High Altitudes and Polarized Light

Discussion in 'Nature' started by stanley_mcmanus, Dec 19, 1997.

  1. Maybe somebody out there can clear this up for me. Several years ago I remember reading a warning about the over use of polarizers at high altitudes. The writed claimed that at high altitudes (8,000+ feet) light is naturally polarized and the use of a polarizer can lead to unnaturally dark skies. He also claimed that this natural polarization was the reason many wide angle scenics shot at high altitudes showed vignetting at the corners since the polarization reduced the light coming in at the edges of the lens.

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    Any truth to this?
     
  2. Some, but not much. The sky tends to be naturally darker at altitude (or
    under very clear conditions, rarely if ever found these days).
    This is because of lower scattering from particulates, water vapor
    dust, etc. in the atmosphere. So if you polarize it too, it gets even
    darker. When you are high up you get above most of the scattering
    junk, so skies get darker. There may be some difference in
    polarization due to depolarization by scattering, but I don't
    think it's a big effect.

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    Wide angle polarized shots may show dark (or light) corners
    because polarization depends on the sun angle wrt to the area
    of the sky being shot. If your lens is wide enough, you see
    areas of different degrees of polarization. The wider the lens,
    the worse the problem is. The darker the sky, the more you notice
    it.

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    Where did you read this. Pop Photog?
     
  3. No, I did not read it in Pop Photo. As I recall it was in a book on nature photography I read several years ago. The author was offering tips on avoiding certain problems and this was one of them.

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    I thought I would throw it out and see if there was any validity to it since I have never heard it from any other source.
     
  4. A few years ago I was using print film in Colorado springs without a polarizer (Kodak Super Gold 100). Even without a polarizer, the sky was almost blue/black in some of the underexposed shots. The polarizer enhanced this effect to a surreal point.

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    The darkening definitely occurs but it depends on the quality of the polarizer -- a Tiffen polarizer would have nearly the effect that a high dollar heliopan or B&W polarizer would -- but you will get a darkening effect primarily when the sun is above 30 degrees from the horizon. Less pronounced effect at high noon or at sunset/sunrise. Since polarization occures at 90 degree angles from the sun angle, polarization is most pronounced at certain times of the season and or day. I was taking the above mentioned pictures in April at around 10:00 am.

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    I would surmise that the effect will be dramatized on some films and not on others depending on the sensitivity of the film to blue. If I had known that the effect was going to be as pronounced as it was I would like to have shot the same scenes with different films and or polariziaing angles. But even the unpolarized shots were a bit shy of believable.

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    Jess
     
  5. Can you offer any support for the notion that the amount of visible polarization effect depends on the quality of the polarizer, i.e. you would get different (less) darkening with a Tiffen than with a Heliopan?
    I'm not going to keep incorrect information in this forum, and from my experience, this statement isn't true. However, I'm open to any references or actual experiences to the contrary.
    I have no doubt that it's possible to make a "polarizer" that doesn't have much effect, but I don't think Hoya, Tiffen etc. fall into that catagory.
     
  6. Actually Bob, it is possible to have different degrees of effectiveness in polarization. This would depend on a number of variables (such as material used to create the effect, the density of the material used, etc). You could even create filters where the level of effectiveness varies with the colour of the light (come to think of it, there is some company doing this). However, in producing a neutral density polarizing filter, the cost comes from getting the effect truly neutral, the quality of the glass and the quality of the optical coatings. I cant see how any manufacturer could obtain a financial (or any other) advantage by producing filters that are not of the standard density (~1 2/3 stops).
     
  7. At the lower altitudes, I believe that scattering from aerosols (particulates, water, etc.) usually dominates - this is called Mie scattering. At the higher altitudes, the air is drier and cleaner (as well as thinner) and the dominant scattering is usually by air molecules - this is called Rayleigh scattering. I would expect the polarization effects associated with Mie and Rayleigh scattering to be different. However, my appropriate textbooks are out of reach at the moment. Maybe after the Holidays...

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    Meanwhile, Season's Greetings fellow photo-netters!
     
  8. The polarization of the sky (and the fact that it's blue) do
    come from Rayleigh scattering in the upper atmosphere. The
    degree of polarization usually reaches about 75% at maximum
    and the peak is at around 550nm. The fact that it isn't
    100% is due to a number of factors including multiple scattering,
    ground reflections
    and aerosols. The more additional scattering (Mie or Rayleigh)
    that occurs in the lower atmosphere, the lower the degree of polarization. So the more junk you have to look through, the
    less polarized the sky tends to be.
     

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