The circular polarizer is one of my favorite filters for a wide array of photography opportunities. Not only can it help to darken a sky when the sun is high above, but it also does a terrific job at cutting the glare on non-metallic surfaces including glass, bodies of water, wet leaves and more. In this video you'll see the effects of a polarizing filter change in real-time as seen through my Canon 6D. I've also included before and after images for comparison.
For traveling photographers, it's common to find yourself at spectacular locations mid-afternoon in less than ideal light. This is the perfect time to use the polarizer to create a more interesting sky and add some vibrance to your scene. Simply rotate the front element of the filter to change the intensity of the effect. Be aware that a polarizer cuts about 1.5 - 2 stops of light from getting into the camera. To compensate, you'll need to adjust your fstop, shutter, and or ISO accordingly.
The next example of the polarizer's power is in the way it cuts the glare on wet surfaces and bodies of water. This particular effect can't be reproduced in post production, and is best achieved in the camera at the time of exposure. When you eliminate the bright glaring areas of a scene, the important details of your subject become more defined. Foliage will also benefit from the filter, becoming richer and more vibrant. By cutting the glare on non-metallic surfaces, the polarizer is very efficient at reducing reflections on surfaces like glass.
When buying a polarizer, the first thing you'll need to determine is the correct filter size. To find this, study your lens for the filter diameter marking. On some lenses, it's on the front, while others show it on the side or bottom of the barrel. If you have a quality lens, it makes sense to invest in a well-made polarizer. With better optics and a durable build, brands like Hoya, Tiffen, and Formatt-Hitech will ultimately provide the desired image clarity.
A couple of other important things to keep in mind: Try not to stack the polarizer on top of a UV filter. Not only will it become very difficult to separate from each other, but the added width may cause a visible vignette with dark corners around your image. To clean the polarizer, I rarely use any liquid chemicals, but rather wipe any fingerprints with a non-abrasive microfiber cloth.