A graduated neutral-density filter, also known as a graduated ND filter, split neutral-density filter, or just a graduated filter, is an optical filter that has a variable light transmission.
Examples of this use include:
- Blurring water motion (e.g. waterfalls, rivers, oceans).
- Reducing depth of field in very bright light (e.g. daylight).
- When using a flash on a camera with a focal-plane shutter, exposure time is limited to the maximum speed—often 1/250th of a second, at best—at which the entire film or sensor is exposed to light at one instant. Without an ND filter this can result in the need to use f8 or higher.
- Using a wider aperture to stay below the diffraction limit.
- Reduce the visibility of moving objects
- Add motion blur to subjects
- Extended time exposures
- In conditions of extreme light intensity, such as sunshine on snowy mountains or on the beach, or when using a camcorder, ND (Neutral Density) filters are recommended as essential.Basic ND filters are used for as below.
- To enable slow shutter speeds to be used, especially with fast films, to record movement in subjects such as waterfalls, clouds, cars, seas etc.
- To decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background.
- To decrease the effective ISO of high speed (ie: above ISO400) and allow it to be used outdoors in blight situations.
To allow DSLR and video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes which would normally cause over-exposure.