Question about rangefinder diopter adjustments

I see that the iii series Leicas and many of the copies have diopter adjustments or focus adjustments, whatever the preferred term is, on the rangefinder window. This has gone away by the M series Leicas although it appears to have been retained on some of the Leica derived cameras like the Soviet stuff (It also seems that on DSLRs they generally have an eyepiece focus adjustment). I use eyeglasses and have a Nicca (Leica iii copy rangefinder) with the aforementioned focus adjustment, and it is unusable for me unless the adjustment is set correctly.
What changed such that the adjustment is no longer necessary on the M cameras, or should I expect to need an add on diopter device if I get a more modern body such as a M2?
Bruce

I'm not an expert. One thing is the screw mount cameras have magnification on the rangefinder, the M bodies don't. I wear glasses and don't have problems with any M. I do adjust the diopter on screw mount cameras and still use my eyeglasses.

mshchem said:

One thing is the screw mount cameras have magnification on the rangefinder, the M bodies don't.

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That had not occurred to me. Not having used a non-adjustable separate rangefinder from viewfinder camera, I wonder how that would work in my case. A bit off topic but: in not too long from now I'll need cataract surgery in my right eye. The patient gets to choose from a menu of corrective choices on the new lens. I will at that point have an interesting consultation about lens choice usefulness for photographic purposes vs usefulness for life in general. I don't suppose that use of old cameras is too high on the MD's list.

Individual One said:

That had not occurred to me. Not having used a non-adjustable separate rangefinder from viewfinder camera, I wonder how that would work in my case. A bit off topic but: in not too long from now I'll need cataract surgery in my right eye. The patient gets to choose from a menu of corrective choices on the new lens. I will at that point have an interesting consultation about lens choice usefulness for photographic purposes vs usefulness for life in general. I don't suppose that use of old cameras is too high on the MD's list.

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I would go for 20/20!!!. A real bona-fide eye doctor should be a Leica person They should understand.