On Thu, 25 Aug 2005, Steve Jolly wrote: > It's very very slight. I could only deduce it by looking at the > reflection of a light fitting, as I described. At the very least the > filter has to affect the focussing of the lens slightly, since its > increased optical depth wrt the air it replaces will marginally > increase the backfocus.
Steve, Or maybe it's concave to make sure it *doesn't* have any effect? If a flat sheet of glass (re Mike Wilson's suggestion) has the effect of changing the angle of refraction noticeably at higher angles of incidence (and this *is* a wide angle lens) then it would make sense to offer up a concave (diverging) lens to counter the converging effect of diffraction? :-) Chris
