Polarizing screens do three things that are useful to a photographer. They darken blue skies, they remove or reduce reflections from non metallic surfaces such as water and glass, and they penetrate haze.

To understand how polarizing screens work, you need to know a few things about the nature of light. Light rays travel in straight lines. Lights rays also vibrate in all directions perpendicular to their direction of travel. When a light ray hits a nonmetallic surface the vibration in only one direction , or plane, is reflected completely. (All vibrations are reflected by a bare metallic surface.) Depending upon the angle at which you’re viewing the light reflected from an object , vibrations in other planes are reduced or eliminated completely. This reflected light-vibrating in only one plane-is called "polarized light." The light from a blue sky is polarized because it is reflected off the nonmetallic particle in the atmosphere.

A polarizing screen will pass the vibrations of a light ray in one plane. Some polarizing screen have handles, and these screens pass the light vibrations in a plane parallel to the handle. When the polarizing screen is passing the light vibration of polarized light, you’ll see no effect on refections or the sky. Rotate the polarizing screen 90 degrees. In this position, the screen will not transmit the polarized light, so it removes reflections and darkens the blue sky. Polarizing screens will work only with polarized light because polarized light vibrates in one direction and the polarizing screen can eliminate that vibration. If you look through a polarizing screen and rotate it until you see it maximum darkening effect, you’ll see some light still reflecting from the scene. This light is the nonpolarized light in the scene and the polarized light vibrating in the plan that the screen will transmit. You’ll use this light to take the picture.

If you want to get the maximum effect with a polarizing screen, the angle at which you view the reflecting light must equal the angle of the sun (or the original light source) to the reflecting surface. For example, if the sun is shining on water at a 60 degree angle to the water’s surface.

A polarizing screen has a filter factor of 2.5 (increase exposure by 1 1/3 stops.) This filter factor applies regardless of how much your rotate the polarizing screen. In addition to this exposure increase for the polarizing screen, you must make any exposure increases required by the nature of the lighting. For example. for the dark sky effect, the scene must be sidelighted or toplighted, so you’ll need to add approximately 1/2 stop exposure the 1 1/3-stop increase required by the polarizing screen itself.

Allow about an additional ½ stop for subjects that show reflections, because reflections often make subjects look brighter than they are.

The only way you can darken a blue sky in color photography without changing the other colors in the scene is to use a polarizing screen. You’ll get the maximum darkening effect when you’re taking pictures at right angles to the sun and the handle of the polarizing screen (if it has one) is pointing at the sun. You can obtain various effects from light to dark by rotating the polarizing screen, and you can see the effect you’ll get by looking through the screen. After you decide which position of the screen produces the effect you want, be sure to keep the screen in that position when you put it over the camera lens. For example, if the handle was in the 3 o’clock position make, sure it’s in the same position when the screen is on the camera. If you have a single lens reflex camera, you can see the effect through the viewfinder while you rotate the screen.

You can improve color saturation with a polarizing screen because it reduces reflections which desaturate the colors. You can also reduce annoying reflections in scenes that include water, glass or other shiny surfaces. Look through the polarizing screen while rotating it to see how you can control the reflections. As we said before, getting the maximum effect with a polarizing screen depends upon your angle to the subject as well as the rotation of the screen. If you can’t remove the reflection completely, try changing your angle to the subject. Don’t expect t control reflections from bare metal surfaces, because the light reflected from these surfaces is not polarized and the screen will have no effect.