Any experts on front of lens filter diffraction? I recently tried using a combination of filters on the same print when enlarging b&w negs, which to my surprise created a double image. Strangely enough I never noticed this when using a low contrast filter, only a high contrast filter (possibly because the double image stands out more..?).
Should I be placing the filter behind the lens (between the light source and the lens) instead of in-front of the lens (between the lens and the paper)?