Tuesday, July 1, 2008
Gadget Bag: Polarizers
If there ever has been a single piece of gear that all nature photographers agree is indispensable, it’s the polarizer
When light reflects off the surface of an object, it scatters in an infinite number of directions. Objects that have multifaceted surfaces—a diamond or a ball of aluminum foil, for example—encourage light to scatter randomly. Relatively flat surfaces, like water and walls, tend to reflect a fair portion of the light waves along more or less parallel paths. For the sake of simplicity, we can equate these parallel waves to the glare commonly reflected from the surface of water or a glossy, painted wall. Although the light waves are moving along similar paths, their energy is scattered perpendicular to their axes.
The popular metaphor used to explain the process involves a picket fence and a jump rope. If you vigorously whip a jump rope that’s tied to a distant tree, the wave created by the length of the rope will be entirely random. But if the rope is threaded through a picket fence, the motion will conform to the shape defined by the two pickets through which the rope passes. In other words, whip a rope through a vertical fence, and the rope can only move up and down, regardless which direction you’re shaking the rope.
If you accept this slightly flawed lesson in physics, you immediately realize one or two other things. First, if the fence is horizontal instead of vertical, the rope will move from side to side rather than up and down. In the case of polarizing filters, they’re constructed using two stacked rings. One ring is threaded and attaches to the camera lens; the other ring holds the glass containing the microscopic (and perfectly aligned) particles that polarize the light that passes through it. Rotating the outer ring is analogous to turning the picket fence on its side.
Polarizers are famous for darkening blue skies and making white clouds pop. Aside from postprocessing in Photoshop, the only way to enhance and darken a sky without unduly influencing other parts of an image is by using a polarizing filter. Strength and effect are adjusted visually by rotating the filter while observing the image in the viewfinder or on the LCD monitor.
To find the area of the sky that will be most strongly affected by a polarizer, make an “L” shape with your index finger and thumb and point it at the sun as though you were shooting a rubber band into space. (Your thumb doesn’t have to be perpendicular to the ground—you can rotate your hand through an arc, just keep you finger pointed at the sun.) If your index finger is aimed at the sun, your thumb will be pointed toward the portion of the sky where a polarizing filter will deliver maximum results.
There are several types of polarizing filters on the market. The most common and generally least expensive is the linear polarizer. This was the most popular type until 1985, when a generation of autofocus SLR cameras that use beam splitters arrived on the scene. That technology required a circular polarizer—the type that reigns supreme today. If you use a linear polarizer where a circular style is required, the result may be incorrect focus and/or improper exposure.
Mainly because of a property called birefringence, it’s possible to change the colorcast produced by certain specially made polarizers during rotation. These usually offer a range of hues between two complimentary colors, for example blue/yellow or red/green. Examples of this type of polarizer are Cokin Varicolor filters, which are available in the above combinations, plus pink/orange, blue/lime and red/blue. It’s also possible to adjust the intensity of one single color. Cokin offers red, blue and yellow Polacolor filters that shift from normal polarization effect to polarizer-plus-color as you twist.
Polarizers are more expensive than many other filters, and because the stacked-ring construction makes them thicker than most, they can cause vignetting when used on some ultra-wide-angle lenses. You can defeat this problem by using a filter system that consists of a filter holder and a threaded adapter for the lens. With this type of system, if you have several lenses with different-sized filter threads, you simply buy different inexpensive adapters instead of buying a dedicated screw-in filter for each.
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