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  1. #41
    markbarendt's Avatar
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    Quote Originally Posted by Bill Burk View Post
    In another thread I had this thought...

    I can't reconcile the ideas of using statistical average when I could quickly spot shadow and highlight and know the actual average.
    One of the joys of using an incident meter is not having to "spot" or "pick" any tone, I get the "average" automatically.
    Mark Barendt, Beaverton, OR

    "We do not see things the way they are. We see things the way we are." AnaÔs Nin

  2. #42
    Stephen Benskin's Avatar
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    Itís important to re-emphasize that the film speed Ė meter exposure ratio is not just chance from of the various elements involved in producing an exposure, nor is it intrinsic to the photographic process. It was chosen. Also keep in mind the film speed value isnít intrinsic to the photographic material either. In A Review of Speed Methods, Todd and Zakia write, ďThe speed of a photographic material is not a fundamental concept, but provides an index number useful for calculating camera settings.Ē The speed point; however, does have a specific reason for itís location. As a result, while the film speed is determined at the speed point, it might be desirable for the exposure placement to be else where.

    The Fractional Gradient method is a good example of this. Itís speed point is located where the slope of the toe of the curve is equal to .3 times the average gradient of the curve. Because of this, it jumps around some, but under the current ISO standardís parameters for film contrast, the fractional gradient (0.3G) speed point falls approximately one stop below the fixed density point of 0.10 over Fb+f.

    As weíve seen, the current ISO speed equation is 0.8/Hm. The exposure value for a 125 speed film would be .8/125 = 0.0064 lxs. If the .3G speed point falls one stop down, the exposure value there would be half of 0.0064 or 0.0032. If we apply that value to the ISO speed equation, we get .08/0.0032 = 250, but thatís not how 0.3G worked.

    They wanted to give the exposure a bit of a safety factor. Meters werenít too common, and many were bad. Many people tended to use either rule of thumb or exposure guides. So, the exposure placement needed to be elsewhere on the curve. Changing the location of the speed point was out of the question because the use of itís location had best correlation with the psychophysical first excellent print test.

    All that is required to shift the exposure placement while keeping the speed point location stable is to change the speed constant which will then change the speed point Ė metered exposure ratio. For 0.3G, they used 0.25 for the speed constant producing 0.25/Hm for the equation (Iím using current nomenclature to help avoid confusion). This would make the film speed for the same exposure values that resulted in a 125 speed value using the current speed stand: 0.25 / 0.0032 = 78.

    The metered exposure is then 8 / 78 = 0.103 lxs. The ratio between the speed point and the metered exposure then becomes 0.103 / 0.0032 = 32 or 1.50 log-H units. So instead of having the metered exposure fall 1.0 log-H above the speed point as with the current ISO standard. The Fractional Gradient Method had the metered exposure fall 1.50 log-H above itís speed point. Thatís a difference of 1 2/3 stops (0.50 log-H).

    The below graph illustrates the exposure placement resulting from the Fractional Gradient Method. The exposure range has been adjusted to represent one stop of flare.

    Click image for larger version. 

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    The speed between the 0.3G speed point and where the shadow exposure falls is considered the safety factor. In addition to the shadow exposure falling 1 2/3 stops above the 0.3G speed point, it also falls 2/3 of a stop above the density of 0.10 over Fb+f. A key point to remember about speed points is that the shadow exposure doesnít necessarily have to fall on them (for black and white negative film).

    Next. How does changing the EI of the film effect the film speed Ė metered exposure ratio? Also, how to adjust the speed constant to create the desired change in the ratio.
    Last edited by Stephen Benskin; 01-10-2012 at 11:30 PM. Click to view previous post history.

  3. #43
    Stephen Benskin's Avatar
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    Now letís take a look at the changes caused by adjusting the filmís EI away from the ISO value. Most general purpose developers, when used correctly, tend to produce speeds in accordance with the ISO film speed, but it is often desirable to want a little extra exposure when shooting. So the photographer under rates the speed of the film. This might be done for a variety of reasons like for a larger safety factor, better shadow separation, or even personal taste. What does changing the EI on the exposure meter do to the film speed Ė metered exposure ratio?

    First of all, changing the film speed setting on the exposure meter does nothing to the actual speed of the film or how the film responses to exposure. For a 125 speed film, the speed point will continue to produce a density of 0.10 over Fb+f when Hm is 0.0064 lxs no matter what the speed setting on the exposure meter. Setting a lower EI on the exposure meter will only shift the exposure placement to the right on the curve.

    Itís common practice to under rate a film by half the ISO speed. With our 125 speed film example this would mean rating the film at 64. Because this isnít exactly Ĺ of 125, Iím going to use 62.5 to keep the results simple. The metered exposure becomes 8 / 62.5 = 0.129 lxs. The ratio then becomes 0.128 / 0.0064 = 20 or a 1.30 log-H range. The two graphs below are examples of the exposure placement of the 125 speed example that has been under rated by 1 stop. One without a flare adjustment and the other with a one stop flare factor.

    Click image for larger version. 

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    Just like with the Fractional Gradient Method, itís possible to adjust the speed point Ė metered exposure ratio when doing the film test. Letís say the photographer likes the results of under rating the film by one stop. Of course, he could always just half the tested film speed, or he could factor it into the sensitometric test.

    It should be apparent at this point that adjusting the film speed by changing the placement of the speed point is the wrong approach. It would not only lose the connection with the exposure limits of the film curve, but it would also lose the good correlation between the sensitometric method and the psychophysical method of speed determination.

    The only credible sensitometric approach is to change the speed constant from the film speed equation while keeping everything else the same. For our 125 speed film, the constant will go from 0.8 to 0.4.

    0.8 / 0.0064 = 125
    0.4 / 0.0064 = 62.5

    Weíve shown how the ratio can be determined from dividing the speed point exposure value into the metered exposure value. It can also be determined by dividing the speed constant into the exposure constant.

    8 / 0.8 = 10 or 1.0 log-H
    8 / 0.4 = 20 or 1.30 log-H
    From the fractional gradient method: 8 / 0.25 = 32 or 1.50 log-H.

    Some of those familiar with the Zone System might be starting to wonder about the Zone Systemís testing method of using a speed point Ė metered exposure ratio of 1.20 log-H.

  4. #44
    Bill Burk's Avatar
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    Interesting that your second diagram showing half film speed with flare...

    Counting down stops/Zones from V.

    Metered/placed Zone I falls only one-third stop to the right of the 0.1 speed point (a sane match for Zone I). And metered/placed Zone 0 falls one-third stop to the right of the 0.3G speed point (a sane match for Zone 0).

    So I prefer a placement that is one-third stop to the left (faster) of the placement in your second diagram. My EI is two-third stops under rated film speed. I expect this to land my Zone I placements on 0.1 density on the film. Anything that would meter Zone 0 I would expect no density (threshold only).

    It seems better to maintain the sensitometry to standard, so that it looks right in comparison to documentation. Then to shift EI at the last minute to accommodate preference/taste.

    No?

  5. #45
    Bill Burk's Avatar
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    Quote Originally Posted by Bill Burk View Post
    It seems better to maintain the sensitometry to standard, so that it looks right in comparison to documentation. Then to shift EI at the last minute to accommodate preference/taste.
    Sounds like I said it ambiguously.

    I mean it is easier to shift EI which affects exposure placement by changing where you place exposure. (I mean it does exactly what I want). So it seems better to shift EI

  6. #46
    markbarendt's Avatar
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    Quote Originally Posted by Bill Burk View Post
    Sounds like I said it ambiguously.

    I mean it is easier to shift EI which affects exposure placement by changing where you place exposure. (I mean it does exactly what I want). So it seems better to shift EI
    This actually brings up something that I have wondered about for a while.

    Why is it that we don't set spot meters to read the camera setting directly from the speed point?

    The math isn't the issue since the formula can be adapted.
    Mark Barendt, Beaverton, OR

    "We do not see things the way they are. We see things the way we are." AnaÔs Nin

  7. #47
    CPorter's Avatar
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    Quote Originally Posted by Bill Burk View Post
    Metered/placed Zone I falls only one-third stop to the right of the 0.1 speed point (a sane match for Zone I). And metered/placed Zone 0 falls one-third stop to the right of the 0.3G speed point (a sane match for Zone 0).
    Where on the horizontal axis of that graph are you claiming that Zone 0 and Zone I lies? Just curious here.....

  8. #48
    markbarendt's Avatar
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    Quote Originally Posted by markbarendt View Post
    This actually brings up something that I have wondered about for a while.

    Why is it that we don't set spot meters to read the camera setting directly from the speed point?

    The math isn't the issue since the formula can be adapted.
    Putting some numbers to this thought for an ISO 125 film.

    When metering from shadows, set meter at maybe EI 2000 (+or-).

    When metering from a face, set meter at maybe EI 64.
    Mark Barendt, Beaverton, OR

    "We do not see things the way they are. We see things the way we are." AnaÔs Nin

  9. #49

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    What is the point of that? It is the same as setting your EI and reading the differences in f-numbers.

  10. #50
    Bill Burk's Avatar
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    Quote Originally Posted by CPorter View Post
    Where on the horizontal axis of that graph are you claiming that Zone 0 and Zone I lies? Just curious here.....
    I'm calling the metered point Zone V and counting three-columns each per Zone

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