Why don’t two circular polarizing camera filters go dark when crossed at 90°?
Asked 10/16/2020
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I expected two polarizers stacked together to block almost all light when one is rotated 90° to the other, like polarized sunglasses do. But when I stack two camera polarizing filters and rotate them, the light only drops a little.
Does this mean the filters are defective, or is this normal for some photographic polarizers? I’ve also seen demonstrations where a polarizer goes dark against an LCD monitor when rotated. How does that relate to camera polarizing filters, and can any photo filter polarize “100%”?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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The bulk of polarizing filters these days for photographic purposes are so-called "circular polarising filters", in brief CPL. They consist essentially of a proper linear polariser and a "quarterwave plate" that "scrambles" the polarised light in order to make it mostly unpolarised again. The reason is that parts of camera optics particularly in SLR and DSLR, like beam splitters, phase-based autofocus, separate exposure meters, analog TTL flash metering will fail to work reliably with polarised light.
The quarterwave plate is not perfect but good enough for those purposes. As a result, CPL filters have a "polarised" and an "unpolarised" side in a manner of speaking, with the polarised side turned to the scene and the unpolarised turned to the camera.
For any purposes intended to try blocking light, you thus need to place CPL filters with their front sides facing each other or with the front side turned to (say) a laptop screen.
If you instead use the backside, the polarised light will first get depolarised by the quarterwave plate and then half of it gets blocked by the polariser plate. Since "depolarisation" is not perfect except for a single wavelength of light, you'll get various hues by rotating the filter then, but no blockage. Orienting it the other way should give you the expected blockage on certain angles.
Originally by user95069. Source · Licensed CC BY-SA 4.0
user95069
5y ago
0
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This is usually normal if your camera filters are circular polarizers (CPLs), not simple linear polarizers.
A CPL contains two parts: a linear polarizer plus a quarter-wave plate. The linear part polarizes the light, and the quarter-wave plate then converts it so the camera’s metering/autofocus systems work properly. Because of that extra layer, two CPLs stacked together often will not behave like two polarized sunglasses and may not go nearly black at 90°.
By contrast, two linear polarizers can extinguish light much more strongly when crossed. An LCD monitor also emits polarized light, so rotating a polarizer in front of it can make it appear very dark; that’s a different test and doesn’t necessarily mean a CPL is defective.
So: failure to go black when stacking two camera polarizers does not by itself prove they are bad. If you want the classic crossed-polarizer effect, use linear polarizers for the test. No real filter polarizes perfectly; there is always some leakage, though good filters usually have strong extinction.
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