Some Light Conversation About a Potentially Polarizing Topic
A polarizer is almost indispensable in certain circumstances, and some photographers are such fans that they leave one affixed permanently to the front of their lens. But as helpful as polarizers are, there are times when you should not use one.
To begin with, let's consider how a polarizer works its magic.
Without going all physics on you, you can think of light as both a particle and a wave. Never mind the particle bit, but it's the wave aspect of light that matters for our discussion here. Light waves don't travel in a straight line, but, much as ocean waves moving with successive ridges and troughs, they roll along paths that averages out to straight. But unlike the open seas, where gravity determines the orientation of those waves, light waves have no mass and aren't limited to any sense of up-and-down in their motion. By the time sunlight passes through the atmosphere and reaches us here on earth, it has had quite a journey. After being scattered by passing through countless water droplets in the air and interacting with dust particles and whatever else may be up there, light lacks a consistent orientation. But reflected light becomes uniformly oriented when it bounces off any sufficiently mirror-like surface. The smoother and more reflective the surface, the more uniformly angled the reflected waves will be. To put this in concrete terms, that's what causes the annoying glare you see when you're trying to photograph something behind glass. Sometimes, it can be all but impossible to tell what's on the other side.
In nature photography, we don't often have to deal with panes of glass, but the smooth surface of a lake can be just as troublesome to photograph without glare. Shiny leaves and other similar objects can also cause problems. To varying degrees, almost anything could be. It's the light reflected off the things around us that allow us to see them at all.
Optically, a polarizer filter functions as a collection of narrow slits that only pass light of a given orientation. It doesn't really have slits, but it helps to think of it as if it did for purposes of explanation. The more a light wave's angle matches the orientation of the slits, the easier time it will have passing through the filter. When you rotate the polarizer, you change the direction of the virtual slits and thereby impact which light waves can enter your lens and which can't. Light at right angles to the filter orientation will find its way almost completely blocked. Thankfully, you can see the effect in the viewfinder, so you needn't worry about figuring out what the light waves are up to, but some level of understanding is necessary to move on to our discussion of the limitations of polarizers.
Let's consider the reflection from that otherwise beautiful, photogenic lake. The newly oriented light waves will be traveling as determined by their incoming direction and what they've encountered along the way. The position of the sun plays a major role in all this. Your shooting position dictates how much the reflective glare impacts your view. And it also limits how effective a polarizer will be in blocking that glare due to the resulting angle. Try as you may, some reflections just can't be effectively stopped. Sorry about that. If your polarizer isn't doing what you want, remove it. All it's doing is costing you a stop or more of light.
Sometimes, that reflection on the lake may carry its share of glorious "golden light" color from sunset or sunrise. At such times, you may want to keep it to enhance the photo. Think carefully before removing colored reflections. It's often the color of the light that makes the photo.
And if the reflections are on something that is supposed to look wet, you may want to be judicious in your polarization as well. It's those reflections that let the viewer know something is wet. Wet leaves should look at least somewhat wet. Wet rocks shouldn't necessarily look the same as dry ones.
Polarizers are also helpful to saturate colors and darken skies. But as above, this magic, too, has its limitations. This technique works by blocking much of the randomly diffused light that results from atmospheric diffraction. This scattered light is what decreases contrast and washes out saturation. By turning the polarizer to favor primarily direct sunlight, you can see colors more clearly. But if you aren't careful, this technique can also be overdone. You can sometimes rotate a polarizer to block so much scattered light that you can turn daylight to look more like twilight. Saturated blue skies can improve an image. Midnight blue skies at mid-day just look wrong.
Even when you don't overdo it, you can still have sky problems. The angle between the sun and where your lens is pointing determines the maximum degree of polarization possible. You'll get the strongest effect when shooting at ninety degrees to the sun. With a telephoto lens, you'll be working with a constant angle and may be able to crank up the polarization enough to compensate for an unfavorable angle. But with a wide-angle lens, the effect will likely be uneven. If one side of the frame is at the perfect angle, the opposite side won't be. A polarizer is generally of minimal use with an ultra-wide lens. But even then, they can work if the glare covers only a manageable portion of the frame. Just hold it up in front of you and look around to gauge the effect before slapping it on your lens.
Polarizers can be a mixed bag for shooting rainbows. Turned one way, and you can darken the sky and accentuate the colors. Turned a different way, and you can make the rainbow disappear completely. And what would be the fun in that?
And if there are no reflections that you need to deal with, leave your polarizer in your camera bag. As I mentioned earlier, even at its most timid orientation, a polarizer will cost you at least a stop of light. One look at a polarizer should let you confirm that. After all, the things do look gray.
After sunset, skip the polarizer in most cases. Light levels are dim enough after dark that there's no sense in making your job more complicated than it has to be. You have to be able to see through that polarizer to focus, too.
Love them or hate them, polarizers are among the few filter types that remain indispensable in the digital world. You can leave your warming filter at home since white balance can do everything it can and with greater flexibility. But it may be impossible to salvage the image later in Lightroom if it's hard to see something through all the glare when shooting.
Most outdoor photographers love polarizers, but I've met a few that eschew their use entirely because of their potential problems. In some circles, the use of polarizers can become a polarizing issue itself. Thankfully, they take little space in your bag. For landscape work, don't leave home without one. But also, don't feel like you need to use one on every shot.