All valid point brought up, good discussion everyone. I remember some of my old lens hoods. There was a rubber one that popped forward and then could be collapsed back. Another was a metal sheath variety with 3 telescoping elements. All were perfectly round and I never noticed vignetting, though I might if I go back and look today. I always leave mine on unless I am doing close-up work.

Kort Kramer - Kramer Kreations