To Polarize, or Not to Polarize

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One of my most frequently asked questions during a workshop shoot is, “Should I use my polarizer here?” Of course that’s an impossible question to answer absolutely because as a creative choice, the polarizer decision is rarely absolute.

While many people believe the sole purpose of a polarizer is to make the sky darker (deeper blue), blue sky is just a byproduct of the polarizer’s function: to cut reflections. In fact, if someone could design a polarizer that only worked on the landscape and did nothing to the sky, that’s the one I’d be using because: 1) I generally don’t care for blue sky in my images, and 2) a polarizer doesn’t usually darken the sky uniformly.

Before going any farther, I should probably explain a little about what a polarizer does, and how it does it.

Put simply

A polarizer cuts reflections. It’s a piece of glass mounted on a threaded ring—the threaded ring screws onto a lens, while the glass part of the polarizer rotates independently, allowing the photographer to rotate the glass 360 degrees on the front of the lens. (Contrary to popular belief, a polarizer is a single piece of rotating glass, not one piece of glass rotating atop a second stationary piece of glass.) The polarizer is designed to rotate because its greatest (reflection cutting) effect is at 90 degrees to the light source; at all other angles polarization decreases as the angle moves away from 90 degrees; it becomes nonexistent at 0 and 180 degrees. By watching the scene through the viewfinder as you rotate the glass, you can see the polarization effect change.

On the surface, cutting reflections might not 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 want 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, and sometimes even the sky’s deep blue that has been 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.
A 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, its 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, wind and 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 filter’s polarizing element, 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.

But back to the original question

Should I use a polarizer? I’m still not going to answer. What I will tell you is that I carry a polarizer for every lens in my bag, and when the sun’s out I virtually always have a polarizer on my lens. But my approach comes with some caveats:

A polarizer cuts the amount of light reaching your sensor by 1 to 2 stops, which means if don’t use a tripod (shame on you), a polarizer requires a faster shutter speed.
You must get in the habit of orienting the polarizer with each composition, or risk doing more harm than good to your image

While I use a polarizer on pretty much all of my daylight images, there are times I remove it:

At night (duh), or whenever the scene is so dark that the polarizer’s cost to my exposure settings exceeds its benefit.
On a wide lens with lots of blue sky, the polarizer’s effect on different areas of the sky can be both obvious and uncorrectable (I can dodge/burn minor differences). On the other hand, I almost always avoid wide shots with lots of blue sky, so this is rarely a consideration.
When the sun is in my frame—for example, when I’m going for a sunstar—the extra glass a polarizer adds increases the likelihood of unsightly reflections.
Photographing a full rainbow with a wide lens, a polarizer can eliminate or diminish part of the rainbow.
When I put on a neutral density filter, my polarizer comes off because stacking filters causes vignetting, the less glass between my subject and sensor the better, and nature abhors stacked filters (every time you stack filters, the photography gods fuse them until you’ve learned your lesson).
Any time I absolutely need the fastest shutter speed possible without increasing my ISO further, the polarizer comes off.
I should probably add that I don’t have a polarizer for my Sony 12-24 lens because ultra-wide lenses like this aren’t threaded for filters. Using a polarizer on an ultra-wide lens requires an awkward, expensive system that provides minimal benefit due to the wide field of view.

One time when I absolutely, without exception, always (have I made my point?) use a polarizer is when there’s no sky in my frame. These are the times I’m using diffuse light to capture the color and texture of leaves, flowers, water, and rocks. All of these things reflect, sometimes subtly and sometimes not so subtly, and that reflection is rarely beneficial.

And finally, a common misconception about polarizers is that their use is either all or nothing (full polarization or minimal polarization). The amount of polarization I dial in depends on the effect I’m going for. For example, each of the four images at the top of this post was captured with the polarizer oriented at a point between maximum and minimum effect by watching the scene as I turned the polarizer, then waiting until I had the combination of reflection/no-reflection I wanted. This allowed me to reveal submerged nearby features while saving the reflection of the more distant subject.

So, when should you use a polarizer? I still can’t tell you, but at least now you have the knowledge to make the decision for yourself.