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Thread: Use of Circular polarizer - colour shift

  1. #1

    Default Use of Circular polarizer - colour shift

    Has anyone noticed a colour shift when using their CPL? I've noticed some shots where I've used a CPL and some areas exhibited a slight purplish/blue cast. I can't post the picture for copyright reasons and I haven't really been able to recreate the conditions.

    It's basically a wide shot of a condo apartment with the sun high right, out of frame. CPL was set to cut glare from water surface in the swimming pool and also to deepen the blue skies. Buildings were a pale yellow. The resulting pictures yielded a very nicely saturated blue swimming pool and skies but my buildings ended up looking a little purplish and it was a pain trying to colour correct in post.

    Can someone point me in the right direction? What have I done wrong and what can I do when using a CPL to avoid a similar problem?
    Last edited by peapilot; 12th October 2007 at 06:34 PM.
    Canon EOS 66 | Kit Lens | Leung Sheung Wing Chun Singapore

  2. #2

    Default Re: Use of Circular polarizer - colour shift

    That's the difference between an expensive polarizer and a cheap one.

  3. #3

    Default Re: Use of Circular polarizer - colour shift

    The issue you brought up is due primarily to the fact that angle of polarization varies continuously with the angle from the sun, and it is exacerbated when wide-angle lenses are used since these lenses cover fairly large angle. For a detailed discussion on using polarizers with wide-angle lenses, you might want to refer to articles found in Luminous-landscape.com and KenRockwell.com.


  4. #4
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    Default Re: Use of Circular polarizer - colour shift

    Quote Originally Posted by peapilot View Post
    Can someone point me in the right direction? What have I done wrong and what can I do when using a CPL to avoid a similar problem?
    There's so many variables in the scenario that you describe, some of which have nothing to do with the polarizer at all, that I don't think you can get an answer that is more than wild speculation. It could e.g. be varying automatic white balance, overexposure, lack of a lens shade, the polarizer merely doing its job, etc.

  5. #5

    Default Re: Use of Circular polarizer - colour shift

    How does the angle of polarization cause a colour-shift? Is the CPL supposed to do that? Or is it the cause of a "cheap" CPL?

    I don't think it's an issue of exposure nor white balance. The colours for the skies and pool and other elements look ok, there was just a strange cast on the building. I noticed a slight shift in the viewfinder while taking the picture, but didn't realise it was that bad till I viewed it on the monitor.
    Canon EOS 66 | Kit Lens | Leung Sheung Wing Chun Singapore

  6. #6

    Default Re: Use of Circular polarizer - colour shift

    How does the angle of polarization cause a colour-shift? Is the CPL supposed to do that? Or is it the cause of a "cheap" CPL?

    I don't think it's an issue of exposure nor white balance. The colours for the skies and pool and other elements look ok, there was just a strange cast on the building. I noticed a slight shift in the viewfinder while taking the picture, but didn't realise it was that bad till I viewed it on the monitor.
    Canon EOS 66 | Kit Lens | Leung Sheung Wing Chun Singapore

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    Default Re: Use of Circular polarizer - colour shift

    Quote Originally Posted by peapilot View Post
    How does the angle of polarization cause a colour-shift? Is the CPL supposed to do that? Or is it the cause of a "cheap" CPL?
    If the light bouncing off a surface partly maintains its polarization, and there's a polarized blue sky which contributes to the illumination of said surface, the colour of the surface will change depending on the position of the polarizer. Same thing if you start with unpolarized light from the sky which gets partially polarized upon reflection by virtue of the viewing angle.

    I don't think it's an issue of exposure nor white balance. The colours for the skies and pool and other elements look ok, there was just a strange cast on the building.
    Did you use a fixed white balance, or automatic white balance? In the latter case, the camera could just make a different choice. It could e.g. be mislead by the blue sky and water and try to correct for what it thinks is a blue cast.

    Exposure wise, if one of the colour channel clips, you get bad colours. This may not be an obvious condition, as the other channels still hold detail, i.e. the overexposed region is not burnt out. After processing (in particular applying white balance), the channel may not even clip in the histogram (but one may see a sharp peak instead). This is often seen in landscape photos where clouds take a pink hue.

    I noticed a slight shift in the viewfinder while taking the picture, but didn't realise it was that bad till I viewed it on the monitor.
    Colour shifts in the viewfinder are normal. The viewfinder of autofocus (D)SLRs is somewhat sensitive to the linear polarization of the light. From the way they are built (i.e. it's not a quality issue, but a fundamental issue), circular polarizers are inherently imperfect; they still have some linearly polarized contribution that is wavelength (colour) dependent. This can lead to some colour change in the viewfinder even in cases where the film/sensor would not be affected.

    To add one more possibility, what would happen if you experienced some moderate lens flare, possibly caused by the filter itself, which originates from polarized light (e.g. sky)?
    Last edited by LittleWolf; 13th October 2007 at 08:06 AM.

  8. #8

    Default Re: Use of Circular polarizer - colour shift

    Quote Originally Posted by peapilot View Post
    How does the angle of polarization cause a colour-shift? Is the CPL supposed to do that? Or is it the cause of a "cheap" CPL?

    I don't think it's an issue of exposure nor white balance. The colours for the skies and pool and other elements look ok, there was just a strange cast on the building. I noticed a slight shift in the viewfinder while taking the picture, but didn't realise it was that bad till I viewed it on the monitor.
    Which CPL are you using? Some CPL are known to cause colour shift.

    BC

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    Default Re: Use of Circular polarizer - colour shift

    Due to the design of circular polarizers, most polarizers affect the various wavelengths of light to different extents, hence a colour shift ( or so I've read from the link below). However, this is mostly unnoticable in good quality polarizers.

    It's difficult to pinpoint your problem because there are so many variables involved. Prehaps you can try again taking another picture with and without the CPL and see if the cast is still there.

    Found this website with some comparision charts on the effect of CPLs with regards to different colours.
    http://www.clarkvision.com/photoinfo...izing_filters/

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    Default Re: Use of Circular polarizer - colour shift

    Quote Originally Posted by Prismatic View Post
    Due to the design of circular polarizers, most polarizers affect the various wavelengths of light to different extents, hence a colour shift ( or so I've read from the link below). However, this is mostly unnoticable in good quality polarizers.

    /[/URL]
    I think that's a very likely scenario. I tend to notice colour shift much more on my Tamron and Tiffen (cheapo filters) and hardly at all with my Hoya (mid-range) and B+W

  11. #11

    Default Re: Use of Circular polarizer - colour shift

    Thank you all for the helpful information. Learnt quite a bit. The CPL used was a kenko one, but I wanted to rule out user error on my part first before blaming my equipment and thus did not name the brand of the CPL first in my previous posts.
    Canon EOS 66 | Kit Lens | Leung Sheung Wing Chun Singapore

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    Default Re: Use of Circular polarizer - colour shift

    I doubt is the fault of the cpl. I read an article before, can't remember which it is now, but light coming from a large area of sky will reach your cpl from different angles, and hence alternating areas of sky will become dark and bright as you change the cpl's orientation. I've witnessed this myself.

    And i think this problem may be avoided if you are facing the sky in the right direction, meaning the sun is at a certain angle from you. Just as facing directly into the sun will result in uniform light cut-off of the sky, then with the sun facing your back, you will achieve the same effect since the suns rays hits the sky in front of you evenly (when u are facing 0 and 180 degrees w.r.t the sun)

    Hence, maybe a few degrees left or right from these two orientations will have less uneven polarisation of the sky, which you want to achieve.

    My 2 cents.

  13. #13

    Default Re: Use of Circular polarizer - colour shift

    pah

    the reason is simple

    if you had read most books, they would not recommend that you do FULL POLARISATION when it comes to taking scenes
    will make the scene look unnatural, etc
    i don't know about cheap or expensive, but i never had that problem because i never did full polarisation, it's just like going all the way when you do saturation

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    Default Re: Use of Circular polarizer - colour shift

    Quote Originally Posted by night86mare View Post
    pah

    the reason is simple

    if you had read most books, they would not recommend that you do FULL POLARISATION when it comes to taking scenes
    will make the scene look unnatural, etc
    i don't know about cheap or expensive, but i never had that problem because i never did full polarisation, it's just like going all the way when you do saturation
    but in SG, most of the time even full polarisation doesn't looks like full.

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