Originally posted by newarts For practical purposes a CPL can be thought of as a linear polarizer followed by a de-polarizer. They are used with cameras that use mirrors and prisms because polarized light interacts with those elements in ways that depend on the direction of the polarization & can mess up metering, etc.
Which is why the older standard polarizer can be used with older cameras. They're just metering the light coming in without doing any else like trying to focus as well with that polarized light.