Neutral Density (ND) filters are designed to reduce transmission evenly across a portion of a specific spectrum. They are typically defined by their Optical Density (OD) which describes the amount of energy blocked by the filter. A high optical density value indicates very low transmission, and low optical density indicates high transmission. ND filters can be stacked to achieve a custom optical density. To calculate the final system OD, simply add the OD of each filter together.
