Formatt Circular Screw in Polariser 105mm Filter
The circular polariser filter produces the same effect as a linear polariser filter, but is used in cameras where polarizing elements are used in the beam-splitters of the camera. If using a linear polarizer with such cameras, the light can become cross polarised (giving the same effect as using two polarisers together).
Circular polarisers are made from a different polarising material than linear and will usually prevent the cross polarisation.
Note that circular polarisers may only be used one way in front of the lens and are marked accordingly.
See enlargement of photo for examples
Circular Screw in Polariser 105mm Filter