I understand what John is talking about. I've worn glasses since High School and when I have sung wearing glasses ('spectacles' for all our UK members) and especially when perspiring (no not because I was nervous necessarily ) but on a hot day, etc., the glasses do have a tendency to slip down on the nose and in theory could possibly pinch the nose a little and constrict the air passages. And if and when that happens your tonal quality could be altered to make you sound like, well; a duck (sort of). I have since switched over to Contacts when Gigging myself because mainly I'm Vain (just kidding of course) "actually I do it because the Stage lights have a tendency to reflect off the lenses of my glasses and it does not look very Professional imo. Anti-Reflective coating on the Lenses will mostly prevent that but Anti-Reflective coating has a tendency to wear off after a while and the coating can actually become smudged to where it starts to be difficult to see through them. And wearing Contacts also will eliminate the possible other scenario that John is talking about. If you do wear Glasses make sure that they have a 'secure' fit to where they wouldn't possibly slide down the nose. It can be embarrassing and you may even lose a Gig contract because the Owner, etc., won't put up with his client sounding like a duck. Just my thoughts.

Best regards,
Mike