When the 72mm Center Filter from Schneider for the 38mm F/5.6 Super Angulon XL lens is used, the rear element is usually so close to the film plane or digital sensor, that light rays do not have room to spread out evenly. This center filter is a finely crafted neutral density filter that is most dense at its center. Its light blocking capability thins out towards the outer edge, thus allowing for a more even exposure from center to edges.
This filter has an 82mm front thread, but normal 82mm filters will cause vignetting if used with this filter. You'll need to use the "extra-wide" 82mm type filters, which are essentially 95mm or 100mm filters built into an 82mm-to-95mm or 82mm-to-100mm step-up ring.