Anyone use full polarizers rather than circular?

Discussion in 'Beyond the Basics' started by citjet, Jan 4, 2010.

  1. citjet

    citjet TPF Noob!

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    Just wondering since these dont seem to be readily available but make sence when working with snow and sky or water and sky together.
     
  2. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    When you say full polarizers I assume you mean linear polarizers?
    The reason is that with the design of the Digital SLR sensor (and the rest of its components) your camera will have metering and AF problems if you use a linear polarizer filter - whilst a circular polarizer will not give you such problems and works perfectly with a DSLR.

    As far as I know the linear and circular should give you very similar results - certainly I have never read of anyone having found a lacking in the circular polarizers when compared to the linear ones
     
  3. Josh66

    Josh66 Been spending a lot of time on here!

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    All I've ever used is Circular, but from what I've read Linear is technically "better", but you can't use them with cameras that have TTL or AF...

    If you have a full manual, mechanical camera - linear polarizers are cheaper and supposedly work better and you won't have to worry about whether or not the AF or TTL will work, because you won't have it anyway...
     
  4. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    Hmm I suppose if it just cuts out the AF and TTL features you could try full manual mode with some guesswork. Meter the scene before adding polarizers - and then deduct two stops from each reading (I think I am correct that polarizers cut out around two stops of light) and then take a shot. If it fails you do at least get instant feedback from the LCD on the camera as well as the histogram - which would give you enough info to adjust the settings and shoot again.
     
  5. Josh66

    Josh66 Been spending a lot of time on here!

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    I'm not sure exactly how much 'better' we're talking about here, so the effort may not be worth it...

    Wow, I just checked B&H...the cost difference is bigger than I thought it would be.

    Circular:
    Polarizer

    Linear:
    Polarizer

    $50 difference on the MRC ones.
     
  6. citjet

    citjet TPF Noob!

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    I did a little searching after I posted the question and found that I could have answered part of my question but I do appreciate the input.

    What I meant was yes, a linear rather than full polarizer. I guess that since the circular blocks only half of the frame, doesnt it make sense to be able to have polarization for the whole frame? So that brought me to Overread's answer of going full manual.

    I do a lot of long exposures of water and would like some shots that both polarize the sky and the wet rocks/folliage around a stream. Since I dont have photoshop and cant do overlays of images yet, that brought me to the question of linear filters.
     
  7. KmH

    KmH Helping photographers learn to fish Supporting Member

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  8. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    Citjet I think you have mixed up circular polarizers with neutral density graduated filters (of the screw in variety). The circular polarizer has its polarizing effect over the whole of the camera frame, so it will capture both sky and water; whilst the NDGrad (screw in filter type) do have a defined line of graduated change in how much light they let through to the camera and they are a poorer choice since the line is always in the middle (which is when things move to using the larger square filters and a filter holder setup).
     
  9. Dwig

    Dwig TPF Noob!

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    The Wikipedia link is good, as far as it goes, but doesn't explain the real difference between a "linear polarizer" and a "circular polarizer".

    A linear polarizer does not polarize the light (making the name incorrect, grammatically). It does not alter the light. It merely selects light of one particular polarized orientation. It somewhat analogous to a doorman who only lets short people into a club. The door man doesn't shorten tall people; he just sends them elsewhere.

    A true circular polarizer, not the camera accessory that borrows the name, actually alters the polarization orientation of the light. It rotates the orientation based on the color of the light.

    Film and digital sensors don't react differently to light of different orientations; they could care less, but many portions of the optical system in modern camera pipe a portion of the light from the image off to other sensors for metering and autofocus. The method used is almost always one that reflects light of one orientation and passes light of another. They can become blind or see an incorrect proportion of the light and thus fail when light is strongly polarized, as it is when a linear polarizer is used over the lens.

    The cure is to use a combination attachment that is called, in photographic jargon, a circular polarizer. These devices have a conventional linear polarizer as their first element, after the front cover glass, and a true circular polarizer as the second, followed by another cover glass. You now have a package that selects light of one orientation, to get the desired photographic effect, and then scrambles the orientation to keep the light exiting the attachment affecting the performance of any metering, AF, or VF system.

    These combination attachments, the so called circular polarizers, vary in quality a great deal. They are laminated packages that must be assembled with their outer surfaces perfectly plano-parallel and the two elements must be very color neutral, a rather difficult achievement for the true circular polarizing element. Cheap attachments are often not plano-parallel, reducing sharpeness, and not particularily neutral in color.
     
  10. Josh66

    Josh66 Been spending a lot of time on here!

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    Sooo...

    Is that why a linear polarizer is "better" than the circular variety...? Because it is the same thing, but with one less filter element (having less of an impact on IQ)...?

    Or do they not cut as much light (because of the missing additional element), letting you use settings closer to what you would have with the filter off?

    What makes them "better", or are they not actually better?
     
  11. Garbz

    Garbz No longer a newbie, moving up!

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    There is no better in any practical sense since the minuscule difference between having a well made linear polariser vs a well made circular polariser borders on nearly unmeasurable let alone field relevant. I have never heard of someone say one type is better than the other, not in the optics field, or the photography field. I have only ever heard that one type is suitable and the other not, with in this case linear being unsuitable.



    Dwig while I'm sure you know how everything works that explanation I don't think really follows the process of polarisation at all.

    A linear polariser definitely polarises the light. By the very definition light is polarised if if the EM wave travels in a single plane rather than in a circle. So even as you explained it you only accept light going in one polarisation direction, the light coming out the other side is definitely polarised.

    A lot of light is circularly polarised which can't effectively be explained by the big man / little man analogy since that analogy is saying that the when the big man isn't getting through the door he is lost, but he isn't really or polarisers would effectively be very dark. A silly version of the same analogy would be a man who is constantly changing size but the door man says you have to be this big once you go through here, but at the same token won't let through big people who don't change size (linear polarised light with the wrong polarisation angle).

    Actually I think I've just confused things further. :lol:
     
  12. Plato

    Plato TPF Noob!

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    Your understanding is incorrect. A CPL is identical to a linear polarizer except for what happens AFTER the polarization. In the case of linear, nothing happens after the polarization. With circular, the polarized rays are then circularized (or unpolzrized, if you prefer) because polarized rays adversely affect autofocus and metering mechanisms.

    Do not interpret this as "undoing" the polarization. We don't really care about the polarization in and of itself. We care only because it blocks certain rays, those reflected from non-metallic surfaces. Once those rays are blocked, our film/sensor does not care if the remaining rays are polarized or not.
     

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