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  1. #21

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    Is there a minimum percentage of the sky that has to be filled with white clouds to make a difference and might it depend on latitude and time of year? Even in high summer here in the U.K. and around noon shadows can tend to be soft even in an unbroken blue sky.

    pentaxuser

  2. #22
    benjiboy's Avatar
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    Quote Originally Posted by David Lyga View Post
    Assume midday, with unrestricted sunlight. In neither case is the actual sun obscured by clouds. Now take these two scenarios:

    1) Scene fully lit under completely blue sky

    2) Scene fully lit under sky with many white clouds (but, again, sun, itself, is not obscured by clouds)

    Do the scenes require the SAME exposure or does the scene with the completely blue sky demand one stop more exposure? I think that the 'blue sky' scene does require more exposure because there is no benefit of the white clouds 'filling' in with more light. And, there are (British) photo books that attest to my assumption.

    Agree or refute: I do not bite. - David Lyga
    That's why God invented light meters.
    Ben

  3. #23
    fotch's Avatar
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    Quote Originally Posted by benjiboy View Post
    That's why God invented light meters.
    Items for sale or trade at www.Camera35.com

  4. #24

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    I find it hard to believe that the contribution of the reflected light by the white clouds would be a stop brighter. When I think about the intensity of the light from the white clouds compared to unobstructed sunlight, it has to pale in comparison. Therefore, I refute your hypothesis.

    Edit: Actually, I have done this. Near noon I have taken incident readings with a completely blue sky. On successive days I have taken incident readings with lots of puffy white clouds but not obstructing the sun. The readings were virtually the same; the worst case was perhaps down by 1/6 stop (most likely water vapor in atmosphere). (my secondary hobby is collecting/testing light meters.)
    Last edited by Fred Aspen; 04-28-2013 at 07:16 PM. Click to view previous post history.
    -Fred

  5. #25
    Ian Grant's Avatar
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    Interesting, when I shoot in Turkey/Greece my meter's near maximum when it's blue sky (the norm most of the year), and a few clouds drop this because there's normally some atmospheric haze.

    I definite don't need to stop down or use a faster shutter speed when there's clouds.

    Ian

  6. #26
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    The intensity of a blue sky varies with the seasons; strongest in Spring-Summer, weakest in autumn-winter, in the southern latitudes. In equatorial regions, the light remains fairly constant and predictable, even with sporadic clouds. A meter reading will only be affected if a large number of clouds fill the sky, thus reflecting light. Where clouds cover the sun either in hazy to overcast light, the light changes from point to diffuse, and an entirely different meter reading again will be obtained. In a nutshell there should not be a change in a meter reading just because of a few clouds, but the more clouds, the more chance of light being reflected and the greater the variation in a metered reading out in the open.
    .::Gary Rowan Higgins

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  7. #27

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    Quote Originally Posted by David Lyga View Post
    Assume midday, with unrestricted sunlight. In neither case is the actual sun obscured by clouds. Now take these two scenarios:

    1) Scene fully lit under completely blue sky

    2) Scene fully lit under sky with many white clouds (but, again, sun, itself, is not obscured by clouds)

    Do the scenes require the SAME exposure or does the scene with the completely blue sky demand one stop more exposure? I think that the 'blue sky' scene does require more exposure because there is no benefit of the white clouds 'filling' in with more light. And, there are (British) photo books that attest to my assumption.

    Agree or refute: I do not bite. - David Lyga
    May be but I am sure that it's not a whole stop. May be not even a 1/3 stop.

  8. #28

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    Quote Originally Posted by benjiboy View Post
    That's why God invented light meters.
    God didn't! You can't blame God for just about everything.

  9. #29
    JBrunner's Avatar
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    Just overexposed two stops, and call it bullet proof.

  10. #30

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    Quote Originally Posted by David Lyga View Post
    Roll film does not allow N-1 (unless the scenes are the same type). Thus, Ansel Adams does not come to the rescue here.

    Try to forget shadows here: we all know that fill light lightens them. I am talking about the SUNLIT portions of the scene and to simplify, let's focus upon ONLY a grey scale fully lit by sun. Would exposures in either case be identical or different. I think different. - David Lyga
    I would think that the difference in fill light provided by blue sky vs. clouds would be minimal in relation to the strength of the direct sunlight in the sunlit areas of your scene. Negligible enough that your grey scales in direct sunlight would be, for all practical purposes, identical. It's only the shadows that would show a significant difference in your scene.

    Two different scenarios:

    1: A subject in direct flat sunlight from a cloudless sky coming from behind the camera position with no shadows at all except maybe for limb effects.

    2. A landscape with significant important shadows (hillsides, tree shadows, etc.) lit by direct sunlight from a cloudless sky.

    Now, add clouds to the above. Scenario one will not change appreciably; the predominant light source is and remains the sun alone. The light added from the clouds is negligible. Scenario 2, however, is going to have more light in the shadows; perhaps significantly more if the blue sky is very dark, as it is at high altitudes, and if the clouds are positioned and illuminated so that they provide significant fill light to the shadows. And the color temperature of the light illuminating the shadows is going to be different as well, affecting possible filter choice. I can imagine up to a stop difference in the shadows in extreme cases.

    (...and, BTW, you can develop roll film to N-1 if you like, or use a different contrast grade paper to achieve basically the same thing. That's not the issue here, and certainly has no bearing on a subject in flat sunlight.)

    Best,

    Doremus

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