ND200 absorptive neutral density filters have an optical density of 2.0, transmitting an average of 1% of the visible spectrum. These filters include identical male and female threads on either end so that they can be stacked with other ND filters to achieve various optical densities. Stacking two ND200 filters together, would be nearly equivalent to a ND400 filter; i.e., visible light transmission would be approximately equal to 0.01%.