Midwest Optical offers two types of Neutral Density Filters: the ND series are filters that use absorptive filter glass, and the Ni series are filters which partially reflect a portion of the light entering the lens. The filters are defined according to their optical density (OD). For example, ND060 is an ND Filter with an OD of 0.60 (the higher the OD, the lower the transmission). The filters have male and female threading, so they can be stacked to test various optical densities. For example, an ND030 Filter mounted together with an ND120 Filter, will result in an ND150 Filter.
