Originally posted by deanallan If a scratch leaves shadows on the lens as shown on OP's attached picture then logic dictates that light has been blocked over the scratched area. Am I wrong? Or am I right? By that can we assume that optical properties of the lens has been compromised?
The issue is whether there is any noticeable difference in photos taken. If you compare the area blocked by the scratches with the total area of the lens, the proportion is tiny. Probably less than 1%. So the effect probably won't be visible to the naked eye. Where-as a UV filter might block 3% of the light, across the entire area, so will have more of an effect.
(3% was for the only filter I could find that gave a figure. I couldn't find transmission figures for the B&W filters you prefer.)
Quote: Would you rather lose a $50 filter or a $500 lens?
You should compare the certainty of the $50 cost against the possibility of the $500 cost. If you have 10 lenses, you can use the money saved by not buying 10 filters to replace a lens if it does get scratched. In practice, scratches are rare. And as argued above, a scratched lens is still usable, and should still have some resale value if you feel you must replace it.