Variable 0.9 to 2.1 Optical Density, 28-Layers of Nano-Coating, 1-Second-Swap Magnetic Filter, Avoids Common. This 3- to 7-stop 77mm Nano-X Magnetic Variable ND Filter from K&F Concept allows you to quickly install the filter adapter ring on the lens thanks to its magnetic properties. The filter utilizes cutting-edge technology to avoid the common X cross on a wide-angle and telephoto lens.