Functionality Grey filters, also referred to as ND filters (neutral-density filters), reduce the entire visible light spectrum evenly throughout the entire filter. Colours and contrast are not affected by this. Reducing the light can have several different useful applications. Increasing the exposure time can produce flowing effects with moving water, or enables shots to be taken with a wide aperture and a long exposure, even in bright conditions. The grey filter used here with factor 4 increases the exposure time / aperture width by 2 stops.