Calumet 52mm Circular Polariser MC Filter High Quality German SCHOTT Optical Glass is not only an extraordinarily effective multiple 6 layer coating, it is also harder than glass, so that it protects filters from scratches, and it is also water and dirt repellent, thus facilitating filter maintenance. Each Digital MC filter has a resilient final coating that is low-reflective, scratch-resistant, and dirt/water-repellant.