Horseman 95mm Center Filter for Super Angulon 90mm Lens Unit

One of the caveats of using ultra-wide-angle lenses is the issue of light fall-off in corners, and uneven illumination across the film plane. Horseman Center Filters are graduated Neutral Density (ND) filters, where the gradation is applied in a circular pattern radiating from the center of the filter, instead of a linear fashion, as in traditional graduated ND filters. This effectively balances exposure, darkening the center and allowing the corners to receive an equal amount of light. This center filter has 95mm threads to affix to a Horseman 90mm f/5.6 lens unit for the SW-617 series cameras.