In conditions of extreme light intensity, such as sunshine on snowy mountains or on the beach, or when using a cine/video camera, neutral density (ND) filters are an essential addition.
Neutral Density (ND) filters are often ignored by photographers, but they have several uses and offer the possibility to achieve otherwise impossible results. ND filters appear a neutral grey and reduce the amount of light reaching the sensor. They have no affect on color balance. They are available in 1-stop (NDx2), 2-stop (NDx4) and 3-stop (NDx4) light reducing grades in a variety of common sizes.
They have four main uses:
1) To enable slower shutter speeds to be used, especially with fast films, to record movement in subjects such as waterfalls, clouds, cars, seas, etc...
2) To decrease depth-of-field by allowing wider apertures to be used, helping separate subjects from their background.
3) To decrease the effective ISO of high speed film (e.g.: above ISO400) and allow it to be used outdoors in bright situations.
4) To allow cine and video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes that would normally cause over-exposure.