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Essential Filters: Polarizers
The polarizer is simultaneously one of the most useful as well as one of the most annoying filters around. Read on for the full scoop.
Whenever light is reflected, the naturally random orientation of light rays becomes much more ordered, or polarized. A polarizer can limit the light that passes through it to only those rays with a  selected orientation by filtering others out. As such, it can reduce or eliminate reflections from non-metallic surfaces like water or shiny leaves. It can also increase color saturation and make the white of clouds and snow-covered mountains stand out more by darkening the blue of the sky. It can cut through some of the late-afternoon atmospheric haze, increasing contrast and making things appear clearer. Rainbows can also benefit from a polarizer by darkening the sky and increasing the saturation of the rainbow's colors. In a reverse twist, a polarizer can also increase reflections if set to block out light from some other predominant direction. A good, basic polarizer should be neutral in color so as to not add an unintended color cast to an image. A polarizer can also be used in a pinch as a simple neutral density filter; at maximum polarization, it cuts nearly two stops of light.
There are two basic types of polarizers: linear and circular. You need a circular polarizer to avoid confusing the meter in auto-focus cameras, whereas either can be used on a manual-focus camera. AF cameras contain a semi-silvered mirror that splits the incoming light to enable calculating both focus distance and exposure. The polarized output of a linear polarizer can play havoc with this splitter and thus the camera's metering system. A circular polarizer is the same thing as linear with the addition of something known as a "quarter-wave retarder" that has the effect of scrambling the output of the polarizer to avoid this problem. You should consider getting a circular polarizer regardless of the type of camera you have. Like all quality filters, a polarizer will last a long time, and even if you have a manual-focus camera now, an upgrade to AF may be in your future.
 Regardless of how a polarizer actually works, using one is really quite simple: just rotate it until you see what you like. You will find that the effect is most pronounced at a 90 degree angle to the sun. If facing directly into or away from the sun, you will see little effect from a polarizer. If using a wide angle lens, keep in mind that the angle of view will be great enough to make one side of your photo more polarized (and thus darker) than the other side since the angle to the sun will vary across the frame. Also, as you should when using any filter with wide-angle lenses, you should watch out for vignetting. For metering, keep in mind that if you should set your exposure after adjusting the filter since a polarizer can eat a lot of light.
 A few companies now make a "warming polarizer" that combines the subject of this week's tip with that of next week's (hint: that'd be a warming filter). This can be a great convenience but should be considered a luxury to be purchased only after you have the other essential filters. The somewhat famous "Moose Filter" marketed by Moose Peterson and THK/Hoya is such a filter, as is Singh-Ray's "Warming Polarizer PLUS." B+W, Heliopan and others have also joined the pack with their own versions of warming polarizers.
Now then, with all the good things I've said about polarizers and what they can do for you, where's the annoying bit I alluded to in the opening paragraph come in? The problem with polarizers is that they are too darned useful and easy to use, and thus too darned popular, and thus tend to be over-used. I've seen people that leave a polarizer on their lens all the time, much as others do with UV or skylight filters. Don't make this mistake. Use a polarizer when it makes sense to, and avoid it when it's not needed or would look to obvious or contrived. A polarizer is indeed essential since there's nothing else that does what it does, but there is such a thing as too much of a good thing. Especially when using Velvia and other super-saturated films, try to keep your polarizing tendencies under control.
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