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.
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
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.