Neutral density filters reduce the intensity of the light passing through a lens without changing the colour balance of the image. They are available in various densities, usually x2, x4 and x8. A x2 ND filter halves the intensity of the light, and consequently increases the required exposure by one stop. A x4 ND filter reduces the intensity of the light to one quarter, and increases the required exposure by two stops. A x8 ND filter reduces the intensity of the light to one eighth, and increases the required exposure by three stops.
Neutral density filters are used extensively by landscape photographers to achieve longer exposures. Small apertures such as f/11 or f/16 are often chosen so that depth of field extends for a few metres to infinity, so long exposures are normal in this environment. Nevertheless, ND filters may be used to slow shutter speeds even further, for example when blurring moving water into a smooth flow. They are also useful for achieving correct flash exposures when a flash unit is very powerful, is used close to a subject, or when a wide aperture is required.
These filters also have good ultra-violet absorption and may be combined to produce the required change of exposure.
Filter factors are generally in the range 2 x to 10 x.