Digital Camera Neutral Density Filters (ND)

They are gray filters that only reduce the intensity of light without altering the color. They are used to reduce the intensity when we can't or don't wish to modify the aperture and speed values. In principle, we need a ND filter when we have to face a large intensity of light which is excessive even for a highest exposure value available on our camera. When using very high sensitivity emulsions in sunny exteriors we can find ourselves in this situation. To avoid overexposure it's necessary to limit even more the intensity of light that reaches the film, which can be done easily with a ND filter of the necessary factor.

ND filters are also used with catadioptric lenses, which don't have a diaphragm, and in conditions of great intensity, in principal we only have the shutter speed to influence the exposure level. In this type of lenses, the ND filter is usually mounted on the back part of the lens and not on the front, which is a much larger diameter.

The use of a ND filter is not only limited to eliminating the excess of absolute intensity, but it becomes necessary when we want to use a large aperture and/or a slow speed. If the aperture we want to select, for depth of field reasons, is inaccessible for the exposure value we are (it can't be combined with a speed in the range we have) or if the exposure time has to be necessarily long, incompatible with the necessary EV, it is the necessary to reduce the intensity with a ND filter.