A polarizing filter is a photographic filter that mutes or blocks polarized light. It works similar to polarized sunglasses. In photography, polarizing filters are used to darken skies, which allows photographers to increase contrast in the sky and normalize light levels between the sky and ground, and to reduce reflections on water, glass and other reflective surfaces. While the effects of many filter types--such as color correction, color subtraction and neutral density filters--can be simulated with digital photo editing software, polarizing filters produce optical effects that cannot be digitally replicated.
Purchase a polarizing filter that matches the diameter of the lens with which you intend to use it. Wide angle lens and particularly fish eye lenses have substantially wider diameters than other types. These lenses need wider and often much more expensive filters to avoid vignetting (dark corners).
Screw on the polarizing filter to the end of your lens. The threads of most lenses and filters are standardized, but you should ensure that the manufacturer of your filter is compatible with the manufacturer of your lens before purchase.
Check your lens to see if the front element rotates when it focuses. Because polarizing filters are affected by their orientation, a rotating front lens element will alter the polarization effect. If your front lens element rotates when focused, switch your lens to manual focus to lock it in place and prevent refocusing.
Rotate the filter in front of the lens until you reach the desired effect. There is no rule governing the rotation of polarizing filters, so continue to rotate the filter until you find the most visually pleasing orientation.
Use your on-camera image preview after taking a few sample shots. Your viewfinder will be much darker with a polarizing filter in place, so it may be difficult to make out the changes when you rotate your filter. The preview screen will give you a better idea of the effect of different filter orientations.