A circular polarizing filter is the one filter that every photographer should have in their camera bag because the effects of a polarizer cannot be duplicated later in Photoshop. The main use of a circular polarizer is to reduce or eliminate glare or reflected light from non-metal surfaces. Here are several situations where a polarizing filter is helpful.
Water and Color
Polarizing filters reduce the glare off of water, which makes it more transparent. If you want to see rocks or other objects that are underwater and make them a part of the image, a polarizer will do this - as shown in the photo at right. The transparency is increased as you shoot from a higher angle into the water and reduced if you are down low and shooting across the water.
If you are photographing plants or any foliage after a rain, or if plants are in an environment with dew or fog, this camera filter will reduce the glare from the water droplets and consequently saturate the colors of the foliage. See photo at left below. The circular polarizer will also tend to darken the shadow areas, making the lighter plants stand out even more against the dark background.
Be careful when using a polarizer to photograph rainbows. Since rainbows are made of polarized light, the colors will be totally eliminated if the polarizer is turned all the way. On the other hand, a minimal amount of polarization will enhance rainbows, making the colors stand out more.
Many skies, particularly skies with clouds, have tiny water droplets in them. A polarizing filter will reduce or eliminate glare from these droplets and make the sky darker and the clouds more distinct. Polarizing filters work best when you are shooting at a 90 degree angle from the sun and do not work at all if the sun is directly in front or directly behind you. I always look at my shadow and try to shoot so my shadow is perpendicular to the direction I am photographing.
A couple words of caution: In some situations, a polarizer will turn part of the sky much darker than the rest so the resulting image will have a sky that is dark on one side and light on the other, instead of having an even tonality. This happens even more with a wide-angle lens. Also, if you are at a high elevation, above 4000-5000 feet, a polarizer will turn the sky very dark or even black. I usually do not use a polarizer above 4000 feet, or if I do, I turn it very slightly.
If you are taking several images to stitch them together as a panoramic you should not use a polarizer. The uneven tonality of the blue sky will not allow the image to stitch together seamlessly.
Light Absorption or Reduction
A circular polarizer, when used at full strength, requires about 1.5-2 stops of additional light. This means that if you are shooting at 1/125 of a second at f/16, you will have to open the aperture up to about f/11 to maintain the fast shutter speed.
As a result of this, I rarely use a polarizer when photographing wildlife or any moving target where I want a fast shutter speed to freeze the action. On the other hand, if you want to soften or blur the action, such as moving water or a waterfall, then a polarizer will be helpful as in the image above on the right. A polarizer will also tend to darken the shadow areas, making the greens darker and increasing the contrast with the moving water.
How to Use A Polarizer
The effect of a polarizer can be readily seen by either placing it on the camera and turning it or by just holding it in front of the scene and turning it. If you do not notice any effect from the polarizing filter as you turn it, then you should take it off and not use it for that scene.
Which Polarizer to Use
All digital SLR cameras will require a circular polarizer and I recommend purchasing the highest quality you can afford. You can also only purchase one to fit your largest lens size and purchase much less expensive step-up rings to attach the polarizer to smaller lenses.
Polarizing filters come in two different thicknesses - regular and thin. You will need a thin one to avoid vignetting the image (creating dark corners) when you use a wide-angle lens. Thin polarizing lenses are more expensive, but if you are only going to purchase one, I recommend a thin one that will fit your largest lens - usually 72mm or 77mm. I use a thin Singh-Ray Warming circular polarizer that is 77mm.
Doug Steakley teaches an outstanding online photo course: Photographing Motion. BetterPhoto's digital photography school online also offers many classes on photography exposure and natural light photography.
Both photos copyright by Douglas Steakley