Filming & Production
ND filter
Also called: neutral density filter
Video looks natural at a shutter speed around double the frame rate, so 1/50 at 25fps. Outdoors in daylight that combination overexposes badly unless you close the aperture right down, which throws away the shallow depth of field you probably wanted. An ND filter fixes it by darkening everything evenly.
Fixed NDs come in strengths measured in stops. Variable NDs twist to adjust, at the risk of a dark cross pattern at the extreme end. Cheap glass tints the image green or magenta, which costs you time in the grade. Some cameras carry internal NDs.