A polarizer is a filter with a special material to block un-polarized light. It blocks reflections and adds contrast to the photo (e.g darkening a blue sky). On DSLR-cameras, a circular polarizer is needed instead of a regular polarizer.

If you’d like to know more about how polarizers work, see the polarizer article on Wikipedia.

Here’s an explanation provided by listener Boris Nienke: “Light normally has any direction (think of it as a waveform. one ray swings up and down, the next left and right and many rays are swinging diagonal in any degree). The polarizer now filters this light. Only light, that is swinging in a specific direction, can pass the filter. So in the end ALL light, that hits the cam inside, is ONLY swinging up and down for example. Now there are the electronic thingys in the cam to meter the amount of light. And most of these tools are build to meter the normal, mixed, light. Now only light with one direction hits the tool and so it get fooled. The circular polarizer works (first) just like the linear. But after the light is filtered out, the circular filter takes the remaining light and swirls it around so that it is not polarized anymore for the electronic.”

_{A circular Polarizer for (D)SLR-Cameras}

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