Range-finder parch white out is no Myth. When faced with a strong light source usually above and at a 45 degree angle down, the RF patch can totally white out. I was experiencing this at wedding receptions where the environment was pretty dark punctuated by higher up hot points of light (like floods shining on the dance floor).
Centering your eye can sometimes lessen this, but in my experience doesn't eliminate it. Plus, by the time you re-orient your eye, the photo op is gone.
The MP finder and later versions of the M7 eliminated this problem with a redesigned baffle system in the RF mechanism.
The RF can be altered by Leica by installing the MP version. I had both my M7s corrected by Leica and the "white out" phenomena totally disappeared.
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