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Old Aug 2, 2005, 2:12 PM   #1
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Here are two shots taken with my D100 using a Cokin Circular Polarizer. The only difference between the shots was the orientation of the filter. No editing has been done except rotate and resize for post.






In the first shot, note that the sky is black. In the second shot the sky is normal considering that both images are dark. In the first shot, the view through the viewfinder showed a dark blue sky as one might expect with a polarizer but the camera recorded a black sky. Looking at the image on the camera LCD, the sky is black.

The lens was a Sigma 28-70mm f2.8 and I had a uv filter attached. The Cokin Polarizer was attached to the uv filter. The Cokin filter is round but is designed to fit in the Cokin P-type filter holder. The only way I can get the filter to fit properly, is to place it with the flange toward the camera body and inserted into the thin slot closest to the camera. This places the printing on the filter toward the camera. I am concerned that this is backward. From my understanding of circular polarizers is that they have a linear polarizing layer in front, and another layer that I don't understand behind it. It seems to me, therefore, that if a circular polarizer is oriented the wrong way, it might not work properly.

Regardless, the Cokin filter works about a hundred times better than the Tiffen circular polarizer I was previously using.

Anybody have any idea what is going on here. Is the filter inserted into the holder wrong? Does the CCD have a polarizing filter in front of it? Any other possibilities.

I was after the effects of the polarizer on the reflections in these shots and it produced some spectular results but it was midday with bright sunshine! We generally don't have black skies under those conditions here in Portland!

Cal Rasmussen

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Old Aug 2, 2005, 9:39 PM   #2
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Technically, I don't much get it myself, but checkout this excerpt from the most extensive article on polarizers that I've seen:

How to Tell a CP from an LP—the Mirror Test Polarizers aren't always clearly marked, but thanks to an RPD tip from Dennis Duke, it's easy to tell whether a polarizer in hand is circular or linear. All you need is a mirror:
Look in a mirror through the polarizer—once with the male threads toward you and then with the threads flipped toward the mirror. If the flip makes no difference, the polarizer's an LP. If the flip causes the polarizer to turn black or nearly so, it's a CP.
How black the CP turns is probably an indication of its quality, particularly with regard to the spectral tuning of its QWP layer.
Dennis also notes that most polarizers not marked as circular are linear. That jibes with my limited experience with a CP and LP from Tiffen.

This pretty much describes the phenomenon that you experianced.
Here's the link for the whole page:

http://dpfwiw.com/polarizer.htm#circular

This'll tell you more than you ever wanted to know.
Grant


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Old Aug 3, 2005, 8:57 AM   #3
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I read the article you linked to and I also found a Cokin website that illustrates the proper way to mount the filter. There is only one way it will fit into the holder and that is how I have been doing it.

The Cokin site does mention that if there is a uv filter on the lens, it should be removed before attaching the circular polarizer. It also mentions that additional Cokin filter elements can be mounted in the holder in front of the polarizer but nothing between the polarizer and the lens.

I am going back to the location where I shot those pictures and try it without the uv filter and see what happens. The uv filter may be creating reflections that are interfering with proper operation of the polarizer.

I will report back on the results.

Cal Rasmussen

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