I was surprised by how many people were sure—incorrectly, of course—that polarizers don’t work like this. Worse yet, these were the very people at whom the piece was aimed! I wanted to help all those photographers who didn’t know you could use your polarizer for so much more than darkening blue skies, but, alas, they wouldn’t listen. Hopefully, they will this time, because the premise at work behind darkening a blue sky is the same as the premise at work behind eliminating shine on rainy days. The reason the polarizer is so useful for both is because, at its core, a polarizer does one thing very well: It reduces or eliminates reflections. And, if you look a little deeper at the consequences of reducing reflections, you’ll understand why a polarizer is so useful for improving detail and color.
In the example of the damp, shiny leaves on a cloudy day, you’ll understand that without the polarizer what the camera sees is a highlight reflection of the sky on the leaves. And where the camera records a highlight, it’s not recording detail or color. Eliminate the highlight reflections and, by default, you’ll see the color and the detail on the leaves come through.
The same goes for blue skies, in fact. The reason a polarizer is so good at darkening a blue sky is because it’s eliminating light-scattering reflections from water vapor and other particles in the atmosphere. Eliminate that glare, and the deep blue of the sky shows through.
This principle works in practically every picture, of course, sunny day or cloudy day, landscape or architecture. Glare is almost everywhere. (I wouldn’t use a polarizer on a portrait, however, because some of that shine on skin is important to make sure your subject looks alive, but otherwise….)
Eliminate glare, and you’ll reveal what’s behind it—the color and the detail of a subject. Look at the example here, a normal sunny day. The sky gets a bit deeper blue with the polarizer, sure. But look at how the color and the detail in the trees and water come through, as well. It’s a great use of the polarizer, and the reason why it’s a filter you should consider using anytime you want to boost color and detail in your pictures.