Circular polarizing filter
From The Glossary of Digital Photography
One of two types of polarizing filters. The other is known as a linear polarizer. Circular polarizers are needed for cameras with automatic focus or through-the-lens metering. Even though circular polarizers are more expensive and not generally as effective as linear polarizers, they are required for most cameras. Linearly polarized light will not work well with automatic focus or through-the-lens metering.
