Thorlabs offers both continuously variable and step variable ND filters. Round filters are available unmounted or mounted. The mounted versions allow the appropriate optical density to be quickly set. The continuously variable round filters can also be purchased in a 30 mm cage-compatible version. Our rectangular variable ND filters are available in one of two sizes and a number of optical density ranges. Additionally, we offer an adjustable, absorptive ND filter in a post-mountable, 30 mm cage-compatible housing.