Roy I have a CP on my Sigma 10-20mm and to be quite honest with you it is a waste of time. As Rob so rightly said its all about the angle you are to the sun, (optimum effect achieved at 90 degrees) to attain this you have to rotate the filter ring, problem is the ring is buried inside the lens hood which has to be detached to make this possible. O.K. for reducing reflections, but little else.
John
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