How to use gray-scale mirror in photography?

Gray-scale mirror, namely subtractive mirror aND nd lens.

If the camera is compared to the eyes, then the gray-scale mirror is equivalent to the function of sunglasses, which can reduce the brightness when the light is too strong.

Just screw it on the lens in the same way as all filters.

Generally used in situations that require long exposure.

For example, if you want to shoot water with exposure time above 1 s, but the environment is too bright, even if the sensitivity is adjusted to the lowest, the aperture is already very small, and the shutter of 1 s will still be overexposed, so you can only use grey mirror.