Polarizers are truly a landscape photographer's best friend. While working one early morning along the coastal range east of my home in Hollister, California, I came across this rather simple landscape of a lone oak tree against a morning sky. At first glance I did not think much of the scene, but once I put the camera to my eye (with a polarizer attached to my lens) I saw a beautiful delineation in the tones and color in the clouds, I set to work on finding a pleasing composition in the fleeting light. I was exactly 90 degrees off-axis from the approaching sunrise allowing for maximum polarization. I simply played with varying degrees of polarization and settled on this image as my favorite.
There seems to be a lot of confusion these days as to what filters are most useful in the digital era. I have to admit, if I was told I could have only one filter in my bag, it would without a doubt be a polarizer. There is really no true way in Photoshop (nor do I no of any plug-in) that can replicate what a polarizer can do. Simply put, you cannot polarize the light after the fact, regardless of what anyone tells you. To the naked eye, these clouds appeared flat and lifeless, but when I polarized the light, magic happened!
Having said all that, the most common mistake I see most amateurs make when using a polarizer is in thinking they have to "max out" the polarization of every image - usually resulting in uneven or unnatural looking skies (especially in the mountains where one can literally tun a sky black by over-polarizing).
The art of using a polarizer is in utilizing it to make a scene retain its natural look, but to give the sky a bit of "pop." Polarizers are also great at taking the glare off water and virtually any reflective surface, thus reducing glare which in turn saturates the scene (think wet leaves when shooting fall color). But you be the judge. I caution photographers working with extreme wide angle lenses (16mm-35mm) to pay attention to their sky, especially as one gets close to 90 degrees off-axis to the light (where maximum polarization occurs). It is very easy to get an uneven distribution of polarization across the entire sky.
There are a couple other items to think about about using polarizers. First, depending on the amount of polarization, you will lose between 1 and 2 stops of light. Second, when shooting towards a light source (sun) it's best to remove the polarizer to help reduce the chance of flare (it's just one more piece of glass in through which light is bent). And third, if you can afford it, purchase a polarizer for all of you lenses and spend the money on a good one. I own Singh-Ray and B+W, and they can top out at over $200 per filter. For wide angles, check into "slim mount" polarizers which will eliminate the vignetting problem when at extreme wide angle (especially if you are shooting with a full-frame sensor).
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