 What single filter dramatically darkens blue sky, removes glare and reflections, saturates color, and acts as a neutral-density filter? The polarizer!
Without going into depth about the physics of light, polarization occurs in open blue sky and wherever there are reflections off shiny surfaces (but not mirrors). By rotating the filter, you can progressively prevent the polarized components of light from passing through (thereby darkening the sky or removing the reflections), while allowing normal light to pass virtually unaffected. The filter acts as a 2X neutral-density filter at its minimum and progresses to 4X ND, depending on the amount of rotation and polarized scene components. Stacking and rotating two polarizers together can give you a variable ND filter that blocks out nearly all light. For darkening the sky with a single filter, the maximum effect occurs in areas of clear (not overcast) sky at right angles (90 degrees) to the light coming from the sun.
We've diagrammed a simple rule of thumb (at right) for determining the maximum sky polarization. Point your thumb in the direction of the sun. Now, the area of greatest effect will be wherever you aim your forefinger like a gun (at 90 degrees) while twisting your wrist side to side. When it comes to reflections, the polarizing filter works best on light that passes through the filter at an optimal angle of 30 to 40 degrees from the reflection surface (see drawing). Most colored objects, plants, and even skin, reflect and polarize light to some degree. Removing these polarized reflections increases the purity of an object's surface color while allowing you to see what's behind glass or water reflections. When using a polarizer in this way, compose your picture first, then choose an angle that will give you the desired polarization effect.
If you want the darkest results from polarization, you should lock exposure before rotating the filter to maximum effect, then expose. Beware of certain zoom lenses that inadvertently rotate the outer lens element (and the filter) as you zoom. With these lenses, zoom first, then manually rotate the polarizing filter.
There are two types of polarizers: linear and circular. Circular polarizers are required for many modern autofocus SLRs for optical reasons too complex to explain here. If you are unsure of the correct type for your camera, buy one marked "circular." It's somewhat more expensive but it works on any camera.
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