The 55mm Circular Polarizer II Filter from Nikon is an upgraded version of the original Nikon Circular Polarizer. This filter is now slimmer in design, leaving the front and rear threads the same size as opposed to the original version that featured a larger front thread. The filter is now multi-coated which is a benefit in that it minimizes any flare and glare that reaches the sensor or film plane of your camera. This means more saturation and clearer images that are more crisp and contrasty.
The front ring rotates with the polarization visible in the viewfinder or on your LCD screen. When shooting on a bright day, this means you can control the "blueness" of the sky as well as having control of how distinct you want the cloud formations. For the outdoor shooter, this filter is a must since it is the only way to make a blue sky bluer. In addition, when shooting into water or glass, you can minimize or eliminate the reflections often associated with landscape or scenic photography. Therefore, if you're shooting a store window or inside a museum into a showcase, glass obstructions will no longer be a problem. A polarizer tends to have its maximum effect when used at right angle to the sun. A polarizing filter, along with a UV filter should be a part of everyone's gear.
Note: Circular polarizers are specifically designed for use with auto-focus SLR cameras (They will also work on manual systems without problems)