Paul Jolly Photography Chapter 18.1

Cross polarization

The use of two polarizing filters on the front of the camera can be used to control the light entering the lens. If the polarizing filters align then the maximum amount of light enters the camera. With the filters rotated with polarizing at 90 degrees the minimum amount of light reaches the lens. This combination appears to be a variable ND filter but the different light wavelengths will be attenuated by slightly different amounts at each setting; reducing the light this way biases the colour to the blue end where the attenuation effect is less making the photograph look colder unless compensated for in post-processing.