Hoya 55mm Neutral Density ND-400 X, 9 Stop Multi-Coated Glass Filter
Photographing solar eclipses and ultra-bright light sources can be extremely dangerous. This filter reduces light values by 9 stops to less than 1/500th of its original intensity and allows safe photography.
It can also be used to achieve super slow shutter speeds in daylight to render moving subjects invisible, and will enable you to use slow shutter speeds, especially with high speed films, to record movement in subjects such as waterfalls, clouds, vehicles, etc.
It will also decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background. And this filter allows you to decrease the effective ISO of high speed film (above ISO 400) so it can be used outdoors in bright situations.
Neutral Density filters have no effect on color balance, appear grey and reduce the amount of light reaching the film. Multi-coating minimizes reflection at the filter surfaces which reduces flare as well as ghosting.