The aurora cr filter set comes with a cr filter holder and four cr format nd filters in the rating of nd 8 (3 stops), nd64 (6 stops), nd 4000 (12 stops), and nd 65000 (16 stops). Rear mount filters are much smaller than front mount filter that typically goes up to 185mm or larger for some canon ef wide angle lenses. Gel filters for the original canon ef lenses are optically inferior with significant resolution deflation and strong color cast. By replacing the gel filter holder with the aurora cr filter holder made from cnc aluminum alloy, gorilla glass based glass nd filters can be used. Multi-layer Nano coatings are applied to filters for both light reduction and surface protection. Canon lenses that can use the cr filters include: Ef 8-15mm f/4l fisheye ef 11-24mm f4l usm ef 14mm f/2.8l usm ef 15mm f/2.8 fisheye ef 16-35mm f/2.8l usm ef 17-35mm f/2.8l usm ef 17-40mm f/4l usm.