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Thread: Question about Polarizer Filters

  1. #1
    Cleophas
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    Default Question about Polarizer Filters

    Hi ppl, I've only recently learned that there are two kinds of them out there. The Circular kind and Linear. How can I tell if what I have is which just by looking at it? I got a Hoya one bought around 93 but it doesn't state which one it is. I used it with my manual system but now that I've switched to AF, I read that iit's better to use a Circular one.
    Anybody care to shed some light on this? Thanks.
    Last edited by Cleophas; 2nd September 2002 at 04:58 AM.

  2. #2
    Midnight
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    Here is an easy way to test whether a polarising filter is circular or linear:[list=1][*]Find a mirror.[*]Hold the polarising filter with the rear threads facing away from you, i.e. hold it "backwards".[*]Look through the polariser with one eye and observe the reflection of the polariser itself in the mirror.[*]If the reflection of the polariser (as seen through the polariser) is black/opaque (or nearly so), then it is a circular polariser. Confirm this by flipping the filter around and repeating the test; this time, the reflection of the polariser should be clear/translucent. The difference should be very obvious.[*]If the reflection of the polariser appears clear no matter which way you look through it, it is a linear polarising filter.[/list=1]Btw, if I'm not mistaken, all Hoya polarising filters will be marked specifically as "PL-CIR" if they are circular polarisers. Otherwise, it is probably a linear polariser.

    It is true that a linear polariser may mess up the autofocus system of SLR cameras due to their beam splitter design, and circular polarisers are designed to overcome this limitation. If your polariser does turn out to be a linear one, try taking a few test shots using AF on subjects at various distances and see how the results are like.

  3. #3

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    i though the difference is between one can turn and the other is fixed?

  4. #4
    Midnight
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    Originally posted by MaGixShOe
    i though the difference is between one can turn and the other is fixed?
    Nope, there wouldn't be much use if you can't vary the polariser orientation, right? Both types of polarising filters can be rotated.

    The only difference is that a circular polariser consists of a linear polariser followed by an additional quarter-wave plate that converts the resultant linearly polarised light into circularly polarised light. Circularly polarised light more closely resembles ordinary unpolarised light and it has been verified that such light will not interfere with SLR autofocus systems. This is the only technical difference, and generally a linear polariser will suffice for compact cameras and non-autofocus SLR bodies.

  5. #5
    Cleophas
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    Whao, thanks alot Midnight!! Now I can find out for sure. This forum is really helpful

  6. #6

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    hmm all i noe is the cir. is for AF cam and the linear for non AF cam
    and the latter is cheaper
    but appearance wise, i duno how to tell them apart

  7. #7
    Midnight
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    Originally posted by MaGixShOe
    hmm all i noe is the cir. is for AF cam and the linear for non AF cam and the latter is cheaper but appearance wise, i duno how to tell them apart
    See my post above for the 'mirror test' to distinguish circular and linear polarising filters. Pretty cool when you try it out for the first time.

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