Medium-gray density mirrors are also called dimming mirrors, which are used to reduce the amount of light entering to achieve special exposure effects. In many cases, due to the intensity of the external light, we cannot get a long enough shutter time and cannot achieve our own photography intentions.
For example, when shooting water in the field, we hope to use a one-second shutter to make the water present a soft cotton-like texture. However, due to the strong light, even when shooting with the camera''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''s minimum aperture and minimum iso, the shutter speed is still around 1/30 second. The solution at this time is to use one or even two nd8 lenses to attenuate the light entering the lens.