[Filligree]

There's a less computational explanation for that. Your glasses sit very close to your face, meaning that any scratches are dramatically out of focus; the degradation gets spread over your entire visual field.

Which is not to say that you wouldn't have gotten used to and started ignoring a scratch you could see, because you're right; you would have. But that's not what's actually going on.

[mikeash]

How do you know that's not what's actually going on? You may be different, but when I wear glasses, they're sufficiently far from my eyes that imperfections are quite visible as localized phenomena. They're not in focus, but they're nowhere close to spread out over the entire visual field. Specks of dust show up as blobs of dust. Scratches show up as fuzzy lines.

I'm going to guess that the person you're replying to is sufficiently familiar with his glasses to know what he actually sees.