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  1. #11
    benjiboy's Avatar
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    The cameras meter inaccuracy is only theoretical until you take some pictures with it.
    Ben

  2. #12
    Stephen Benskin's Avatar
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    Quote Originally Posted by Trask View Post
    Well, some older meters were calibrated to 12%, but none of the ones under discussion here.
    Meters aren't calibrated to any reflectance / percentage. Let's not perpetuate myths.

  3. #13

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    Quote Originally Posted by Stephen Benskin View Post
    Meters aren't calibrated to any reflectance / percentage. Let's not perpetuate myths.
    No but they are calibrated with a factor K12 or K14 etc..
    If a reflective light meter is said to be calibrated with a K14 it means that it was calibrated to read a luminance of 14 candela per square meter as EV0 for ISO 1. Or in other word, 0.14 candela per square meter as EV0 for ISO 100.

  4. #14

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    To be fair the 14% or 18% reflectance does come in to play when one compares readings from an incident meter to that of a reflected light meter. An incident light meter with a flat receptor and calibrated to C250 would read EV0@ISO100 with an illuminance of 2.5 Lux. If one places a gray card of 18% reflectance and make a reflected light reading with a meter calibrated to K14 it would read EV0@ISO100. If one places a card of 16% reflectance a reflected light meter with a calibration factor of K12 would read EV0@ISO100.
    Last edited by Chan Tran; 04-25-2010 at 05:26 PM. Click to view previous post history.

  5. #15
    Stephen Benskin's Avatar
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    Chan Tran,

    Yes, there is a lot we could get into. I just wanted to make a quick point without hijacking the tread.

    As to the difference in light meters, calibration can get off, but added to that is the spectral sensitivity of the photo cell, and with older meters made before the mid-sixties, the color temperature of the calibration light source was 2700K whereas it later became 4700K. This can make a big difference depending on the color temperature under which a test is conducted. Anyone remember or remember reading that there used to be different speed ratings for incandescent and daylight? The calibration color temperature is the reason for it.

    Steve
    Last edited by Stephen Benskin; 04-25-2010 at 08:35 PM. Click to view previous post history.

  6. #16
    Mats_A's Avatar
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    Quote Originally Posted by Chan Tran View Post
    No but they are calibrated with a factor K12 or K14 etc..
    If a reflective light meter is said to be calibrated with a K14 it means that it was calibrated to read a luminance of 14 candela per square meter as EV0 for ISO 1. Or in other word, 0.14 candela per square meter as EV0 for ISO 100.
    Wow! The quality and diversity of comments on this forum is amazing. And humbling. Where else could one get to know something as esoteric as this? I just love this site.

    r

    Mats
    Digital is for communication, film is for documentation.


    http://www.flickr.com/photos/studiopirilo

  7. #17
    benjiboy's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    Chan Tran,

    Yes, there is a lot we could get into. I just wanted to make a quick point without hijacking the tread.

    As to the difference in light meters, calibration can get off, but added to that is the spectral sensitivity of the photo cell, and with older meters made before the mid-sixties, the color temperature of the calibration light source was 2700K whereas it later became 4700K. This can make a big difference depending on the color temperature under which a test is conducted. Anyone remember or remember reading that there used to be different speed ratings for incandescent and daylight? The calibration color temperature is the reason for it.

    Steve
    As I wrote earlier the O.P needs to use the ******* camera and look at the results before assuming there's a problem.
    Ben

  8. #18
    Stephen Benskin's Avatar
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    Quote Originally Posted by Chan Tran View Post
    To be fair the 14% or 18% reflectance does come in to play when one compares readings from an incident meter to that of a reflected light meter. An incident light meter with a flat receptor and calibrated to C250 would read EV0@ISO100 with an illuminance of 2.5 Lux.
    Using a flat receptor can be a little misleading as the constant for dome needs to be used to calculate incident/reflective average reflectance approximation.. According to the ISO standard, that would be C=30 (30*10.76 = 323) as opposed to the C =250 or 320/10.76 = 23.23 and K = 1.16 (1.16*pi = 3.64.

    3.64/30 = 0.12 or 12%

    This is also based on the Sun being at an approximate angle of 41 degrees. This only works for the standard model for an average scene and it in no way means the meter "sees" 12% reflectance or that what the meter points at will reproduce that tone as 12% reflectance on the print.

    The basic value of K is also 12.5 or 1.16*10.76 = 12.48 (rounded up to 12.5). This can be confirmed using the constants equation K = P/q as defined in Connelly,D, Calibration Levels in Films and Exposure Devices, Journal of Photographic Science, vol 16, 1968. Where P is Eg in the basic exposure equation Hg = Eg * t and q is the light loss constant in the classic exposure equation q*Lg*t/A^2 = Hg. P = 8 (rounded down from 8.11 to 8.2) and q=.65.

    K = P/q or 12.48 = 8.11/.65
    Last edited by Stephen Benskin; 04-27-2010 at 02:32 AM. Click to view previous post history.

  9. #19
    Stephen Benskin's Avatar
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    Forgot to mention that through the lens meters don't have a K to speak of. They read the actual light transmitted through the lens and don't need one.

  10. #20

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    Yes, agree with Mats, thanks for letting me in on this discussion. The more I learn, the less I know.

    Also agree with Ben. Real Pepsi challenge is putting some film through it, which is in the works. Thanks everybody.

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