In photography and cinematography, a neutral density filter or ND filter is a filter that reduces or modifies the intensity of all wavelengths or colors of light equally, giving no changes in hue of color rendition. A ND filter should be one of the main accessorizes that should be on your list of "MUST PURCHASE" items. The purpose of a standard photographic neutral-density filter is to refuse unwanted amount of light that should not be present before a image is taken. Let me show you a chart when you can understand and how ND filter work. Keep this in mind that 90% of the time when using a neutral density filter will be in outdoor conditions. Rarely used in indoor conditions.