The Tiffen 6.6 x 6.6" Hard Edge Graduated 0.9 ND Filter is a graduated neutral density filter that helps to specifically 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 hard-edged line of transition between the density and clear areas. Neutral density filters do not affect the coloration of the image and the 0.9 density provides a 3 stop reduction in light from entering the lens.
This filter is constructed from Water White glass and 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. This filter measures 6.6 x 6.6" / 167.6 x 167.6mm.