The Tiffen 6.6 x 6.6" 5 Cool Blue Hard-Edge Graduated Filter helps to specifically add a cool, purplish-blue coloration to skies and other bright areas of an image while allowing a portion of the image to remain neutrally colored. The filter's coloring is densest at the edge and tapers to clear by the middle, with a hard-edged transition between the density and clear areas. The 5 color density provides more cool blue coloration than the 4 density filter.