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The viewfinder optical change brightened the framelines. However, it also introduced some flare and narrowed the exit pupil of the viewfinder ocular so you have to position your eye more carefully to see the framelines evenly illuminated, which becomes problematic with glasses.
The framelines aren't "better" or "worse" to my eye: the M4-2's simpler frameline setup just makes looking through the viewfinder a cleaner experience to me. Only one frame for 50 and 90mm, a double frame for 35/135. Coupled with the wider exit pupil, it's a more relaxed view.
Thanks! What do you mean by wider exit pupil?
Also, you mention the optical change only came in for the second batch. How would you know 1st from 2nd batch M4-2's