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ND is an abbreviation for "Neutral Density", which means they have no effect on color but reduce the overall amount of light. These filters are of course handy for reducing the overall light level when it exceeds the AE range for high-sensitivity film, but they are also invaluable in other cases, such as when you want to use a lower shutter speed or a wider aperture. The ND400 is recommended for shooting the sun.
