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 58mm threads to affix to a Horseman 45mm f/4.5 or 65mm f/4.5 lens unit for the SW series cameras.
