> fra: mike wilson <[EMAIL PROTECTED]> > > > > From: Chris Stoddart <[EMAIL PROTECTED]> > > > > 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 > > Either way, the filter would be a _neccessity_ in the light path to form a > sharp image. I have a 300/2.8 with rear filters. The manual says that a > filter _must_ be in place at all times. > > This is all very interesting. >
Remember that with a perfect filter having two parallel plane surfaces the light beams are only shifted slightly in the radial direction without changing directions relative to the axis. In front of the lens this does not make any significant difference (a very small change in the closest focusing distance) but in the back this shift may result in problems, for example by loosing the ability to focus at infinity. DagT
