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Photography Question 
Roger Galburt

Polarization Filters

Can anyone enlighten me about the use and difference between circular and regular polarization filters. When is one better than the other?

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8/31/2002 7:56:58 PM

John A. Lind
BetterPhoto Member Since: 9/27/2001
The "regular" polarizers you refer to are called "linear" polarizers. A "circular" polarizer is actually a linear polarizer with a "quarter wave plate" added to the back of it. Only the linear polarizer part of a circular visibly affects light and the work exactly the same in modifying light for making the photograph.

"Circular" polarizers became necessary when single lens reflex camera designers began using "beam splitter" mirrors in them. These mirrors are used to pass some light through the mirror to light meter and/or auto-focus sensors mounted behind the mirror. Before these designs, the SLR mirror reflected all the light up into the prism and light meter or AF sensors were in the prism housing. There are some technical advantages to having them behind the mirror. A beam splitter mirror is also called a "half-silvered" mirror. The method for half-silvering a mirror effectively makes it a linear polarizer. The combination of a linear polarizer on the lens and a half-silvered mirror in the camera can cause problems for the light metering and/or AF sensors behind the mirror, especially light meter sensors. The quarter-wave plate added to the back of a linear polarizer that makes it a "circular" polarizer fixes this problem.

The "why" gets rather technical about what linear polarizers do and how a quarter-wave plate added to the back of one works. The light we see around us is usually randomly polarized. Only light of a specific polarization is passed through a linear polarizer. If two linear polarizers are placed back-to-back and one of them rotated until they are both aligned in the same orientation, the first one does all the light blocking and the second passes through all the light that reaches it. If one of them is rotated out of alignment with the first, the second one begins to block more light. At a 90 degrees orientation to each other, all light is blocked.

This is the problem created by using a pure linear polarizer on a camera with a half-silvered beam-splitting mirror. The linear polarizer on the camera lens is rotated for the effect it has on the image to be photographed without any regard to alignment with the orientation of the polarizing done by the half-silvered mirror to perform its beam-splitting. The result is the mirror blocking more light than it should. Any metering done using sensors behind the mirror will result in overexposure. If enough light is blocked an AF sensor will not have enough light to function properly.

Proper light meter and AF sensor operation behind the mirror in a camera design with a beam-splitting mirror assumes light striking the mirror is randomly polarized for proper sensor operation. The quarter-wave plate added to the back of a linear polarizer solves this problem. All the "filtering" for the photograph is done by the linear polarizer in front. The light that passes through it is now linearly polarized in a specific orientation. The quarter-wave plate circularly polarizes this light. Circular polarization is better described as a "corkscrew" effect with polarization orientation continously rotating. To a linear polarizer, circularly polarized light is affected the same as randomly polarized light; equal amounts in all orientations. Because of this, the proper proportions of light are reflected into the prism housing and passed through to the sensors by the beam-splitting mirror.

Bottom Line:
They both affect light exactly the same way for making a photograph and are both used exactly the same way. If your camera does not have a beam-splitter to divide the light into portions for the prism/viewfinder and sensors behind the mirror, you can use either one. If your camera does have a beam-splitting mirror with sensors behind it, you must use a circular for the sensors, especially light metering ones, to work properly (i.e., as designed).

-- John

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9/2/2002 3:24:56 AM

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