Some people couldn’t care less how a polarizer works—they’re satisfied knowing what a polarizer does, and how to make it happen. But if you’re like me, you also need to understand why things behave the way they do.
A polarizer cuts reflections. On the surface that not might seem so desirable for someone who likes photographing reflections as much as I do, but reflections are a much bigger part of our visual experience than most people realize. Virtually every object reflects at least a little, and many things reflect a lot more than we’re aware. Worse still, these reflections often hide the very surface features and color we most love to photograph.
When reflections hide an object’s underlying beauty, a polarizer can restore some of that beauty. I use a polarizer when I want to capture the submerged rocks or sand hidden by the reflection atop a river or lake, the rich color overwhelmed by glare reflecting from foliage, or the sky’s deep blue washed out by light scattered by atmospheric molecules.
Put a little less simply…
In reality, reflections are merely collateral damage to your polarizer. What a polarizer really does is eliminate light that’s already been polarized. To understand what’s really going on with a polarizer, read on….
While an unpolarized light wave oscillates on every plane perpendicular to the wave’s motion, polarized light only oscillates on one perpendicular plane (up/down or left/right or 45°/225° and so on).
Polarization can be induced many ways, but photographers are most interested in light that has already been polarized by reflection from a nonmetallic surface (such as water or foliage), or light that has been scattered by molecules in our atmosphere. Light scattered by a reflective surface is polarized parallel to the reflective surface; light scattered by molecules in the atmosphere is polarized perpendicular to the direction of the light.
Polarization can also be induced artificially with a polarizing filter (“polarizer”), a filter coated with a material whose molecular structure allows most light to pass, but blocks light waves oscillating in a specific direction. When unpolarized light (most of the light that illuminates our lives) passes through a polarizer, the light that enters the lens to which it’s attached has been stripped of the waves oscillating in a certain direction and we (through the viewfinder) see a uniform darkening of the entire scene (usually one to two stops).
But that uniform darkening is not usually what we use a polarizer for. (I say usually because sometimes we use a polarizer to reduce light and stretch the shutter speed in lieu of a neutral density filter.) Photographers are most interested in their polarizers’ ability to eliminate reflective glare and darken the sky, which occurs when their polarizer’s rotating glass element matches the oscillation direction of light that has already been polarized by reflection or scattering, cancelling that light. By watching the scene as we rotate the polarizing element on the filter, photographers know that we’ve achieved maximum polarization (reflection reduction) when we rotate the polarizer until maximum darkening is achieved—voila!
The exception that proves the rule
Most photographers know that a polarizer has its greatest effect on the sky when it’s at right angles (90°) to the sun, and least effective when pointed directly into or away from the sun (0º or 180°). We also know that a rainbow, which is always centered on the “anti-solar point” (a line drawn from the sun through the back of your head and out between your eyes points to the anti-solar point) exactly 180° from the sun, can be erased by a polarizer. But how can it be that a polarizer is most effective at 90° to the sun, and a rainbow is 180° from the sun? To test your understanding of polarization, try to reason out why a rainbow is eliminated by a polarizer.
Did you figure it out? I won’t keep you in suspense: light entering a raindrop is split into its component colors by refraction; that light is reflected off the back of the raindrop and back to your eyes (there’s a little more bouncing around going on inside the raindrop, but this is the end result). Because a rainbow is reflected light, it’s polarized, which means that it can be eliminated by a properly oriented polarizer.
About this image
Long before achieving international fame as the background scene for Apple OS X High Sierra, North Lake near the top of Bishop Canyon in the Eastern Sierra has been beloved by photographers. Each autumn this little gem of a lake teams with photographers longing for even one of the following conditions: peak gold and red in the aspen, a glassy reflection, or a dusting of snow.
I visit North Lake multiple times each autumn, sometimes with my workshop groups, sometimes by myself. I’ve found pretty much every possible combination of conditions: snow/no-snow; early, peak, or late fall color; and a lake surface ranging from mirror smooth to churning whitecaps.
One sunrise early October of 2010 I hit the North Lake trifecta. Crossing my freezing fingers that the reflection would hold until I was ready, I lowered my tripod on the rocky shore and framed the aspen-draped peak and its vivid reflection. I used a couple of protruding rocks to anchor my foreground, slowly dialed my polarizer until the entire lake surface became a reflection, and clicked. But rather than settle for that shot, I reoriented my polarizer until the reflection virtually disappeared and a world of submerged granite rocks appeared. I clicked another frame and stood back to study the image on my LCD.
As much as I liked the rocky lakebed version, I knew there was no way I could pass on the best reflection I’d ever seen at North Lake. So I returned my eye to my viewfinder and very slowly dialed the polarizer again, watching the reflection reappear across the lake and advance toward me until the entire mountain unfolded in reverse atop the lake. Stopping just at that midway polarization point, I had the best of both worlds: my pristine reflection and an assortment of submerge rocks.
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