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Old Jul 21, 2006, 7:22 PM   #1
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I'm having a problem understanding how a Circular Polarizing filter works. Can someone with experence with these filters try to explain how they work. I have used a linear filter before but the circular rotates and I don't understand why. Thanks
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Old Jul 24, 2006, 10:40 PM   #2
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From a photographers point of view they work identically except that a linear polariser will not work very well with an auto-focus system. The difference has nothing to do with a circular shape or rotating filter.
A circular polariser produces circularly polarised light (think of the light-wave as a corkscrew coming towards you - you will see a circle) wheras a linear polariserer produces, surprise, surprise, linearly polarised light (think of looking at a sine wave end-on - you will see a line). The circular polariser is comprised of a linear polariser which does the polarising plus a second layer that turns the linearly polarised light into circularly polarised light.
Auto-focus systems work by splitting the light using a half-silvered mirror and then recombing it. When the autofocus system gets it right the split image combines nicely back into one.
With linearly polarised light think of a vertical line, which is what your autofocus will see with the sine waves coming head-on. Split this in two so that you have two images of the line, and then try and reflect the two back together. It will be quite clear when the two lines are on top of each other and this will mean that the image is in focus. No problem. But what if the light isn't polarised in that direction? If it was at 90 degrees you would have a horizontal line. Split this in two and instead of two parallel lines you now have two overlapping lines sliding along each other. Adjusting focus will make this long line shorter or longer. Focus is achieved when it is at its shortest as now the two images of the line are directly on top of each other. This is much harder to do accurately than with the vertical line where it was either obviously two lines (out of focus) or one (in focus). For this reason, autofocus will not work properly with linearly polarised light when it is polarised at certain angles.
With the circular polarised light the autofocus sees the light like a corkscrew head-on, giving a circle instead of a line. Split your circle and recombine for focus. Unlike the line, the circle is the same no matter whether it is vertical or horizontal or at any other angle, and so auto-focus will always be able to work.

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