A neutral density filter is a colourless, optically grey filter designed to reduce the intensity of light passing through a camera lens uniformly across the entire visible spectrum, decreasing the amount of light reaching the film or image sensor without introducing any colour cast, shift in colour balance, or selective alteration of any particular wavelength of light. Unlike coloured filters, which achieve their effects by selectively absorbing certain wavelengths while transmitting others, a neutral density filter absorbs all wavelengths of visible light equally, acting as a spectrally neutral attenuator that simply reduces overall brightness while leaving the colour characteristics of the transmitted light unchanged.
The primary practical value of a neutral density filter lies in its ability to decouple the relationship between the available light level and the combination of aperture and shutter speed settings required to achieve a correct exposure, giving the photographer creative freedom to use aperture and shutter speed combinations that would otherwise result in overexposure in bright lighting conditions. Without a neutral density filter, photographing in bright sunlight with a slow ISO film or sensor setting limits the photographer to relatively fast shutter speeds and small apertures to avoid overexposure - constraints that may conflict with the creative intentions of the shot. By placing an ND filter in front of the lens, the photographer can reduce the effective light level reaching the sensor by a known amount, freeing them to use a wider aperture for reduced depth of field and subject isolation, or a longer shutter speed for deliberate motion blur effects, without overexposing the image.
Neutral density filters are available in a range of densities, each reducing the light transmission by a specific amount expressed either as an optical density value, an ND number, or in stops of light reduction. Common ND filter strengths include ND2 or ND0.3, which reduces light by one stop; ND4 or ND0.6, which reduces light by two stops; ND8 or ND0.9, which reduces light by three stops; and stronger filters such as ND64, ND400, ND1000, and beyond, which reduce light by six, eight, ten, or more stops respectively. The strongest ND filters - sometimes marketed as big stopper or extreme ND filters - reduce light so dramatically that exposures of many seconds or minutes become possible in normal daylight conditions, enabling the distinctive creative effects of extreme long exposure photography including the silky smooth rendering of moving water, the ghostly blurring of moving clouds into smooth sweeping forms, and the complete disappearance of moving people and vehicles from busy urban scenes.
Graduated neutral density filters, commonly abbreviated to ND grad or GND filters, are a closely related variant in which the filter is clear on one half and progressively darkens to a neutral density on the other half, with a gradual or hard edged transition zone between the two areas. These filters are widely used in landscape photography to balance the exposure between a bright sky and a darker foreground in a single frame, reducing the brightness of the sky to bring it within the dynamic range of the film or sensor without darkening the foreground, a correction that is particularly valuable in scenes where the luminance difference between sky and land is too great for the recording medium to capture detail in both simultaneously without either burning out the sky or underexposing the foreground.
Variable neutral density filters offer an adjustable density range within a single filter, achieved by rotating one polarising layer relative to another to vary the degree of light attenuation continuously between a minimum and maximum density. This adjustability makes variable ND filters particularly convenient for videographers and photographers who need to fine tune their light reduction quickly and precisely in response to changing light levels, avoiding the need to carry and swap multiple fixed density filters.