Commanding the Light: Polarizers

Over the years I have collected a number of polarizers, but you really only need one, big enough for your biggest diameter lens (in my case, 77mm) and a step-up ring which will allow you to put bigger filters on smaller lenses.

Now you see it, now you don't: the light emitted by computer monitors is strongly polarized. — Now you see it, now you don’t: the light emitted by computer monitors is strongly polarized.

In recent entries I talked about the use of filters in black-and-white film photography, and ways to emulate them using digital image files and editing features such as Adobe Photoshop’s channel mixer.

Unlike black-and-white filters, which pass their own color, but don’t pass opposite colors, polarizers pass light that is polarized in the same direction as the polarizer, and don’t pass light that is polarized at a 90˚ angle to the filter’s setting. I could go on about the mechanics of this process, but in photographic terms, results matter more than anything else.

The two main purposes of a polarizer are to control reflections, and to manipulate the blue part of the sky. There are other uses, but these are the reasons to carry a polarizer on a regular basis.

A polarizer can be used to suppress reflections, like this one of the street in my car window.

Polarizers can also be used to improve the appearance of sky areas in an image, since blue sky light is usually more polarized than clouds or objects on the ground.

There are a couple of serious downsides to using a polarizer:

It absorbs between one and three EV of light, meaning one to three f/stops or shutter values, and
Light isn’t usually polarized evenly over the area of the image, which can result in a darker area of, for instance, the sky, which can be hard to fix in post-production
Beware the “hot spot,” particularly with wider-angle lenses, like in this 18mm image at New Mexico’s Plaza Blanca. The uneven darkening of the sky from clumsy use of a polarizer can be difficult to remove.

Using polarizers is pretty straightforward on a digital SLR: rotate the movable ring on the front of the filter until you see the result you want. On bridge/crossover cameras, it’s more complicated, since the exposure system of the camera will make the image in the viewfinder or display on the back of the camera lighter or darker to compensate for the action of the polarizer. With cameras like that (in my case, the Minolta DiMage 7i and the Fuji S200EXR and HS30EXR), I typically let the camera focus and set exposure, then I manually lock the exposure, then rotate the polarizer for the best effect.

Polarizers use a literal “rule of thumb,” meaning that if you point your thumb at the sun, and keep your index finger at a 90˚angle to it, anywhere your index finger can point will be the area of greatest polarization of the sky.

Also of note: when rotating your polarizer, turn it in the direction your would screw on a filter, or you might end up accidentally removing it while trying to use it.

In my day-to-day news and sports photography, I don’t use a polarizer very often, but in my travels, particularly in the American West, I find that careful use of this filter can dramatically improve my photographic expression.

A polarizer and careful attention to exposure can yield beautiful, dramatic skies like this one near Shiprock Peak in northwestern New Mexico.