Durable Construction The filters are constructed from German SCHOTT optical glass that has been ground and polished multiple times for a low refractive index to deliver accurate color reproduction. They're protected from damage with a waterproof, oil-resistant, and anti-scratch coating. Double-Sided Coating The filter is protected on both sides by its multilayered coating, and the optical glass contains two additional coatings, one to help reduce reflections without color shifts, and another to help absorb red light. Additional Features Lightweight aviation aluminum frame Nonslip, quick release design