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In over 30 years of making optical filters, Ira Tiffen created the Pro-Mist, Soft/FX, Ultra Contrast, GlimmerGlass, and others, netting him both a Technical Achievement Award from the Academy of Motion Picture Arts and Sciences and a Prime-Time Emmy Award. Elected a Fellow of the SMPTE in 2002, he is also an Associate member of the ASC, and the author of the filter section of the �American Cinematographer Manual.�

Polarizers can produce real imaging magic, if you understand how they do what they do. As a film or video maker, you must know how to manipulate light in so many ways. Usually, it�s a matter of illumination and exposure, light and shadow; hue and saturation, hard light and soft. But, what happens when the issue is reflection? The options dwindle rapidly, often leaving only one solution and means of control: the polarizer.

The polarizer is a unique optical entity, treating light, as it does, by how it vibrates. Not by the wavelengths that define its color, but by the direction in which it vibrates. Now, most light we see, and that we record, vibrates in all directions, 360 degrees around the axis of its travel. Polarized light, by definition, vibrates in only one plane, one direction. Why is this difference important?

It happens that light, which reflects from many surfaces, becomes polarized in doing so, at least in part. Let�s look at blades of green grass in bright sunlight. The grass is green because, first, there is green in the sunlight illuminating it. Second, it is green because it happens to have the property of reflecting green and absorbing all other colors. Without both of these characteristics, the grass won�t appear as what we consider green. That said we know that the light that reflects as green tends to remain unpolarized, in this case from the grass, and also from many other surfaces. So far, fairly intuitive, about what you�d expect. Until we look a bit further�

When the light intensity is of sufficient strength, as in bright sunlight, there is a secondary reflection of interest. This is light that reflects off the very surface of the object, in this case the blades of grass, without ever being converted to green, retaining its original (sunlight white) coloration. We call this �reflected glare,� which often is polarized. Two overlapping reflections, one green, the other white; the former un-polarized, the latter polarized. What is the result of this mix? The white reflected glare washes out the colored light, reducing its saturation. Deep green grass appears paler than it is.

Here�s where the polarizer can work its magic. Mounted on the lens, in a manner that allows for rotation, it filters the light entering the camera. The grass-green light vibrates in all directions, so that no matter how the polarizer is rotated, it will get through. The reflected glare, however, is another story.

Rotating the polarizer, so that its polarization axis is perpendicular to the direction of vibration of the glare, allows the filter to absorb most, if not all of it. You can actually see this effect through the filter. Once removed from the image-forming light, the green grass can record as deeper, more saturated in color. Truly saturated color in bright sunlight without having to paint it all in post. An otherwise difficult feat of prestidigitation, made easy with a polarizer.

Now the true �imagician� also knows that the trick doesn�t function without setting the proper stage. You have to know how to make the magic work. With polarizers, that largely involves understanding reflection angles.

For example, let�s look up at the sky. Often blue, our view of it is filled with myriad water droplets in the atmosphere. Each is a reflecting surface, producing glare that washes out the true blue of the sky. This, too, is a job for the polarizer.

Here, the important factor to remember is that the angle of the sun determines how strongly the glare is polarized and how much saturation you can restore to the sky. The basics: keep the sun at your side; the closer you are to aiming directly at or away from the sun, the less the effect of the filter. In addition, you can readily determine the area of greatest effect by making a right angle with your thumb and index filter. Point your index finger at the sun, and rotate your thumb around its axis to point out the band of deepest blue.

For example, at the equator at noon, with the sun directly overhead, the band of deepest blue is all around just above the horizon. At dusk or dawn, facing toward the sun on the horizon, deepest blue is in a band from your left side to your right, forming an arc straight above you. In providing dramatically bluer skies (if the sky is blue to start with), the polarizer will also bring out white clouds in greater detail, without any change in their color. Two caveats: be aware that using a wide angle lens may show enough of the sky to make this band visible; also, be careful when panning so that you don�t find the sky going from lighter to darker as you change your camera�s angle to the sun.

But wait, there�s more! Reflections occurring from water and windows can also be reduced or removed. Don�t aim straight at the window, for instance, but from an angle of about 33 degrees off to the side. Again, you can see the effect through the filter. Using this method, you can reduce distracting glare from a window to allow a clear view of what�s on the other side; see further beneath the surface of a lake; provide a good view of the driver through a car windshield; and much more.

Some things to remember. You may not want to remove all reflected glare from, say, a pond, so that the frog you are filming can retain some of its watery context. Remove all of the reflection, and it may not appear to be in water at all. Rotate the filter until you get just the effect you want. Same is true of bluer skies�sometimes it isn�t desirable to go all the way.

As for the car windshield, if at first you find that the filter doesn�t give you a wide enough effect, move further away and zoom in to retain framing. This will increase the area of the windshield surface that is closer to the best reflection angle, improving the glare reduction from left to right.

Another thing to know: there are two key types of polarizers, linear and circular. Having nothing to do with their shape, the terms refer to how polarization is handled. Linear polarizers are the traditional types that transmit light vibrating in only one direction. Circular polarizers are linear polarizers with an added clear layer known as a retarder, which is on the side of the filter facing toward the lens, and effectively de-polarizes the light again before it enters the camera.

Many camera designs have internal optics that polarize some of the light, and which cause adverse effects, such as loss of autofocus and/or accurate metering, with linear polarizers. So for these types of camera systems (and your camera manual or dealer should be able to tell you which ones they are) you should use the circular polarizer. When in doubt go circular.

Remember to mount the filter in the proper orientation, with the retarder facing the lens. Filters with threads only go on in one way. Square filters and round Series sizes without threads should be labeled by the manufacturer to show the proper orientation. It�s simple: if you don�t see the polarization effect, the filter should be re-mounted facing the other way.

Finally, polarizers are available with different exposure compensation requirements. The traditional polarizer requires about one and two-thirds stop compensation. More recently, filters needing only about one stop are being offered. They may not be quite as polarization-efficient, but they are worth knowing about when lower light levels call for it.

And now, you are readier-than-ever to make some imaging magic of your own.

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Check out this article in the November 2006 print edition of StudentFilmmakers magazine, page 8.