The Cokin A121 filter darkens part of the image by 3 divisions of the aperture (ND8). The filter has a harder color transition than its softer counterpart (A121S). It is used most often when we want to align exposure or highlight some part of the frame. Cokin filters are mounted using a holder (holder), which is attached to replaceable mounting adapters, with diameters that can be adapted to your lens. System A (size S) is recommended for use with compact cameras with lens filters or amateur video cameras.