ND (Neutral Density = reduction of light) filter has an effect to reduce the amount of light entering the lens.

Even under the condition of high level of light such as outdoors in daytime, by mounting an ND filter, it becomes possible to make an aperture an open state when photographing a portrait and to photograph a quickly moving object with a slow shutter speed.

In a streetscape with many people, it is possible to eliminate people by a long time exposure using an ND filter having a strong effect.