Neutral density or ND filters have the sole purpose of cutting down the amount of light as a whole. A perfect neutral density filter would transmit all wavelengths of light equally, so there would be no color change. Though not actually perfect, it can be used equally well in both color photography or black and white photography.
It is made in a wide variety of densities. Depending on the manufacturer, there are currently two rating systems commonly used to designate the density of neutral density filters.
- A straightforward system of whole numbers whereby an ND-1 indicates a need for a 1-stop exposure increase; ND-2 is two stops; ND-3 is three stops; and so on.
- In the other, which is scientifically oriented, the decimal number .3 indicates a 1-stop exposure increase need; .6 is for two stops; .9 is for three stops; and so on. [read]