These clear/ND graduated filters by Tiffen help to darken skies and other bright areas of the image. The filter is densest at the edge and tapers to clear by the middle, with a soft-edged or hard-edged line of transition between the density and clear areas. Neutral density filters do not affect the coloration of the image.
ND .3 reduces incoming light by 1 stop.
ND .6 reduces incoming light by 2 stops.
ND .9 reduces incoming light by 3 stops.