As a unit of measurement for exposure, a stop signifies an increase (or decrease) of the amount of light reaching the film by 2. Therefore a one stop increase will allow twice the exposure, and a one stop decrease will signify half as much light is reaching the film. ND filters tend to be rated by their average optical density – 0.3 is 50% – ie. It will reduce the amount of light transmitted by the filter to 50% of its original intensity – a one stop reduction.