A variable ND filter is a photographic filter whose main purpose is to reduce the amount of light reaching the camera’s sensor. The abbreviation “ND” stands for “neutral density,” meaning that the filter reduces light evenly across the entire color spectrum and thus does not alter the colors in the resulting photograph. Unlike traditional ND filters with a fixed density value, such as ND8 or ND64, the variable version allows you to smoothly adjust the level of darkness by rotating the front part of the filter. It works on the principle of two polarizing filters, whose mutual rotation regulates light transmission. For the photographer, a single filter thus replaces an entire set of fixed ND filters and allows for flexible use of long exposure times to blur motion, such as flowing water, or to use low f-stops for a shallow depth of field even in very bright light, without overexposing the image.