Expert: John Oliverio - 8/12/2004

Question
Hi John,

Sorry I wasn't specific enough with my original question.  I know that the circular polarizer is to accomodate some AF systems.  But what I wanted to ask was how does it do that?  In other words, how does a regular polarizer work differently than a circular one?

kind regards,

Sam

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Followup To
Question -
Hi John,

Could you explain the difference between a circular polarizing filter as opposed to a "regular" polarizer?

thanks for your time.

Sam
Answer -
Samuel,

Circular polarizers were designed to accomodate the auto-focusing systems of early 35mm AF SLR's.  There is no difference in the effect they have on your image.  If you aren't sure which one your camera uses, just play it safe and get the circular.  

John

Answer
I found this explanation on DPfwiw:

Circular polarizers (CPs) are 2-layer affairs—a linear polarizer (LP) backed with a special birefringent optical layer known as a quarter-wave plate (QWP). The QWP is bonded to the LP with its fast optical axis oriented 45° to the LP's passing axis. In this configuration, the QWP transforms the linearly polarized light emerging from the outer LP layer into circularly polarized light, which to the polarization-sensitive metering and auto-focus components found in many modern SLR cameras is indistinguishable from unpolarized light.

John