Is it a Photoshop plug-in?
Nope. It's a lens screw-on--a polarizing filter. Yes, a good, old-fashioned polarizing filter! While Photoshop, usually with the help of a plug-in or two, can emulate the look of a polarized sky, it just can't see through reflections. Polarizers can.
To prove this simple point, I went out one early autumn day, armed with my Canon 20D, 18-55mm f/3.5-4.5 lens, and a B+W Polarizing filter, to photograph some picture-perfect puffy little clouds.
I drove to a local park, and the first thing I did when I parked in an open area was to shoot through the front windshield. The glass reflected some of the sky and clouds, and didn't reveal much of the interior (see photo above left). Then I put the Polarizing filter on (above right), and turned it until many of the clouds dissapeared from the window and I could see into my car.
How do polarizers work? For a quickie technical overview, I quote my colleague, Jim Bailey: "Polarizers work by capitalizing on the fact that light vibrates in waves. Most of the time, light vibrates in all directions, but under some circumstances much of the energy in a beam of light is forced to vibrate predominantly in only one direction. This is called polarization.
Polarizing filters are made to transmit light that’s vibrating in one direction only. By rotating the filter you can select which portion of the scene’s light will darken or lighten."
And, a polarizer removes glare and reflections from shiny surfaces. This happens to a greater or lesser degree, depending on your shooting angle in relation to the reflective surface and light source, and how you've turned the filter.
Field comparison: Photoshp vs. Polarizer
Other than reflections, can Photoshop mimick the "polarized" look? Let's find out!
Near my car was a softball diamond, already being reconfigured for football season, with a reddish fence and blue garbage can that I felt set off the dramatic sky. (Unfiltered scene is at right.) A bit of polarization would make the clouds and sky contrastier. Based on experience, I knew that I should shoot away from the sun to get the most pronounced polarization effect. If I'd shot the southeastern sky that morning, I would not have seen much, if any, polarization effect.
And so I shot the scene twice--first, without any filtration, then with the polarizer. It less than a minute to screw on the polarizer, increase the exposure by two stops (since the neutral density of the filter reduces light transmission) and shoot three bracketed exposure (one at the metered exposure, one 1/2 stop more, one 1/2 stop less). Scroll to the end of the article to see the result.
Phony Photoshop Polarizers
Now, let's see what Photoshop can do, and we'll compare the results at the end.
Step 1: Select
In Photoshop Elements 3.0, I used the selection tool to select the sky area, since that's the only area I wanted to change, and a polarizer would leave the non-sky part of the picture alone.
Step 2: Enhance
Then I chose Enhance > Adjust Brightness/Contrast > Levels, and moved the left slider until it was under the beginning of the active area of the histogram. I moved the right slider to the left. This punches up the tonal range of the image, which was a bit muddy straight out of the camera. Then I moved the middle slider to the right until the sky was a few shades darker.
Step 3: Fine-Tune Enhancements
Now the sky looked dramatic, but it was an unnaturally saturated of blue, especially when compared with the true-polarized photo, which I'd opened for a reference. To correct this, I went to Enhance > Adjust Color > Hue/Saturation. I moved the Saturation slider to the left, partially desaturating the sky. This brought the sky back to a more natural look.
After about 10 minutes of experimenting, I was done. OK, so I cheated a bit--I kept the real polarized image open for comparison and adjusted the color until it was pretty darn close. If you don't have a polarizer with you, you won't have this kind of instant reference. There are plug-ins that can more quickly achieve a polarizer look, such as Nik Color Efex Pro.
In general, it's better to have a high-quality image with the look you want right out of the camera than to rely on post-processing on your PC. So, I strongly suggest that you bring a polarizing filter with you. It takes a lot less time to attach one to lens and just shoot then it does to fix the photo after the fact. In my case, it saved nine minutes.
Which was polarized? The image on the left was shot with a polarizing filter. The one on the right was shot without one, then adjusted to emulate a polarized look in Adobe Photoshop Elements 3.0. The differences are subtle. Can you see them?