The Cokin Hard-Edge Graduated Fluorescent Blue 1 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. The Fluorescent Blue 1 is lighter (1 2/3-stop) and more subtle than the Fluorescent Blue 2 (2 2/3-stop).