-P - 1 meter (some M4-P units have a M6 mask, that shows coverage at .7m)
Around this time Leica introduced several 50mm lenses that focused to .7 meters (70cm). In response they also changed the area of coverage that the framelines indicated to .70cm.
Minimum area of coverage: 50mm
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M6 - .7 meter
M6ttl - .7 meter
M7 - .7 meter
MP - .7 meter
While this makes perfect sense from an engineering standpoint, it is something of a fiasco in real life.
As mentioned, when you focus a lens from its closest focusing distance to infinity there is a slight change magnification.
Due to this issue, Leica decided that the frame lines should indicate the absolute minimum amount you can expect to get on your negative. This is the area of coverage at the closest focusing distance of the lens.
Take a look at the attached illustration.
Starting with the M6 the 50mm frame-lines indicate the area of coverage when focused at .7 meters (70 cm). This was due to the introduction of many lenses that could focus as close as 70cm, as opposed to 1 meter.
Prior to that, in cameras like the M4, the 50mm lines indicated the area of coverage when focused at 1 meter. With the older lenses 1 meter was the principal close focus distance. The difference (error) between the coverage indicated at 1 meter and 5 meters wasn't that great and in general we didn't notice it. It became somewhat more noticeable at infinity. If you were smart you used the outside edges of the thick framelines for framing at infinity.
Starting with the M6 the difference between the area of coverage indicated at .7 meters and 5 meters is quite a bit larger and very noticeable. The difference is even more apparent between the area of coverage indicated at .7 meters and infinity. It doesn't help that Leica also made the actual framelines thinner. Someone figured out that at working distances (5m - infinity) the M6 framelines indicate the area of coverage of a 60mm lens, instead of a 50.
Unless I absolutely need to use a built in meter, I will not use a 50mm on the newer bodies. The framing error is far too big for my taste. I hope, I pray that some day Leica will offer a 1 meter frameline mask for the M6/ttl/M7/MP. Apparently they did this with the MP3, but of course Leica priced it out of the reach of the vast majority of us (rats!).
Focal Lengths in Viewfinder
M3 - 50/90/135mm
M2 - 35/50/90mm
M4/M5/M4-2 - 35/50/90/135
M4-P/M6/M6ttl/M7/MP - 28/35/50/75/90/135
The ttl/M7/MP also vary depending on magnification
.58 - 28/35/50/75/90
.72 - 28/35/50/90/135
.85 - 35/50/90/135
The 28mm framelines will simply not be visible in the viewfinder at .85 magnification. Some people have trouble even seeing the 35mm on the .85, because they are so close to edge of the frame.
The .58 doesn't support 135mm, because the small magnification does not allow you to accurately focus such a long lens.
By adding the x1.25 magnifier you can change the magnification as follows:
.58 -> .725
.72 -> .90
.85 - 1.0625
