Polarizers: fact vs fiction? I recently added an auto focus camera (Minolta) to my gear and I bought a circular polarizer (Heliopan) to go with it. It was costly and the booklet that came with it said it effects were not as strong as a linear polizer. I often shoot art, painting that are varnished and I polarize the lights and camera to contol the reflections. What will happen if I use a linear polarizer with the auto focus? Will the auto focus still work? I use an off camera flash meter for this application so metering is not a factor. I have gotten conflicting theories from various people. Thanks Stephen

-- Stephen Mitchell (mitmad@telus.net), January 11, 2002

Answers

Maybe. It depends on the orientation of the polarizer. The AF sensors are behind a beam splitter, and a beam splitter is a form of polarizer. If the polarizer in front of your lens is oriented approximately perpendicular to the beam splitter's polarization, it will cut off light to the AF sensors. If it's oriented approximately parallel to the beam splitter's polarization, AF will work fine.

But I'm curious about why a circular polarizer would have less effect than a linear polarizer. I don't believe that to be the case, all else being equal (though polarizers DO vary in quality, so I'll readily believe that one polarizer is more effective than another).

A circular polarizer is simply a linear polarizer with a quarter-wave retarding plate behind it. The linear polarizer is what does the filtering, and this is the same regardless of what type of polarizer is used. Adding the quarter-wave retarder behind the linear polarizer should not lessen the effect.

For more theoretical discussion, see http://www.cliffshade.com/dpfwiw/polarizer.htm

I'm further curious why one would use AF to photograph artwork, since it's presumably static, but that's another issue entirely.

-- Richard Cochran (rcochran@lanset.com), January 11, 2002.

Hello Richard,"Using auto focus for art work", with all the polarizers the light gets very dim and low contrast so focusing is less than easy. Plus my eyes are not as sharp as they once were. Thanks for the reply. Stephen

-- Stephen Mitchell (mitmad@telus.net), January 11, 2002.

You can use a linear polarizer with any camera - the film doesn't care if its a circular polarizer or linear. As long as [i]you[/i] can assure that the subject is in focus and the scene is metered correctly, you'll be ok. (with most cameras, only the spot meter and AF sensor are affected by the wrong type of polarizer)

-- Dave Herzstein (dherzstein@juno.com), January 11, 2002.