Circular ND filters provide continuously variable, linear attenuation of incident light by rotating the filter about its center. The transmitted intensity is varied as a function of the optical density range. An inexpensive, ergonomic hub assembly holds the glass filter securely to the mount block. The Delrin® hub ensures a protective surface against the filter. A convenient ¼-20 tapped hole in the aluminum mount block permits a variety of mounting configurations. The filter is rotated manually by turning along the outside edge, front knob, or rear turning screw. A 4" long, 0.5" diameter stainless steel post is included.