The reason for photography is: how should ND (reduction mirror) be used?

ND mirror is a subtractive mirror, also called grey mirror, which can reduce the light input without affecting other effects. Generally used for daytime shooting, the aperture is larger in high brightness environment, and the shutter speed cannot be further improved. For example, if you shoot a waterfall, you can shoot a smooth splash with a slow shutter. The aperture is expected to be the best around F8, so you need the cooperation of nd mirror.