How a polarizer works

Gary Hart Photography: Autumn Reflection, North Lake, Eastern Sierra

Autumn Reflection, North Lake, Eastern Sierra (2010)
Canon EOS-1Ds Mark III
Canon 17-40L
1/5 second
F/16
ISO 200

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.

Put simply…

A polarizer eliminates 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 know. 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….

Essential terminology

  • Oscillation is motion relative to a fixed point. For example, when you snap a whip, the whip “oscillates” along its length. Without external interference (e.g., friction from the atmosphere or other objects), motion in one direction along the whip will have an identical motion in the opposite direction (e.g., up=down, left right, and so on), and that motion will move forward along the whip.
  • wave is oscillation along or through a medium (such as air, water, or space). The bulge that moves up and down (oscillates) along a cracked whip is a wave. For the liberal arts folks, (in this context) wave is a noun, oscillate is a verb. A wave is measured by its wavelength and frequency—the higher the “frequency,” the shorter the “wavelength.”
  • Frequency is the number of times a wave peak passes a discrete point in a given unit of time (usually one second: “per second”).
  • Wavelength is the distance from one wave peak to the next at any instant frozen in time.
  • A transverse wave oscillates perpendicular (90°) to its direction of motion. To imagine the motion of a transverse wave, picture an ocean wave, which oscillates up and down as it advances through the water. Now think about a bottle floating in the open ocean—bobbing up and down with each wave, it’s up/down motion is perpendicular to the wave’s forward motion, but when that wave has passed, the bottle is in the same place it was before the wave arrived. (Waves don’t move bobbing bottles across the ocean, currents do.)
  • Visible light is electromagnetic radiation that reaches our eyes as a transverse wave somewhere in the wavelength range the human eye can register, about 380 to 740 nanometers (really small).
  • Sunlight (or more accurately, solar energy) reaches earth as a transverse wave with a very broad and continuous spectrum of wavelengths that include, among others, the visible spectrum (lucky for photographers), infrared (lucky for everyone), and ultraviolet (lucky for sunscreen vendors). The oscillation of solar energy’s transverse wave is infinitely more complicated than an ocean wave because light oscillates in an infinite number of directions perpendicular to its direction of motion. Huh? Think about the blades of a propeller—each is perpendicular to the shaft upon which the propeller rotates, so in theory you can have an infinite number of propeller blades pointing in an infinite number of directions, each perpendicular to the shaft. So a light wave oscillates not just up/down, but also left/right, and every other (perpendicular) angle in between.

Polarization

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 at 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.

I Use Breakthrough Filters


Dialing In My Polarizer

Click an image for a closer look and slide show. Refresh the window to reorder the display.

 

2 Comments on “How a polarizer works

  1. Hi Gary:

    Just a short note to tell you how much I enjoy reading your Eloquent Nature emails and viewing your photos.

    I have a quick question: Would it be possible for one (me!) to use one or more of your photos as my desktop background? Oops, second question: Do you know how I would do that?

    Thanks, Gary! Keep up the great shooting!

    Mary Ryan

    >

    • Thanks for reading and viewing, Mary. I don’t offer high-res jpegs to use as a desktop background, but you’re welcome to use any image of mine that you find online, providing my copyright watermark is visible, and it’s only used on your desktop. The easiest way is to do a screen capture, which depends on the computer platform you’re on (Mac or Windows). The best thing to do would be to google it—you might also find help on YouTube.

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