Grey filters are also referred to as ND = Neutral Density. The filter is color neutral, reducing the amount of light passing through the lens.
Photographers use it, for example, to reduce the depth of field or to lengthen the exposure when photographing motion, out-of-focus effects.
With ND filters, you can try one more nice effect. If you need to photograph a scene without moving objects (people, cars), use a long exposure from a tripod in daylight and you can "erase" the moving objects from the photo.
It is also used when shooting with flash to prevent overexposure. Or in cases where you can't move the aperture or shutter, e.g. for artistic intent.
It is used not only by photographers but also by filmmakers. It allows them to keep aperture and shutter priority settings. In bright daylight, there is no need to increase the aperture number, so you retain depth of field and don't have to worry about setting the time to keep the movement in the video smooth.
Filter specification of the German brand DÖRR, DIGILINE SLIM VARIABLE series:
ND: 2 to 400 (variable, 1 to 8.5 f-stops)
Front thread: Yes
Material: glass/aluminum alloy
Frame: Slim (extra thin)
