Now this explains something I always wondered about - why lenses with behind the lens filters provide a 'normal' filter for when you 'aren't using a filter.' Seems to be standard for both the behind and between types (like the Sigma 16mm fisheye I once had which 'broke open' to fit filters). The only explanation I'd seen was that 'the lens is optimised for having a filter in place, so...' If one used a Nikon 39mm NC (No Change) in a rear mount telephoto does the effect still occur - that is, is it a function of the glass itself rather that the filtration? Why would it occur if one is focusing through the lens? (Incidentally, if anyone needs to know, I use the Nikon 39's on my Leica CL Summicron-C and Elmar-C lenses - they're the only ones that will fit the 39mm thread on those two) AndrewF > I would not advise the use of inside, behind-the lens filters, >specially for neophytes. It will result in out of focus negatives. l >Most photographers will never notice, and the rare few that do will >attribute the degradation to the filter, but this is not the case. > > Almost no one realizes that using a filter behind the lens throws >the image out of focus. When using behind-the-lens filters, focusing >the camera is nearly impossible with these in place, so people >"logically" focus, then insert the filter -- throwing the image out >of focus -- and then make the exposure. > > As Father Ansel clealy states in the Biblel...oops, I mean The >Negative, one must compensate for this focus shift by focusing >backwards 2/3 the thickness of the filter material. Who knows >how much that is ? Who can move the focusing knob precisely that >amount ? No one I know of. For all the trouble one goes to for >sharpness with large format, I recommend keeping the filter up front, >and buying a decent shade for it. > > --- Luis > > >
