Shooting Through the Sunstone—
The Many Uses of Polarizing Filters
By Tamia Nelson
May 5, 2009
Twenty-five years ago last Thursday, spring was in the air. Soft rains were awakening the land from its winter sleep, and the rich perfume of new life was everywhere evident. Almost everywhere, that is. All I could smell in the windowless cell that served as a geology lab was the dust of ancient rock. That arid lab was the room where my biweekly class in optical mineralogy met, and the prof's lecture reflected the setting. It, too, was dry as dust. Then the prof held up a thin slab of a clear, colorless mineral. "This is Iceland spar," he said. My eyelids drooped. I was dangerously close to falling asleep. But the prof had more to say: "Crystals just like it were used by the Vikings in navigating the northern ocean." That got my attention. My drooping lids snapped open, and I had no trouble staying awake for rest of the hour. Here's what I learned: Iceland spar is strange stuff. It polarizes light. Thanks to this quirk of nature, you can use a piece of it to find the sun's position in the sky even on a cloudy day. And if you're navigating by the sun in latitudes where clear skies are rare—as the Norsemen often did—that's a very big deal, indeed. To geologists, Iceland spar is simply a transparent form of the mineral calcite, but to the Norse it was magic. They called it the "sunstone." It's easy to see why.
Not everyone agrees, however. My professor knew a good story when he came across one, but scholars are still arguing whether or not the sunstone was used as an aide to navigation by the Northmen. There's evidence in the sagas to suggest that it was, but the sagas are…well…sagas, wonderfully embellished accounts written down long after the fact. Still, my prof got one thing exactly right. It's a great story. And it might even be true. In 2005, studies aboard an arctic research vessel lent some weight to the sunstone theory. If you're curious, you'll find the details in the Proceedings of the Royal Society.
Luckily, backcountry photographers don't need to take sides in this academic dispute. We don't even have to understand the phenomenon of polarization. We just need to know how and when to use it. First things first, though. Not all lens filters are created equal. In fact, not all filters are polarizing filters. The cheapest and most common after-market filter is the ultraviolet (UV) filter. One lives on each of the lenses for my digital SLR. These lenses aren't cheap, and the UV filters protect them from fingerprints, spray, and scratches. They also reduce haze, and that's good, too. But they're not polarizers.
OK. If UV filters reduce haze and protect costly lenses, what's the point of a polarizing filter? Or, to put it another way…
When Does It Pay to Polarize?
Back in the day, when photography meant film photography, and you didn't know what you had until you got your slides back from the processor, it wasn't unusual to return home from a trip with roll after roll of disappointing shots. The details of wave and water were lost in a dazzle of light. Intricate cloudscapes faded into a featureless sky. The brilliant reds and yellows of the autumn woods looked muddy, dull, and dowdy. And there was very little you could do about it but try again on another trip and hope for better luck. "Disappointed" doesn't begin to describe how you felt.
Nowadays, of course, you can rescue bad shots with good software. Sometimes. The old GIGO principle still applies: Garbage in means garbage out. So it helps to be lucky. And it doesn't hurt to make your own luck if you can. That's where the polarizing filter came in, back in the years BD (Before Digital). It still does, particularly if you're shooting any of these common backcountry subjects:
The details: Unlike UV filters, polarizing filters have two elements. The outer element rotates. The inner element doesn't. It's attached to your lens (or screwed down over the resident UV filter). And the best way to learn what it does is to mount one on your DSLR—polarizing filters for point-and-shoot cameras are rare, and rarely used—take a squint through the viewfinder, and rotate the outer element through a full 360 degrees while focusing on a variety of subjects. (Make sure that the fixed element stays put while you experiment. You don't want your filter falling off!) Notice how the degree of polarization changes with the orientation of the outer element. There are no fixed rules here. Getting it right is a matter of trial and error. The optimum rotation in a given setting depends on the intensity and direction of the ambient light and the nature of your subject. Experiment. You'll soon discover that a polarizing filter can reveal hidden details in a cloudscape, sharpen the outline of distant hills, and intensify colors, whether of sky and woodland, autumn hillsides, or spring wildflowers. Nor is a polarizer just a fair-weather friend. In winter it tempers the blinding brilliance of snowy slopes, disclosing what the dazzle conceals.
Sometimes a picture really is worth a thousand words. So I've taken some shots to illustrate the polarizing filter's many uses. Here are the first:
The shot on the left was taken with zero polarization. Compare it with the shot on right, made with the filter adjusted so as to reduce glare to a minimum. Same light. Same place. And only a minute or two separating the two exposures. But see the difference! (In these and all the other photos in this article, I used my Pentax K200D digital SLR outfitted with a Pentax DA 18-55mm telephoto lens. The polarizing filter was screwed down over a UV filter.)
Next, let's look at a couple of photos showing what a polarizing filter can do to minimize unwanted reflections:
The scene is a shallow backwater on The River. In the shot on the left—as before, it was made with zero polarization—the water's surface acts as a mirror, reflecting the sky and the trees while concealing most of what lies beneath. With the filter element rotated just a bit, however, you can see below the surface. (That's why anglers favor polarized sunglasses.) Moreover, the sky's reflection is a deeper, richer blue than before.
And what about snow scenes? Yes, winter's grip on Canoe Country has loosened, but who knows? We might still have a late-season storm, and General Winter has been known to launch probing attacks well before the autumnal equinox. (When paddling "north of 50," we've wakened to a snow-covered camp in mid-August.) Is there any good news in all this? Sure! While your polarizing filter won't warm you up, it can bring out the fine detail in the dazzle of new snow, even as you shiver in the unseasonable cold. Take a look at this:
The shot on the left (no polarization) shows a featureless expanse of white; the one on the right (optimum polarization) highlights every hump and hollow. And there's more: The polarizer makes the shadows darker, the sky bluer. In short, the polarized shot more accurately reflects the scene as you see it. You say there's no snow where you paddle? Ever? No problem. (And lucky you!) The sunstone's magic works with clouds, too.
The sun was veiled by altocumuli in both photos, but you can only see the fully developed skyscape in the (polarized) shot on the right. You get a couple of bonuses, as well. The polarizer reduces the glare on the water, while intensifying the colors. Putting it another way, the colors are more saturated in the polarized shot. Here's another example:
These tiny flowers are Carolina spring beauties (Claytonia caroliniana), among the earliest Canoe Country blooms, but the scalelike leaves don't belong to the flowers. They're ground-cedar (Diphasiastrum complanatum or Lycopodium complanatum, take you choice), a variety of clubmoss. By the way, shutterbugs have a family connection to the clubmosses. In the early days of photography, clubmoss spores were used as flash powder. In fact, "lycopodium powder" is still used to develop latent prints at crime scenes, so they can then be captured on film. But that's getting away from the subject at hand. Notice how the polarizer pushes the colors in the right-hand shot, making the greens greener, the magenta deeper, and the duff in the background rustier.
Glare is another problem for photographers hoping to capture spring's new growth. Take a look a these riverside ferns:
Compare the colors in the (unpolarized) photo on the top to those in the second shot. The glary highlights in the first photo are absent in the second, and the colors are more saturated. Admittedly, these differences are subtle—but they are significant.
Once you've seen what a polarizing filter can do, you'll be tempted to use it in every shot. But don't yield to temptation. Mae West was wrong. Too much of a good thing isn't always wonderful. Want a for-instance? Just look at this pair of pictures:
The photo on the left was shot with no polarization. (The reflections in the still water give the game away.) While the polarizer reduces glare in the second shot, it makes it hard to discern the water's surface. Moreover, because I metered for the foreground, I overexposed the sky, losing much detail. Which photo is better? That depends. If I needed to draw attention to the haze enveloping The River, I'd choose the first photo. On the other hand, if I wanted to emphasize the shallows and show off the rich browns of the riverside vegetation, I'd pick the right-hand photo. On purely aesthetic grounds, I lean toward the first, unpolarized shot. The moral of this story? That's easy:
"Polarizer" isn't a Synonym for "Panacea"
As the preceding photo pair demonstrates, glare can be good—or if not good, at least not bad. And that's just the beginning. Take another look at some of the earlier photos. Pay particular attention to the shots of the two canoes and the pictures of the woodland pool just above them. The polarizer makes the green hulls of the little pack canoes look almost black, while it lightens the leaf litter around the pool until it's nearly white. Why did this happen? In the first case, I metered for the snow; in the second, for the pool. The polarizer did the rest. Photography is like ecology—everything is connected to everything else. So it's important to consider the whole picture before pressing the shutter.
The sophisticated through-the-lens meters in modern digital cameras can actually make things worse. Their sensors are sometimes thrown off by polarizers. So if the photo you're taking is important, meter manually and "open up" 1½ stops (or EV 1.5). You don't know how? You'll find instructions in the booklet that came with your camera. If your owner's manual is anything like mine, it won't be a riveting read, but it certainly gentles the learning curve. Alternatively, employ an old shutterbug's trick that's made easier (and cheaper) by digital imaging: bracket your exposure by taking three or four shots of the same scene, metering on a different area of the subject each time. Then pick the best.
The sun's orientation also determines how well polarizing filters work. Their influence is most striking when the incident light comes from the side. On the other hand, if the sun's behind your back, front-lighting your subject, or if your shot is strongly backlit, your polarizer will have little effect. Here's an example of the latter case:
There's little to choose between in these backlit shots of fern fronds, is there? If I hadn't annotated the photos, I'd be hard pressed to tell the polarized shot (bottom) from its unpolarized counterpart. I might just as well have left my polarizer off the lens. And the same goes for this photo of The River that I took a month earlier:
The low morning sun is just out of the picture on the right, and my polarizer did nothing to reduce the glare from the water. Diffuse light—sunlight filtered through the trees along the riverbank—helped highlight the topography on the scalloped ice pans, but I needn't have risked frostnipped fingers to put the polarizer on the lens. The good news? The filter didn't do any harm.
Don't misunderstand me. Polarizing filters have their limitations, but they're still worthwhile additions to your backcountry photography kit, and I'd recommend them to any paddling shutterbug. Getting the right filter for your camera can be tricky, though. It's not quite as simple as you might think. Here are a few points to consider when…
Buying a Polarizing Filter
You've heard this before: Read the manuals that came with your camera and lenses. These will tell you what size(s) and type(s) of filter to buy. Different lenses may require different sizes, and not all makes of lens take screw-on filters. (All my Pentax lenses take the same size of filter. This is a Very Good Thing.) Maybe you still have the polarizers you used with your old film camera, and maybe they're the right sizes for the lenses on your new DSLR. It's tempting to save money by recycling your old filters, I know. But this is one temptation you probably ought to resist. The autofocus lenses that accompany most DSLRs can be thrown off by old-style linear polarizing filters. There's just one answer to the problem: Buy a new circular polarizer. These look the same as linear polarizers, so be sure to double-check before you part with your cash. The bad news? The newer filters cost more than the old-style linear polarizers. They're still pretty cheap, though. You can get a good circular polarizer for about what it used to cost to have a couple of rolls of slide film developed. That's not too bad.
Unfortunately, this isn't the only trap waiting for the unwary photographer. (I told you that buying a polarizer wasn't simple, didn't I?) Wide-angle lenses sometimes require special polarizers. As the photo above illustrates, standard circular polarizers are pretty thick, fat enough to create a dark halo around the edges of your pictures. This is called "vignetting," and while it's sometimes done deliberately, it's not something you'll want in all your shots. Here's a comparatively benign example:
Notice the dark corners? That's vignetting. Polarizers designed specifically for wide-angle lenses minimize the problem by slimming down, but these fashionably slim accessories don't come cheap. For most backcountry photographers, they're luxuries that we can live without. Happily, as I've already noted, basic circular polarizers needn't break your budget. You don't have to pay top dollar to get a perfectly serviceable filter. My 22-dollar Tiffen has served me well for months now. It's the (Twenty-)Two Buck Chuck of photo gear, and I've had no cause for complaint.
I don't think you will, either, whatever make of filter you buy. And once you get it, put it through its paces. Take a series of comparison shots like I did in working up the illustrations for this article. A reminder: There's no reason to remove the filter when you don't need (or want) polarization. Just rotate the outer element until the effect is lost. It's that easy.
The Norsemen may have navigated through the waters of the Northern Ocean with the help of their magical sunstone. Or maybe they didn't. Either way, though, the sunstone's descendants help modern-day photographers make the most of the sun's light. That's magical enough for me, and if you put a polarizing filter in your bag of tricks, it will work its magic for you, too—whatever waters you ply.
Copyright © 2009 by Verloren Hoop Productions. All rights reserved.