Looking at this question from a slightly different perspective, does anyone have any objective criteria which would inform us at which point of scratch/ internal dust/ other, that the performance of e.g. a Leica lens looses its erformance advantage/ characteristics. such that you would be better either replacing it whether with a new cheaper alternative or minty Leica? The problem being that we tend only to have one model of a given focal length hence unable to make the comparisons for ourselves.
Or does it matter if we are happy with the results as they appear before us?!!!
Peter (no scratches)
As an old fart who has had cameras with many lenses that had scratches, and even chips taken out, I can attest that these blemishes on the front element don't impact the IQ until they just covering the front element. (I know you techy types will argue this point on the micro level), But, in the real world observation of the negatives/prints, They just don't show up, without a microscope, by the time the image is projected/recorded on the film, and it printed on paper, that level of defect is eradicated.
Years ago, I was hanging out with my Newspaper Photographer friend, who gave a Nikon F3 with a 200 f2.8 ED Nikkor that had a large scratch (like someone took a sharp knife and laid down a 2" gouge dead center on the front element.).. IT HAD NO EFFECT on the negatives sharpness, on any part of the negative as far real world use in printing on Photographic paper or newsprint.
So, We may want perfect old glass, we may not need it... unless you shoot in very bright sunlight..towards the Sun, Or, photographing a stage with bright spot lights.... Then, you may get some flair or softness...maybe...
OTOH... You can get a very good deal, knowing the few limitations of the lens with a lot scratches.
Now, if they are on the rear element.... Pass on it. that is where the image is focused with, and that will cause problems. The Front Element is more for light gathering, the internal elements guide and sharpen the image to the back group, that focus's the image on the film plane.