The one accessory you’re likely to find in any professional nature photographer’s camera bag is a polarizer. No matter the camera type or brand, from large-format film shooters to DSLR users, everyone has a polarizer, and many pros carry a few different types. The reason is simple: Polarizers make a scene look better by increasing saturation, reducing or eliminating glare and, in particular, darkening down a washed-out, milquetoast cyan sky to a crisp slate blue.
Singh-Ray Gold-N-Blue polarizer; Tiffen polarizer
Polarizers remain popular even in this age of Photoshop because their effects are very difficult to simulate. There are a few software programs that do a decent job, but no postprocessing will beat the real thing. As good as they are, to use polarizers effectively, you need to know their limitations and quirks. The filters work by only allowing light waves that are oriented in a narrow range of directions to pass through it. Think of a picket fence. In nature, light rays are oriented in all different directions. A picket fence, where the pickets are close together, would block all of those rays except for those traveling in a very vertical orientation.
Whenever light reflects off a surface, it becomes polarized, and the reflected light is often seen as glare. For example, here in California, we sometimes get a magnificent poppy bloom in the spring. The poppies open up as the day heats up and the sun climbs in the sky, and you often get a lot of glare from the sunlight reflected off the petals, which have a slight sheen. That glare is all polarized, and by applying a polarizer to the camera, the glare can be reduced or eliminated, and the rich orange color comes through. The same effect can be seen when you’re photographing a wet rock. The light reflecting off the rock can create an objectionable glare, but by using the polarizer, you can eliminate it almost completely.
When it comes to darkening down the sky, light is reflecting off of tiny water droplets in the atmosphere. All of those reflections are polarized, but because there are so many droplets, the effect is light rays traveling at all different angles. When you apply a polarizer, you can filter out most of those rays and you get a darker sky. But polarizers don’t give the same effect everywhere in the sky. The effect is strongest at 90º from the sun. When you get closer or farther from the sun, the effect drops off significantly.
1 Unless you have a manual camera, use a circular polarizer. Linear polarizers create problems for AF systems.