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  1. #1
    Stephen Benskin's Avatar
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    Comparison of Reflectance from 18% and 12% Zone Models

    This is kind of interesting. I've put together three exposure models comparing the subject reflection densities and Reflectances for Zone V = 18%, Zone V = 12%, and the standard model of exposure. As anyone familiar with the Zone System knows, Zone VIII falls three stops above the metered exposure point (Zone V). Zone I falls four stops under the metered exposure point for a total subject luminance range of 7 stops (2.10 logs). The standard model of exposure is slightly different as it doesn't base exposure from the metered exposure point but from 100% Reflectance with a 7 1/3 stop (2.20 logs) subject luminance range.

    If Zone V equals 18% Reflectance, what would Zone VIII equal? How about when Zone V equals 12%? And there are other implications depending on the model used.

    Click image for larger version. 

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    A little fun and a little food for thought.
    Last edited by Stephen Benskin; 12-26-2011 at 10:42 PM. Click to view previous post history.

  2. #2
    Bill Burk's Avatar
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    The numbers at 18% are not pretty - but that's what Ansel Adams wanted.

    At 12%, the percent reflectances are convenient and percentages progress through sensible numbers, like 95% for the white side of the card.

    The standard model seems to have a clean series of relative densities.

  3. #3
    Stephen Benskin's Avatar
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    So, you don't have a problem with a Zone VIII as having a Reflectance of 144%? Doesn't that sound somewhat unrealistic to you?

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    I'm 150% sure zone VIII doesn't have a 144% reflectance

  5. #5
    Bill Burk's Avatar
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    I don't have a problem with it because it might assume a long range subject - reflectances of a part of the scene in shadow plus another part of the scene in open daylight - so a natural scene could exceed 100%

  6. #6
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    I'm 150% sure zone VIII doesn't have a 144% reflectance
    I know you are just being funny, but think about it. It's the math. 18% is a reflection density of 0.74. Add three stops and that equals an RD of -0.18 or 144% Reflectance for Zone VIII.

    This is simply putting numbers to the Zones. If not 144% then what?

    From Bill Burke
    I don't have a problem with it because it might assume a long range subject
    This is for a normal 7 stop scene when Zone V is at 18%.

  7. #7
    Bill Burk's Avatar
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    I just measured, in bright daylight, from top-hat black hole to white wall is 7 2/3 stops.

    A reflection grayscale I made only measures about 4 stops. It has "less than ideal" characteristics.

    But I believe even a reflective target with ideal characteristics, from 0 - 100% would be less than 7 stops.

  8. #8
    Stephen Benskin's Avatar
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    Quote Originally Posted by Bill Burk View Post
    But I believe even a reflective target with ideal characteristics, from 0 - 100% would be less than 7 stops.
    From 100% - 0.8% Reflectance is seven stops. I guess technically the range could be infinite as you would never reach zero if you are always dividing by 2.

    Remember, the use of Reflectance in relation to luminance usually involves a Lambertain surface which is a perfect diffuser. 100% Reflectance doesn't mean 100% reflection - (illuminance x reflectance) / pi = luminance.

    Scenes with Reflectances of 100% and above incorporate non Lambertain surfaces and specular reflections. There's also relative Reflectance like when shooting a scene with multiple lighting conditions, like the cave and the beach. You have a a normal scene in bright sunlight and then a normal scene in dark shadows. If you are shooting the inside of the cave, the exterior scene will appear to have Reflectances easily over 100%.

    However, that still doesn't answer of Zone VIII being at 144% when Zone V is considered to be metered at 18% gray.

  9. #9
    Bill Burk's Avatar
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    Well... isn't Zone V 12% gray? After all... Zone V is what the meter reads and that's supposedly 12-13%

  10. #10
    Stephen Benskin's Avatar
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    Quote Originally Posted by Bill Burk View Post
    Well... isn't Zone V 12% gray? After all... Zone V is what the meter reads and that's supposedly 12-13%
    Yes it is!!! Nice catch. This is just one way to prove it's not 18% using very little math. Zone VIII isn't supposed to have specular reflections. So, it shouldn't be over 100%. As Zone VIII is three stops over Zone V and if the meter "reads" 18%, then Zone VIII is over 100%; therefore, the meter can't be reading 18%. Another photographic myth bites the dust.

    I almost worked myself into a corner as exposure meters don't "see" Reflectance, but it's possible to extrapolate an equivalent number based on the reflection meter's and incident meter's calibration. This is also the beginning of a more precise way to determine exactly what a meter "sees". Apply the average illuminance of Daylight to the different reflection densities (basically log Reflectance), plug them into the camera exposure equation, and see which one produces the correct exposure at the metered exposure point for a given EI.

    And for the record, it's 12%.

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