Defect of the lens rim blackining
Schneideritis is the common name for a kind of defect. The defect are bright spots in the black paint on some lens surfaces. The unused lens surfaces of better lens systems are blackened to minimize stray light. And this blackening could degrade over the years!
Here an image of an affected Schneider Kreuznach C-Claron 190mm f/5.0 lens blackening. In this lens it can not be seen on the painted outside of the lens.
Large format camera users noticed this some years ago mainly on their Schneider Kreuznach lenses, and named it "Schneideritis".
But not only Schneider lenses are affected.
Here how it looks like:
Brighter dots - but only where the lens is black. These dots are not somewhere without black paint.
Here I scratched some paint of this lens surface, one could see it looks very much like the defective spots.
These defective spots results in a bit more stray light. I am not sure wheter this could be seen or measured in an image.
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