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Thread: Physics, Polarisers, the Sun and the Sea

  1. #1
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    Default Physics, Polarisers, the Sun and the Sea

    Hi all,

    Recently my beach photos were coming out a bit washed out cos I didn't exp compensate up a stop or 2 for the reflection from the sand. My next trip I plan to do this plus also bring along my circular polarising filter.

    Anyone good with physics? From my memory of physics lessons long long ago, a large source of light at the beach is light that is reflected from the sand. Polarising filters alter this by causing light waves to travel in a single plane only, which has the additional effect of reducing intensity, thats why they tend to reduce light a stop or 2 if i'm not wrong.

    So the problem is that at the beach lets say I want to polarise and make my skies bluer and cut down water reflection. Do I still need to exposure compensate up for reflection from the sand? This keeping in mind that the "rotation" of the polariser to cut off water reflection etc. may not be the same plane by which the light coming of the sand is?

    Or since the polarising filter already cuts off light intensity (sometimes I have to exp compensate up with it) I can just use my metered value?

    I vaguely remember something in physics that says most waves striking the horizontal surface (incl sand and sea) reflect in the same plane (vertical i think). My physics not too good as you can see. In that case the plane by which I have no water reflection would be the same plane where I cut off sand reflection.

    hehe. Confused!
    cheers,
    allsmilez

  2. #2
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    Default Re: Physics, Polarisers, the Sun and the Sea

    just trial and error if you're on digital. for me, i use the sunny 16 rule outdoors

    better still, use a light meter
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  3. #3
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    Default Re: Physics, Polarisers, the Sun and the Sea

    If you capture both the water and the sky in the frame, you maight not be able to cut reflections and have deeper blue skies at the same time. Maybe 2 polarizers stacked would do the trick. Adjust one to darken the sky and the other one to remove glare from the water surface....since it is so bright at the beach you might be able to make it with 2 stacked polarizers.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  4. #4

    Default Re: Physics, Polarisers, the Sun and the Sea

    Quote Originally Posted by roygoh
    If you capture both the water and the sky in the frame, you maight not be able to cut reflections and have deeper blue skies at the same time. Maybe 2 polarizers stacked would do the trick. Adjust one to darken the sky and the other one to remove glare from the water surface....since it is so bright at the beach you might be able to make it with 2 stacked polarizers.
    care to explain more?

    i thought polarizers filter filer off the light wave from certain angle, if stack 2 filters, will the light from the sky pass through first filter but block by second filter?

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    Default Re: Physics, Polarisers, the Sun and the Sea

    Quote Originally Posted by Seventh
    care to explain more?

    i thought polarizers filter filer off the light wave from certain angle, if stack 2 filters, will the light from the sky pass through first filter but block by second filter?
    The light from the sky and the reflection from the water surface may not be polarized in the same direction. If that's the case then a single polarizer cannot achieve both the effects of deepening the sky and cutting glare from the water surface (can only pick one of them).

    If 2 polarizers are stacked, they can be adjusted separately, one to deepen the sky and the other to cut glare fromt eh water surface.

    Mostly speculation based on what I know about polarizers. Have not tried stacking polarizers myself, I must say.

    - Roy
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  6. #6

    Default Re: Physics, Polarisers, the Sun and the Sea

    It's not easy using 2 polarizers. Most of the time, the 2 polarizers will block each other, thereby having no light passing through.

  7. #7

    Default Re: Physics, Polarisers, the Sun and the Sea

    not to mention that 2 polarisers will cause vignetting on the len.
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    Default Re: Physics, Polarisers, the Sun and the Sea

    Quote Originally Posted by Blur Shadow
    It's not easy using 2 polarizers. Most of the time, the 2 polarizers will block each other, thereby having no light passing through.
    Something like that, they called it variable ND filter. That's actually 2 polarizer (I forgot, are they need to be all circular-polarizer, or one circular and one linear polarizer will do).

    Regards,
    Arto.

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    Default Re: Physics, Polarisers, the Sun and the Sea

    Quote Originally Posted by serchim
    not to mention that 2 polarisers will cause vignetting on the len.
    Agree, on the wide angle lens...

    Regards,
    Arto.

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    Default Re: Physics, Polarisers, the Sun and the Sea

    After a good night's sleep I can now think about this problem with a brain that is closer to fully functioning...

    I believe I am still right about a single polarizer not being able to achieve both effects of deepening the sly and cutting reflection from the water surface. However, I am wrong in stating that 2 polariszers can be stacked and adjusted individually to get the desired effects.

    The only way to do that seems to be taking 2 shots with different polarizer settings and then blen them in photoshop.

    Stacking polarizers creats the effects of Polarizer + variable ND. The first (front most) polarizer must be a linear polarizer. It can be used to cut reflections or deepen the sky colour. The second polarizer can be linear or circular, depending on the camera (if not sure, use circular). It will further reduce the lignt intensity, and the amount of reduction is depending on its relative angle to the first polarizer.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  11. #11

    Default Re: Physics, Polarisers, the Sun and the Sea

    how about taking snow scenery, can use normal CPL ?

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