can polarizing filters work as ND filters?

Discussion in 'Photography Equipment & Products' started by cgennoe, Jun 22, 2008.

  1. cgennoe

    cgennoe TPF Noob!

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    Hey!!

    I read in a magazine today that my circular polarizer filter can also work as a ND filter to the affect of two stops worth of light. Is this true?

    Thanks a million!

    Colin
     
  2. Mav

    Mav TPF Noob!

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    sorta. They'll work more as a grad-ND than anything, and you'll usually lose around 2-stops overall.
     
  3. cgennoe

    cgennoe TPF Noob!

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    thanks! If i understand you correctly, they'll work similar to a ND gradient filter by allowing light to pass unaffected on one half (useful for the foreground in a sunset) and reduced by two stops on the other half (to reduce the relatively bright exposure of the sky in a sunset)?
     
  4. Steph

    Steph No longer a newbie, moving up!

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    Not really. A polariser will affect the entire image. Have a look here to see the effect of a polariser or use Google to find other examples.
     
  5. Mav

    Mav TPF Noob!

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  6. Garbz

    Garbz No longer a newbie, moving up!

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    I think you are talking about different things. Polarisers may effect different polarisations of light depending on the incidence angle, and this would produce a gradient effect in the sky for instance where one area would be darker than the other.

    What I think the magazine and the OP are talking about is the fact that halfwave quarterwave polarisers like the Circular Polarisers we stick on our lenses have a 1-2 stop insertion loss. That is regardless of polarisation you lose 1-2 stops of light evenly across the image. This is due to the non ideal nature of this type of polarisers and would behave like a standard ND filter. The downside is it also polarises the light which may not always be what you want.
     
  7. Helen B

    Helen B TPF Noob!

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    Just to elaborate on Garbz' post:

    An ideal polarizing filter, whether linear or circular, would act as a 1-stop ND filter for 'unpolarized' (randomly polarized) light. That's because it stops light that is polarized in a certain direction, and randomly polarized light will have 50% of its energy in any given direction of polarization.

    Anything more than one stop loss of randomly polarized light is because the polarizing foil is less than perfect. It could also be less than one stop, but that would be unusual.

    If you mount a circular polarizer the wrong way round it will lose a lot of its effectiveness as a polarizer, and act as mostly ND. It will, however, send polarized light to the camera, which may fool the exposure meter and/or the autofocus.

    You can make a variable ND filter with two polarizers: two linears (if your camera is OK with polarized light); one linear and one circular; or two circulars with the first circular the wrong way round.

    Best,
    Helen
     
  8. Overkill-F1

    Overkill-F1 TPF Noob!

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    Helen, I haven't tried it, but I was lead to believe if you stack two polarizers and rotate them to the right (wrong?) position, the light is almost stopped by the filters. I didn't think one of the filters had to be reversed. I'm referring to linear polarizers.
    ...Terry
     
  9. Tiberius47

    Tiberius47 TPF Noob!

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    My polarising filter acts as a neutral density filter, cutting two stops of light.
     
  10. Steph

    Steph No longer a newbie, moving up!

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    A polariser will cut the amount of light entering the lens (the amount is variable) but the effect of the filter is not really neutral. Polariser and ND filters are not the same thing and are not always interchangeable.
     
  11. Helen B

    Helen B TPF Noob!

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    That's right. It's only if the first polarizer is a circular that you should reverse it for maximum effect (I hope that was clear from my post). Two circulars often work when both are the right way round, but the combination is likely to have a colour cast, and it won't achieve the greatest possible density when crossed. A circular polarizer is effectively a linear polarizer followed by a depolarizer. It's difficult to get 'ideal' performance from the depolarizer (a quarter-wave retardation plate) because it is wavelength dependent.

    A backwards circular polarizer followed by a forwards circular polarizer acts like a variable ND filter (even though it may not be truly neutral) with little polarizing effect. About 8 stops of ND is usually possible.

    Edit: A linear polarizer doesn’t have a forward direction – it doesn’t matter which way round it is because it has only one active component, the polarizing foil. A circular polarizer does behave differently when used different ways round because it has two active components (a linear polarizing foil and a quarter-wave retardation plate) that do different things, and the order in which those things are done makes a difference.


    Best,
    Helen
     
  12. 250Gimp

    250Gimp TPF Noob!

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    So..If I understand correctly Helen B, I can use a CPL on the lense with a cheaper linear polarizer stacked on top to get an effective max. 8 Stop ND, or does it have to be to CPL's to get that?

    Cheers
     

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