Is it a coated or uncoated lens? I have a 50mm f1.5 Xenar which looks like someone cleaned it with sandpaper, and a big black fleck of paint internally. The latter is no big deal at all. The former is so bad that it makes it a soft focus lens and little else (not that it was sharp focusing to begin with). Depending on how bad, scratches will cause a loss of contrast, maybe some loss of sharpness, maybe a bit of flare (see loss of contrast above). I'd suggest, as others already have, just leaving it alone.