ND neutral density filters are designed to reduce the amount of light coming through the lens by 4 stops. This allows you to significantly lengthen shutter speeds, experiment with motion blur and soften the motion of flowing water or clouds. It also makes moving persons or vehicles vanish from the scene when shooting with longer exposure times. Alternatively, these filters allow you to shoot at wide open aperture in strong lighting conditions when otherwise the camera's fastest shutter speed would be exceeded resulting in an overexposed image.