Light rays which are reflected by any surface become polarised and polarising filters are used to select which light rays enter your camara lens.
• They allow you to remove unwanted reflections from non-metallic surfaces such as water, glass etc.
• They also saturate colors and increase contrast.
•Polarizers come in two variations but have the same effect. Linear polarizers work with older camera meters; Circurlar polarizers are designed for today's newwer slpit-beam metering systems used in auto focus cameras. Check your camera's manual.
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As per a Popular Photography article the combination of the LIN polirizer with the CIR polirizer on the lens First enables you to get the effect of a variable ND filter,on DSLR cameras. The use of a lens shade is restricted because you need to rotate the Lin POL without disturbeing the CIR polirizer