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  1. #1
    Stephen Benskin's Avatar
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    Quantifying Exposure

    Occasionally I like to review what I think I know about an area of the photography. It's lead to new insights and challenged a few long held beliefs. With this in mind, I find it interesting that there is often some uncertainty among photographers regarding the fundamental question of what the exposure meter “reads” and it’s connection to film exposure. The discussion usually centers broadly around percentages, Zones, or other nonspecific terms, but does this really explain it? This is a subject that I’ve covered before, although it’s never been the topic of a thread.

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    Is it not reading the illuminance (intensity) of reflected light and then using Exposure = Illuminance x Time?

    Edit: I feel like I'm missing something here...
    Last edited by Michael R 1974; 05-09-2013 at 08:46 AM. Click to view previous post history.

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    There comes a point in photography when one can so embroiled in the technicalitys, testing equipment and materials that we lose sight that it's about taking good pictures .
    Ben

  4. #4
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    Is it not reading the illuminance (intensity) of reflected light and then using Exposure = Illuminance x Time?

    Edit: I feel like I'm missing something here...
    Your basically right, but I'm talking more about image illuminance, Eg. Although the subject illuminance is part of it. So the equation is more like Hg = Eg x t. So what then is Eg, and what is Hg (g = mean)?
    Last edited by Stephen Benskin; 05-09-2013 at 09:37 AM. Click to view previous post history.

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    I'm still confused. When you say Hg are you talking about the image plane? What is image illuminance?

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    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    I'm still confused. When you say Hg are you talking about the image plane? What is image illuminance?
    Yes, H is the illuminance at the film plane. E is the camera illuminace. You used the classic simplified exposure equation E = I x t. Are you able to put it to use? What I mean is that that equation is in most photo books, but none of them actually seem to define it or show how it works with examples. So, we think we know what it is about but only in the abstract and not in practical terms.
    Last edited by Stephen Benskin; 05-09-2013 at 10:23 AM. Click to view previous post history.

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    I'm still not sure what you're getting at - ie what sort of use to put it to. Re the equation, I learnt it from introductory Kodak sensitometry publications in which there is some brief discussion of the principles, units, that sort of thing. But I never really dug into the details. Perhaps that's why I still can't figure out how to do a proper film speed test with a contacted step tablet (in the camera or on the enlarger baseboard) without a calibrated sensitomter.

    Then of course there are the usual issues related to the potential differences between the metered exposure (or the exposure set on the camera) and the actual exposure at the film plane.

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    A specific subject luminance will result in a specific illuminance at the film plane. It would depend on aperture, focal length, focus distance, angle from lens axis, lens flare, light loss due to glass absorption etc.. I think that's what Steve was talking about. This illuminance x the exposure time will result in a specific density depending on the film characteristic and processing.

  9. #9
    Stephen Benskin's Avatar
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    Let's not forget exposure meters. People talk about them seeing 12%, or 18%, or Zone V. These terms don't really define how the meter works or how it relates to camera exposure.
    Last edited by Stephen Benskin; 05-09-2013 at 11:25 AM. Click to view previous post history.

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    Chan Tran: That's all fairly obvious so I'm still suspicious. Underlying all the variables is the usual question of whether or not the various image exposures at the film plane based on the metering and camera setting will generate the expected film densities (ie the first transition in the tone reproduction diagrams), but I don't think that's what Stephen is getting at.

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