A: It is the one filter whose effects cannot be recreated in post-process.                     What a polarizing filter does:

• Reduces haze
• Removes reflections
• Increases color saturation.  The effect is particularly evident in skies and in foliage.
• Neutral density. The natural loss of 1-2 stops of light, without shifting color, makes a polarizing filter a good ND filter as well.

Notes:

• The effect of polarization (outdoors) is most prominent at a 90-degree angle to the sun.  Make an “L” with your thumb and pointer finger.  Point your thumb at the sun, your pointer is now showing you the direction that the polarization will be most noticeable.
• If you use a  lens <about 28mm (in 35mm film terms) the sky could be unevenly polarized, much darker in one area than another.

What size do you need?

• If you have the Canon Kit Lens (18-55) -HOYA 58CIR 58mm Circular Polarized Filter
• Nikon Kit Lens (18-55) -Hoya 52mm Circular Polarizer and Filter Wallet
• Nikon Kit Lens (18-105) -Tiffen 67mm Circular Polarizer
• All other lenses – Circular Polarizers If you search your lens on Amazon it will show in the Frequently Bought Together section a UV filter that matches your lens diameter, use that to find the correct polarizing filter