The Cokin WA (wide angle) blue center spot filter has a clear center and diffused blue edges. These Diffused edges will be produce blur while leaving the center sharp. The filter effect is directly influenced by the f-stop and the focal length. A lens shot at f/1.4 will see a greater blurring effect than f/8.0 and a 85mm lens will see more blur than a 28mm. Additionally, a longer focal length Lens will visually increase the size of the center spot area because it sees less of the filter area.