When photographing subjects which are backlit against a bright blue sky, the exposure required for the subject can often result in the sky being somewhat overexposed and washed out.
|Sometimes this effect can be negated by using a polarizing filter (also known as a polarizer) to darken the sky and even out the exposure. A simple linear polarizer works fine with most compact digital cameras and manual-focusing SLRs, but DSLRs require a slightly more expensive circular polarizer.
In the first set of images, you can see the effect of using a polarizer on the image of a tree against a bright sky. The effective exposure was the same in both cases, but the polarizer has darkened the sky in the shot on the right, while not affecting the brightness of the tree.
One thing to watch out for is an uneven darkening of the sky when shooting with a wide-angle lens. The sky is not uniformly polarized. Maximum polarization occurs at 90 degrees to the sun and if the field of view is wide enough, the effect shown in the second set of images (shot with an 18mm 18-55mm lens set at 18mm on a Canon EOS 40D) can be seen. In this case the polarization was greater higher in the sky, resulting in an uneven, overly dramatic transition of dark to light blue.