Quantifying Exposure

Discussion in 'Exposure Discussion' started by Stephen Benskin, May 9, 2013.

  1. Stephen Benskin

    Stephen Benskin Member

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    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.
     
  2. michael_r

    michael_r Subscriber

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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...
     
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  3. benjiboy

    benjiboy Subscriber

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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 .
     
  4. Stephen Benskin

    Stephen Benskin Member

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    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)?
     
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  5. michael_r

    michael_r Subscriber

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

    Stephen Benskin Member

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    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.
     
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  7. michael_r

    michael_r Subscriber

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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.
     
  8. Chan Tran

    Chan Tran Member

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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. Stephen Benskin

    Stephen Benskin Member

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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.
     
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  10. michael_r

    michael_r Subscriber

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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.
     
  11. michael_r

    michael_r Subscriber

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    Ok now I think I see where you're going.
     
  12. Stephen Benskin

    Stephen Benskin Member

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    That is what I'm getting at. It should be obvious, but how many people are comfortable with it. In camera testing, such as the Zone System method, use the exposure meter for calibrating the film speed, yet few can define what the meter is actually doing or what the exposure at the film plane should be.
     
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  13. michael_r

    michael_r Subscriber

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    Well, I'll admit, for example, the section in Henry's book on meters is fairly tough for me to get through every time.
     
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  15. Bill Burk

    Bill Burk Subscriber

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    I have a couple simple questions.

    Is it just that the meter wants to put the same amount of light on the film as sensitometric testing would put on the film?

    Then the meter has the extra work to try to back out the optics of the meter and factor in the optics of the camera and estimate the camera flare. (while an in-camera meter is excused from some of that work)?
     
  16. Stephen Benskin

    Stephen Benskin Member

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    Henry was somewhat non-specific in that section.
     
  17. Stephen Benskin

    Stephen Benskin Member

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    That's bascially how the two are related.


    Yes, hand held exposure meters have to estimate the value of q, while in camera meters measure the actual luminance passing through the optical system.
     
  18. michael_r

    michael_r Subscriber

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    Why do you say that? It gets into quite a bit of detail on calibration, flare etc.
     
  19. Stephen Benskin

    Stephen Benskin Member

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    Not very specific for my taste, and while we're talking about it, not very accurate either.
     
  20. michael_r

    michael_r Subscriber

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    In any case it doesn't matter much to me since there is not much I can do about it anyway. I always get a laugh out of the meter modifications. I'll get right on that as soon as Leica builds me a precision 4x5.
     
  21. Stephen Benskin

    Stephen Benskin Member

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    Take a look at his flare graph. He starts it at 0.10 over Fb+f and has the amount of flare based from that point, when it is actually based off a stop below. A flare factor of 2 will not double the exposure at 0.10. His exposure testing also uses Zones and stopping down 4 stops.

    What about the technique of shooting a step tablet with a camera, whether it is inside like with Shaffer's method or outside like WBM? The camera is the exposing device and the exposure meter determines the exposure. Wouldn't it be beneficial to understand what the film plane exposure should be (Hg)?

    Let's say we are testing a 125 speed film. What should the exposure be at the speed point? What should the exposure be at the metered exposure point? What is the difference between the speed point constant and the exposure constant?
     
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  22. Chan Tran

    Chan Tran Member

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    As Ben said, some of us can get unnecessary too technical but if you feel like you're missing something then yes you do. While it's simple Exposure=Illumninance x Time but the meter reads the luminance( and not Illuminance) of reflected light and thus there is a relatively complex process to arrive at what will be the Illuminance at the film plane.
     
  23. michael_r

    michael_r Subscriber

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    That isn't what I meant by missing something. I know the variables involved. I meant it might be a Benskin-esque "plain sight" trap.
     
  24. michael_r

    michael_r Subscriber

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    Stephen, for the speed point we know the exposure should be Hm in S=0.8/Hm when the ISO conditions are satisfied. Determing Hm is what I find tricky. Simplifying without ISO requirements, even when just targetting an arbitrary fixed density speed point with a step tablet test (in camera, out of camera, whatever), I find it somewhat less than straight forward.
     
  25. Stephen Benskin

    Stephen Benskin Member

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    I'm speaking more theoretically. Under the ISO conditions, what should Hm be if the film speed was 125? What would the metered exposure point be? And what is the difference between the two?

    This isn't as hard or overly technical as some are suggesting, and by understanding a few basic rules of exposure, it's possible to evaluate the validity of a test method like Schaffer's or WBM. To start with, what are the exposure instructions for the WBM method? It's a simple job of comparing the expected results with the two known exposure points in the above question.
     
  26. Stephen Benskin

    Stephen Benskin Member

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    Books don't seem to cover this. The more technical books assume their readers are familiar with the values of the variables and don't bother to show examples. More general photography books usually don't attempt to cover it, so the opportunity of working with actual numbers associated with exposure falls through the cracks. I think this deprives people of a very useful tool for analysis or simply for a better understanding of the process. How can someone think to properly analyze something like the ISO speed standard when they don't have the necessary tools.