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  1. #31
    wiltw's Avatar
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    1. The ISO standard for calibration of reflected meter has a manufacturer chosen Constant -- essentially this is a VARIABLE value, selected by the meter manufacturer.

    2. The ISO standard for calibration of incident meter has a manufacturer chosen Constant -- essentially this is a VARIABLE value, selected by the meter manufacturer.


    ...so the fact that meter #1 does not match meter #2 is inherent to the ISO standards for the two meters!

    3. Although 18% grey tone is 'middle of the tonal range', one can determine that really one needs to meter 12-13% grey card, for #1 to truly match #2 in reading.

  2. #32
    Stephen Benskin's Avatar
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    A very detailed explanation of the constants can be found in the tread "Is the K factor relevant to me or should I cancel it out?" Basically, the exposure constants K and C do two things. Hand held meters are separate from the camera's optical system. The light loss caused by the camera lens needs to be factored into the exposure calculation, and because the meter isn't reading through the camera lens, the actual value must be averaged. Second, the exposure calculation needs to factor out the influence of the physicality of the meter, ie spectral sensitivity. There is a rather involved equation for determining K. The following is a page from the K factor thread which shows the equation for K.

    What's considered the average reflectance comes from K / C. 1.16*pi / 30 = 0.12.

    Click image for larger version. 

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    Last edited by Stephen Benskin; 02-23-2013 at 09:23 PM. Click to view previous post history.

  3. #33

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    In which decade was your meter last calibrated?

  4. #34

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    Quote Originally Posted by Kirk Keyes View Post
    In which decade was your meter last calibrated?
    Kirk. Never. Is it expensive and where would one go in the UK, for example?

  5. #35

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    I have no ideas where to go for the UK. But most good camera repair shops should have a calibrated light soure to check your meter with.

    "Never"
    But that's kind of my point, how can one expect a measuring tool like a light meter that more than 50 years old, to be accurate? Add to that all issues Steve Benskin points out above...

  6. #36
    Stephen Benskin's Avatar
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    Even when calibrated, meters are only "accurate" under certain conditions. Not that bad if you consider the actual purpose of the meter. These are exposure meters and not light meters.

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  7. #37

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    Quote Originally Posted by Kirk Keyes View Post
    I have no ideas where to go for the UK. But most good camera repair shops should have a calibrated light soure to check your meter with.

    "Never"
    But that's kind of my point, how can one expect a measuring tool like a light meter that more than 50 years old, to be accurate? Add to that all issues Steve Benskin points out above...
    You may be correct. The thought that they didn't last for ever never cross my mind until now. Perhaps it's time to get it serviced.

    But. It does have user-calibration setting on the back. This is used to zero-set it.

    And then. If it is out of calibration then that does not necessarily explain why the two readings differ. The sensor is the same one but in one case uses a diffusion cone and in the other not. Would this meter be sophisticated enough to have two separate calibrations, one for each mode of use?

  8. #38

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    Many meters are calibrated against a 12 percent reflectance standard, not 18 percent. That probably accounts for the one zone difference. You will probably find that your reflectance and incident light readings pretty well agree for an average landscape, although there can be differences depending on the landscape.

  9. #39
    jcc
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    It's possible too that the position of the dome leaves it partly in shadow, so it gets a lower reading than the incident meter, which is averaging a planar card.

  10. #40
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    getting back to the OP, the instructions from the grey card itself reads:
    "Meter readings of the gray card cshould be adjusted as follows- 1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop. 2) For light subjects use the indicated exposure; for very light subjects decrease exposure by 1/2 stop 3) If the subject is dark to very dark increase the indicated exposure by 1 to 1.5 stops"

    So to answr your question, the grey card is actually designed to be lighter than 'middle grey'.
    Go not to the elves for counsel, for they will say both yes and no.

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