The Tiffen 4 x 4" 4 Cool Blue Soft-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 soft-edged transition between the density and clear areas. The 4 color density provides less cool blue coloration than the 5 density, but more than the 3 density filter.
This filter is made using ColorCore technology, a process that involves laminating the filter substrate between 2 pieces of optical glass and grinding flat to a tolerance of 1/10,000th of an inch. It measures 4 x 4" / 101.6 x 101.6mm.