The Cokin WA (wide angle) Blue Center Spot filter has a clear center and diffused blue edges. These diffused edges will produce blur while leaving the centre 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 centre spot area because it sees less of the filter area.How to use Center Spot ?The shorter the focal length, the more visible the zone. As for the transition from sharp to soft, it depends on the aperture: the wider it is, the more diffuse the transition will be. For a correct adjustment with auto-focus reflex cameras, the choice of an AF central collimator is obviously required. We also recommend working with apertures of /5.6 or /8 in order to keep a sufficiently blurry transitional zone. A strong backlight brightens the effect of the filter.Uncolored Center Spot filters create blur around the central subject in a completely natural-looking way, giving the subject prominence in the image.Grey and Colored Center Spot filters work by blurring and darkening the periphery of the image in a more or less noticeable way. The intensity and the quality of the central subjects lighting are clearly emphasised.Oval Center Spot filters are made for larger or taller subjects.Specs:- Filter Dimensions: 84mm x 84mm x 1.6mm- Filter Construction: C39 Optical Resin- Filter : 077