Polarizer Camera Filters

Polarizer filters are among the most versatile filters that are used. Below are some of the basic functions of polarizer filters:

  1. Polarizer can remove most reflections from nonmetallic surfaces such as glass, some foliage, water, plastic, and fabric colors.

  2. Polarizer filters are the only filters that darken a blue sky and increase contrast between sky and clouds.

  3. As polarizer filter can remove blocking reflections, it can also increase the overall color saturation.

  4. Polarizer filter can reduce overall scene contrast.

  5. Polarizer filter function much like a neutral density filter.

Polarizer filters are made of a myriad of extremely small (microscopic or smaller) crystals oriented like a Venetian blind. The crystals block light waves vibrates at angles other than parallel to the crystals. Light waves vibrating parallel to the crystal array pass between them. Much as the light passing through the slats in a Venetian blind is controlled by changing the orientation of the blind slots, so turning the rotating polarizer in its mount acts to block the polarized light attempting to pass through.
The effect of the polarizer filters on a blue sky is to darken it at certain angles. The most efficient angle is a 33 arc made by the filter and the reflecting surface. As the polarizer filters block much of the light attempting to pass through it, exposure must be increased to compensate for the light loss. Generally, polarizer filters will impose around 1.5 f-stop light losses but this will depends on situation.
Other than darkening blue skies and increasing sky/cloud contrast, polarizer filter can also increase color saturation of many objects by reducing diffuse reflections on the subject's surface. Employing a polarizer does not change or add color; its use allows color to be recorded as a richer, deeper, and unmasked hue, all but free from color-robbing reflections. It may seem strange, in view of the increases in color intensity of polarized subjects, but these filters can actually reduce overall scene contrast, particularly in scenes containing major areas of blue sky. By darkening a blue sky, a polarizer diminishes its brightness. The highlights represented by the sky and any shadows in the scene are brought closer together with a net result of a lower contrast range.
An added benefit of this contrast-controlling technique is that shadow detail can be given more exposure without overly lightening the sky. In essence, polarizer filters act much like a moderate neutral density filter and reduce contrast by about one full f-stop. Polarizer filters can also remove reflections from glass and water surfaces. The amount of surface reflection that is eliminated depends upon the angle of the polarizer to the subject. There are still further uses for these ubiquitous filters. They cut through light and moderate haze by blocking the haze, allowing polarized light to pass through the lens.
There is no one best way to employ a polarizer. The only clear recommendation is not to overuse it. Even though polarized color is often intense, it cannot save an otherwise weak picture. The best way to judge the effect of polarization is by direct observation. To achieve maximum effect, move the camera 90 to the sun. You should be able to observe the filter effect right in the viewfinder as you rotate the filter in its mount.
In black-and-white photography, the filter is used as you would a red contrast filter. Expect significant increases in contrast between the darkened sky and lighter objects in the scene. Polarizer filters are brought into use to control overall black-and-white scene contrast on bright, sunny days by absorbing and reducing secular highlights, thus bringing all light values in a scene closer to one another.
To find the part of the sky that will be most affected by polarization, point your forefinger at the light source with your forefinger and thumb circumscribing a 90 arc. As you rotate your thumb, it will describe an arc indicating the polarized portion of the sky.
 

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