Do You Really Need a Circular Polarizer?
Conventional wisdom says you can use a linear polarizer on a manual focus camera, but you need a circular one for auto-focus cameras. However, sometimes you hear of people who have tempted the polarizer gods by using a linear polarizer on their new AF camera and lived to tell about it. And they have the photos to prove it. So how can this be?
Inside most modern cameras is a beam splitter that directs some light to the viewfinder and some to the meter. Based on how it functions, the light that reaches the meter is effectively polarized by the beam splitter. If you add a polarizer to your lens, light gets polarized twice on its path to the meter. If your polarizer is a linear one, it is now working at cross purposes with the beam splitter. The degree to which the angle of polarization differs between these two changes the amount of light that can reach the meter.
A circular polarizer solves this problem by mean of what is known as a "quarter wave retarder" that re-scatters the polarized light making it uniform again no matter what angle the polarizer is turned to.
These days, virtually all cameras made have either semi-silvered mirrors or prisms that act as beam splitters. A few might not though so it isn't strictly an auto-focus versus manual distinction that determines whether you need a circular polarizer. The simplest thing to do is to always get a circular polarizer, especially if you plan to upgrade cameras in the future.
Keep in mind that if you shoot print film, even large exposure errors from this or other causes are often covered up (made up for) by the photo processor when they print your negatives. Stick with a circular polarizer nonetheless to get the best quality results you can.