The Tiffen 6.6x6.6" Circular Ultra Polarizing Filter is a wonderful filter designed to bring significantly more "snap" to your images. If a traditional polarizer doesn't give you enough "zip", the "Ultra" polarizer gives you a little more. It will bump up your blue skies even more - making the difference between a good photo and a great image.
Light rays which are reflected become polarized. Polarizing filters are used to select which light rays enter your camera lens. They can remove unwanted reflections from non-metallic surfaces such as water or glass and also saturate colors providing better contrast. The effect can be seen through the viewfinder and changed by rotating the filter. The filter factor varies according to how the filter is rotated and its orientation to the sun.
Circular polarizers are specifically designed for use with auto-focus SLR cameras (They will also work on manual systems and video cameras without problems).