Photo Tip: Polarizing Filters
The polarizing filter is arguably the single most valuable filter in nature photography. It is certainly in the top three. It is handy in other fields of photography, too, and is useful for black and white work as well as color. It can intensify the colors in a scene, make white clouds really stand out against a blue sky, and reduce glare from the surface of water or glass.
It can work its magic because of two properties of light. One property is that light can be thought of as travelling forward in waves, with the wave crests perpendicular to the direction of motion. The orientation of the wave crests is called the polarization of the light. The other thing to know about light is that, while some processes in nature give rise to light that is a random, jumbled mix of all polarizations (this random mix is called unpolarized light), some processes favor one particular polarization over the others resulting in polarized light.
(Fully polarized light -- light with each and every wave crest oriented in the same direction -- is quite rare, so this "polarized" light of which I speak is actually only partially polarized. Partially polarized light has all polarizations present to some degree, it's just that one polarization predominates. I will drop the word "partially" for simplicity, but it's good to remember that it's always implied.)
A polarizing filter blocks all light of one polarization, passes all light of the perpendicular polarization, and passes some but not all of the light for in-between polarizations, as is depicted in the sketch below. This lets the polarizing filter affect the appearance of a scene that has polarized light in it.
So what happens in nature that makes polarizing filters useful? Two things. First of all, reflected light (except reflections from metal) is polarized. This means a polarizing filter can block glare. If you are shooting a lake, for example, this filter will reduce the skylight bouncing off the surface and let you see into the water and record its true color. If you are in town shooting through the window of a photogenic shop from the sidewalk, the filter will reduce reflections of the outdoor street scene on the shop window, providing a clear view of what's within. A more subtle ramification of this glare-cutting feature is that a polarizing filter can intensify colors in a scene. This is because reflections of a white or blue sky mix with the true colors of a scene, washing the color out to a more pastel shade. Eliminate the reflections and you eliminate the washing-out.
The second thing that makes a polarizing filter useful is that the blue sky is polarized, so the filter can darken it. Film is very blue sensitive, so it often records a blue sky as lighter than you experienced it. The filter can help to record the scene more faithful to how you saw it. It can also jack up the drama in a scene, especially if white, puffy clouds are present. One troublesome fact is that the amount of (partial) polarization in the sky is not constant, but varies across it. There is no polarization in the directions directly toward or opposite the sun, and polarization is at its maximum at 90 degrees away from the sun. This means that shooting with a polarizing filter on a wide angle lens, which views a huge chunk of sky, will show a changing shade of blue across the sky, an effect which can be unpleasant.
Here are some examples of a polarizing filter in use in my nature photography.
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Left: without polarizer. Right: with polarizer.
Note how the filter deepens the blue sky and intensifies the other colors.
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Left: without polarizer. Right: with polarizer.
Note how the filter kills off the blue skylight reflecting off berries and
leaves and intensifies the colors.
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Left: without polarizer. Right: with polarizer.
Note how the filter intensifies the blue sky, making the clouds stand out.
Okay, wow, so the polarizing filter is the greatest thing since sliced bread. How do you use the thing? (After all, the human eye is not sensitive to polarization, so you can't know how to orient the filter on the lens just by looking at it.) Its use couldn't be simpler. After you screw the filter onto your lens, you'll find that the polarizing element itself is on a ring that rotates. You simply look through the viewfinder, rotate the polarizing element until you see the effect you want ... and that's it. Go ahead and shoot. At first you may find that the effect on film is larger than what it seemed through the viewfinder, but you will quickly learn from experience how to judge the viewfinder image to achieve the result you want.
One other thing you will quickly learn is that a polarizing filter blocks almost two stops of light, even from an unpolarized object. You are essentially putting a two stop neutral density filter on your camera, and will have to live with the consequences of longer shutter speeds or wider apertures. This is the filter's biggest drawback. (Of course, if you want longer shutter speeds, say to blur running water, you can use your polarizer to your advantage as an impromptu neutral density filter.)
Before you go off and buy a polarizing filter, let me give you a few last pieces of information. For one thing, polarizing filters come in two types, "linear" polarizers and "circular" polarizers. The names have nothing to do with their shape. Linear polarizers and circular polarizers do exactly the same thing photographically. In fact, a circular polarizer is nothing more than a linear polarizer with an extra layer on the back side of the filter to mix up the nicely polarized light the filter has produced. Since all the photographic work of the filter is done by blocking light with the front-end linear polarizer, the circular polarizer does precisely the same things to an image that a linear polarizer does. Since that extra layer on the circular polarizer adds to its cost, why should you ever choose to buy one? Because modern cameras can have their autofocus, exposure meter, or both thrown off by the polarized light coming through a linear polarizer. By scrambling the polarization of the light after it has been filtered by the linear polarizing element, the circular polarizer eliminates this pitfall. If you are certain your camera will work fine with a linear polarizer, by all means get one and save a few bucks. But otherwise, my advice is to pay for the circular polarizer. Then you know you're safe. And besides, you won't have to trade in your polarizer if you get a new camera body.
The next thing you need to know is that polarizers can come with an unwanted color cast. Ideally they should be gray, so as not to alter the overall color balance in your photograph. But life is not ideal, and some polarizers are manufactured with a color cast to them, often a sickly green tint, as shown below in a photo of four polarizing filters I own. Because of this, a polarizing filter is just about the one thing I won't buy via mail-order -- I need to eyeball it in person before buying to make sure it has a neutral gray cast. (This color cast won't matter much, by the way, if you only shoot prints, because the color can be corrected in the printing. But the added color is death to slides.)
Four polarizers shot on a light table. Each has its own color.
The worst is the top center one -- what an icky green.
The final thing I want to say is that a polarizing filter tends to be thicker than most filters, and so can cause vignetting with a wide-angle lens. Vignetting is a darkening of the corners of the image as the lens actually sees the rim of the filter. To eliminate this problem, manufacturers have made polarizing filters with extra-flat profiles.
With these warnings in mind, by all means go out and get a polarizing filter. A nature photographer can't be without one, and many other styles of photography benefit from its use as well. Cut glare, intensify colors, and darken blue skies to make your photographs represent what you saw and felt when you tripped the shutter.
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