Quote:
Originally Posted by dvdowns
Why the heck does a circular polarizer need to be rotated ? how is it made ?
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You have to rotate the CP because it's still a linear polarizer at heart, and the polarizing effect depends on how it's oriented.
A circular polarizer has 2 layers. The first layer is the polarizer material and is
exactly the same material as in a linear polarizer. The magic comes in the second layer, called a
quarter wave plate, which takes the output of the linear polarizer layer and applies a twist to the wavefront so the light reaching the film (ahem, or sensor) is circularly polarized.
Most autofocus sensors, and some lightmeters, use prisms to sample the light which themselves act as polarizers. If you use a linear polarizer with such cameras, it's possible to have the LP "crossed" with respect to the polarization of the AF or meter -- with the result that the camera's electronics may see the light correctly or not at all. Adding the quarter wave plate ensures that the camera's bits will always see the same amount of light regardless of the orientation of the polarizer.
Circular polarizers give exactly the same results as do linear polarizers, and a CP can be used with any camera.