Anyone who has ever spent time around bodies of water of any size knows that under certain conditions reflected light can make viewing conditions difficult. Everyday glare can make it hard to see what's on or beneath the water's surface. The same thing happens with other reflective surfaces, including ice and glass. Sometimes you can partially address the problem by changing your position relative to the reflective surface. But this isn't always possible or effective.
As a boy in the 1920s, Edwin Land became fascinated with the problem of blinding glare caused by car headlights of the day. While Land later became best known to the general public for creating the Polaroid camera, his early work was focused on polarization. By the age of 19, Land invented a new material that could filter out stray light, so that light waves reaching the viewer come from a single direction, eliminating glare. While the motor vehicle industry eventually rejected his invention as a costly solution in search of a problem, other industries quickly found a multitude of uses for this revolutionary material: from consumer sunglasses to military goggles to photographic gear. One photographic accessory that grew out of Land's invention is the circular polarizer (CPL) filter. A CPL filter works pretty much like polarizing sunglasses for your camera.
Modern polarizing filters are threaded attachments that screw on the end of your camera lens, and they come in a variety of sizes. A CPL filter operates by rotating a thin ring on the end of the filter, which turns one piece of glass relative to a second piece integrated in the filter. As the filter is turned, tiny polarizing crystals embedded in the glass block stray light waves in different areas of the scene. You can see the effect produced by looking through the viewfinder of your SLR or DSLR camera as you rotate the filter ring. A fuller, more technical description of how CPL filters work can be found here.
Here are some examples of how a CPL filter can eliminate or minimize reflections, shown with and without the filter rotated in position.
The images above were taken within moments of each other under overcast conditions, and have the exact same, basic edits applied to both. Notice that in the top photo the river bed is mostly hidden by the glare, particularly in area closest to the camera position. When the polarizer is rotated to apply its effect to that area of the photo, the glare is significantly reduced. Angling enthusiasts were quick to adopt this technology that could show them where fish congregated underwater.
The reason a polarizing filter is still important to photographers nearly a hundred years after its invention is that the filter actually determines what the camera lens can see. The resulting image is then faithfully recorded on your sensor or film. While you can do amazing things today in Photoshop, there's no easy way to copy details like these digitally back into a photograph. There's no "Glare Compensation" slider control yet in Lightroom. So this is one instance where getting things right in camera is still absolutely essential.
Another vivid example of what polarizing filters can do is seen in the images below. As in the previous examples, the images are taken from the same spot within moments of each other. I even left the stray cigarette butt in the scene for that authentic, outdoor feel.
CPL filters can also be used to darken blue skies for greater contrast. This is one area that I haven't found as useful in my own photography, as the filters tend to darken only a portion of the sky. Moreover, it's trivial to darken blues in Photoshop - or even to drop in a replacement sky from another image when mother nature isn't cooperating with your photography plans. In preparing to write this blog entry, I made some example sky photos with the CPL filter but the resulting images were kind of boring. Part of the issue may be that I'm using an inexpensive "ZEIKOS" CPL filter that doesn't deliver the best sky results. For a better example of what a good CPL filter can do for you, take a look at this image (not mine).
In summary, if you regularly shoot outdoors, I would certainly recommend investing in a CPL filter. There's considerable debate in various photographic forums about which brands offer the best value. You generally can't go wrong with anything made by B+W, although you'll pay more for the higher quality. My recommendation is to read some reviews before dropping money on any filter. Good sources for CPL filters include Amazon, B&H Photo and Adorama.
Also, be sure to note the thread sizes of all your lenses. These will likely vary, but range from about 49mm to 77mm for typical consumer lenses (not to be confused with the lens focal length), and you'll see this value stamped somewhere on the end of the lens. Once you determine the largest lens you're likely to use a CPL filter on, buy that size. You can purchase inexpensive "step up" adapter rings that will let you use your larger filter on smaller lenses. This is far more economical than buying a separate filter for each size that you need.