The canon 52mm drop in circular polariser allows you to add filters to your canons super telephoto lenses ie canon ef 300mm f28 l is usm lens canon ef 400mm f28 l is usm lens canon ef 500mm f4 l is usm lens and canon ef 600mm f4 l is ii usm lens the canon drop in polarising filter has a screw on its external side top that allows you to rotate the circular polariser for specific lighting condition