The Cokin B1 hard-edge graduated blue is a 1 2/3-stop blue filter can be used to enhance dull, flat, or hazy skies. The filter is densest at the edge and graduates to clear at the middle, with a hard-edged line of transition. Hard-edge filters are great for images with a defined horizon line. The filter is Rectangle so you can selectively choose where the blue Edge sits in the framing of the image.