What is a neutral density filter?
Neutral density (ND) filters are simply filters which block a certain percentage of light from passing through. In other words, they re darkening filters. The neutral refers to the fact that a proper ND filter does not colour the light inadvertently. (ie: a true neutral density filter does not introduce any colour casts to the image) Such filters are useful for, for example, shooting outdoors in bright sunlight when you happen to have fast film. They re also handy for extending shutter times. For instance, nature photographers often like shooting waterfalls or moving water with very long shutter times in order to achieve the bridal veil effect of blurring motion. To do this with ordinary film requires an ND filter to cut back the amount of light hitting the film. Users of mirror lenses also use ND filters in the unusual case that they need to cut back on light, since mirror lenses lack adjustable aperture diaphragms. ND filters are typically specified in either decimal values or numeri
