I am guessing that this focus shift becomes much more exaggerated with smaller formats and shorter lenses? If I don't have the rear filter in place on the back of my 15mm nikkor, it will NOT focus. I learned that the hard way. r andrew fildes wrote: > > 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?
