This allows motion blur to be achieved with a longer exposure time than would otherwise be permitted by the brightness of the illumination and the film speed or sensor sensitivity. Examples are images of running water or smear effects with panning camera for car, motor bike or horse races. Alternatively such a filter provides less depth of field with a larger aperture, e.g. for focusing portraits in front of a blurred background, even with lenses with a shorter focal lengths than that of telephoto lenses. Another application is the light reduction in situations where the brightness of the motif is too high for a correct exposure with the shortest exposure time available at the given film speed or sensor sensitivity. For instance images of snow or desserts in bright sunlight. As the effect of ND filters cannot be replaced by digital imaging software at a later date, these filters offer many advantages also for digital photography.
ND filters must not be used as solar filters for observing the sun! For this purpose special solar filters are necessary with a much stronger absorption power not only within the visual range but also in the even more dangerous infrared range (which can burn the retina!).