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When I bought my polarizer, a friend told me that I should get a circular one, because the linear ones can mess with the autofocus. Is this true? What should each be used for?

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3 Answers 3

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A polarizer works in the way that it will let through only the light that is polarized in the same direction as the filter is currently turned. It is true that many AF systems have problems with this. To solve this, circular polarizer have a layer "inside" the polarizing filter that "un-polarizes" the polarized light so that the AF can function properly. The linear polarizer does not have this extra layer. The wikipedia article explains this quite well, I think.

As far as I know, there are no cases when you would want a linear polarizer instead of a circular one. Anywhere a linear polarizer will do the job, the circular one will work just as well, but not the other way around.

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The inside layer doesn't unpolarize the light — it just polarizes it in a strange, non-intuitive way. –  mattdm Mar 6 '11 at 15:43
    
The terminology used on photo sites is all messed up. What they call a "circular polarizer" is in fact a linear polarizer stacked with a circular polarizer. The second layer is not a de-polarizer as your post would suggest. –  Szabolcs May 23 at 13:46

Yes, it's true - sometimes. Some AF systems work OK with a linear polarizer under some or all conditions, and others fail all the time. You'd have to try it and see.

When would you want a linear polarizer?

  • They are often cheaper.
  • Since there's only one polarizing layer, not two, light transmission is often greater.
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Because many AF and metering systems have problems with polarized light, circular polarizer has an added layer which "un-polarizes" the light that comes out. Therefore you should only get circular polarizers in usual cases.

The exception is when you want to use two polarizers to create a tunable ND filter. In that case, the front polarizer has to be linear, and then you can control the darkening effect by adjusting relative position of the two polarizers.

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The inside layer doesn't unpolarize the light — it just polarizes it in a strange, non-intuitive way. –  mattdm Mar 6 '11 at 15:44
    
@mattdm: So it like rotates polarization of the light? Because if it did another "round" of polarization, it would further reduce light transmission, wouldn't it? –  che Mar 8 '11 at 10:03
4  
yeah, there's a quarter-wave plate which transforms the polarization of the light wave into a sort of helix shape. It's not another filter layer, so there's no more light loss (other than the addition of more optics). –  mattdm Mar 8 '11 at 12:46

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