"There are 2 types of polarizing filters: the linear and the circular. This does not describe their shape or the light they filter away. Photographically, they produce the same effect. But the linear polarizer should not be used with "through the lens" (ttl) metering or auto focus. A linear polarizer filters away light polarized in a certain direction and that's it. Ttl metering and auto focus systems are fed with light through a beam splitter, and if the "direction" of the light does not allow or reduces the amount of light picked up by the splitter, your metering and AF would be off. The circular polarizer on the other hand not only does what the linear polarizer does, it also has a special scrambler plate that mixes the light back again so that the light is more or less distributed evenly in its "direction" (ie. it de-polarizes the light!). This secondary process would allow the ttl metering and AF to work optimally."