Trying to use a Hoya +4 filter (52mm with a 58 -> 52 step-down ring) on a Sigma 70-300 Super II Macro lens on a Canon 350D
However, i always have problems autofocusing. Image in viewfinder always appear blur (i.e. very soft on the edges.) before i take the shot, thus the camera cannot autofocus based on that image. I understand i must set the smallest aperture available, thus i'll use the depth-of-view preview button and manual focus with it.
Any idea how to use auto-focusing in such a scenario?