Sure, diffraction effect of blackening would be same as stopping down a very tiny amount more. No where near the bad effect of an untreated scratch or crack or worse. JCO
-----Original Message----- From: Fred [mailto:[EMAIL PROTECTED] Sent: Thursday, July 29, 2004 12:03 AM To: [EMAIL PROTECTED] Subject: Re: Test Results was - Scratch on rear element > When you fill in a scratch with black ANYWHERE on ANY LENS SURFACE you > are in effect "stopping down" a very slight amount, the only side > effect is slower lens speed but still full quality. IF you leave > scratched or filled clear you are creating an improper lens suface > which will bend light rays in the wrong direction(s) which is far > worse than the slight "stopping down" effect of blackening it out. Well, yes and no, I think. While an untreated scratch will cause incorrect diffraction (and also may cause an increase in flare), a blacked-out scratch must also cause a small increase in diffraction effects. (Still, I would prefer treating the scratch - I've done this on two user/beater lenses, and I've also done so on the scratched mirror of a Schmidt-Newtonian telescope.) Fred
