Circular Or Linear Polarizing Filter?

Q. Do I need a Linear or Circular Polarizer?

As a general rule if you have an auto focus camera the manufacturer will recommend that you use a Circular Polarizer.  Most modern SLR cameras use a polarizing half mirror system, this is called a split beam metering system. A circular type Polarizer is required for the meter to function correctly.

Q. My Polarizer has a rotating ring. Is this a circular Polarizer?

Not necessarily, this is a rotating mount polarizing filter. You rotate the Polarizer to achieve the desired amount of polarization.

Q. How do I tell if I have a circular or linear Polarizer?

The most reliable way is to look at the filter ring. It should be marked to indicate that it is a circular type. Look for any of the following markings on the ring: CIR., CIRCULAR. CPL., C-PL 

The Following is information from the THK web site
Light rays which are reflected by any surface become polarized and polarizing filters are used to select which light rays enter your camera lens. PL (Linear Polarizing) and PL-CIR (Circular Polarizing) filters have the same effect, but it is important that you choose the correct version for your camera. They allow you to remove unwanted reflections from non-metallic surfaces such as water, glass etc. They also enable colors to become more saturated and appear clearer, with better contrast. This effect is often used to increase the contrast and saturation in blue skies and white clouds. HOYA's polarizing filters do not affect the overall color balance of a shot.