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  1. #21
    Stephen Benskin's Avatar
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    The exposure constant is the numerator in the film speed equation. While the speed point is chosen because of it's relevance, it might be desirable to place the exposure at a different place. Any number other than one actually determines a point other than the speed point for the exposure placement. What if the ISO speed equation was 0.4 / Hm? This would offset the exposure by a stop.

    BTW, I don't see the ZS EI as being designed for the discriminating taste of the photographic elite. It's just the safety factor from before 1960. All anyone has to do is take the ISO film speed and cut it in half.

  2. #22
    Bill Burk's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    BTW, I don't see the ZS EI as being designed for the discriminating taste of the photographic elite. It's just the safety factor from before 1960. All anyone has to do is take the ISO film speed and cut it in half.
    That's a short-circuit (which I occasionally do) by taking the usual outcome without stepping through how to get there...

    Steps for testing for Zone System film speed are well (and inconsistently) documented... There isn't any intention to include a safety factor. Its a specification of quality that shadow detail is required.

    And it may not correlate to the first excellent print criterion, the negative that fits Grade 2 under Zone System, may well be exposed a stop above the "first excellent" print.

  3. #23
    Bill Burk's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    The exposure constant is the numerator in the film speed equation..
    OK, that's the constant 0.8 and it's the same between ZS and ISO right?

  4. #24
    Stephen Benskin's Avatar
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    The Zone System doesn't have one.

  5. #25
    Bill Burk's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    The Zone System doesn't have one.
    So the Zone System defines its benchmark log-H point in terms of the EI which corresponds to the "desired" shadow density quality, which in early versions was a visual determination where you could just detect detail difference between Zone 0 and Zone I on the print.

    So many of our variables and constants are not defined... but might be inferred if the components could be isolated somehow... Say by doing flare tests and film sensitometry and working backwards to the same meter settings.

  6. #26
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    Quote Originally Posted by MattKing View Post
    It would surprise me if the ISO standard speed was the same as the EI used by ZS practitioners.

    It surprised me too-----but I began using the results as is and I've been using them for quite a while now and have no reason to argue with them. I test precisely as described here, in short, it's an in-camera exposure of a step tablet contacted to a sheet of 4x5 film, yielding flare-free step data.

    Tmax 100 & D76 1:1-------Tmax 100 & XTOL 1:1 tested with the same results:
    N+2 EI 125
    N+1 box speed
    N box speed
    N-1 box speed
    N-2 EI 80


    Tri-X 320 & HC-110(1:63)
    N+2 EI 400
    N+1 box speed
    N box speed
    N-1 EI 200
    N-2 EI 160

  7. #27
    Ian Grant's Avatar
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    Quote Originally Posted by MattKing View Post
    It would surprise me if the ISO standard speed was the same as the EI used by ZS practitioners.

    Because I think that the vast majority of photographs are taken by people who would most likely choose prints that are different then the prints preferred by most ZS practitioners.

    Most people prefer lighter prints with more contrast. People who like "fine" prints tend to have different tastes.
    The ISO standard allows the film speed to be test by either the ASA/BS system (which Kodak had relaxed) or the more practical DIN system used by Agfa.

    So with Agfa films like APX100 where the speed was tested using the DIN standard my Zone system tests gave me an EI thge same as the ISO speed, and many others found the same. The opposite was true with Tmax100 where I needed to use50 EI instead of the box 100 ISO.

    It's also worth remembering that the developer used for the ASA tests gave slightly better film speed and shadow detail than D76.

    Ian

  8. #28

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    While I appreciate and understand all the sensitometry data involved here, I really believe the answer to Bill's original question can be expressed in less technical terms.

    First, we haven't addressed one important factor in the ZS; that of adjusting development time to include a wide tonal range of 9 Zones (or 10, depending on how you're counting) for "Normal." This, I believe, is somewhat more range than the ISO tests result in. Therefore, a ZS N-development is usually less than the ISO standard development. This results in a skosh less effective film speed.

    Secondly, we Zonies use our spot meters as arbitrary tools to place shadow values and see where other values fall. In practice, this ends up having very little or nothing at all to do with speed points or metering points (or middle grey for that matter). It is more an "I know what my shadows will look like when I place them in Zone III (or II or IV), because I've tested, and that's where I want them" thing. That coupled with "and now I have to adjust development a bit to get the highlight values where I want them" (many times callously disregarding the slight changes in effective film speed with shortened or lengthened development!) and we can consistently get negatives that are "in the ballpark" enough to be able to be printed well, even if we need to print a grade higher or lower than our target grade. For me, using the meter to help me visualize print values is paramount.

    Furthermore, I think the Zone System emphasis on shadow detail and shadow metering, coupled with the relatively greater range of Zones and the resulting need to develop a bit less results in E.Is. that are slower than box speed for two reasons. First, is the slight bit of film speed loss with reduced development to accommodate more Zones together with the use of compensating developers/techniques, which also results in film speed loss. Then there is a tendency to place of the shadow values higher than might be done in the ISO standard in order to get "detail." Since no one takes up the Zone System without having a desire for richly-detailed shadows, and since most of them are shooting large-format film and aren't overly concerned about grain, there is a tendency in the entire process of determining a personal E.I. and then metering and placing shadows to err on the side of overexposure.

    I know that when I test for E.I. and there is any doubt at all exactly where that Zone I density falls, I'll just rate the film a third-stop slower. What the heck, one third of a stop makes very little difference in the field. Similarly, if I'm not sure exactly where I want a shadow to go, I'll err on the side of overexposure.

    In the end, a personal E.I. is not a film-speed determination, but an accommodation and adjustment of all elements of one's photo-making procedure in order to get repeatable and usable results in rendering of shadow detail. It only stands to reason that, consciously or unconsciously, we tend to err on the side of overexposing, thus building safety factors into our systems without even being aware of it.

    FWIW, I shoot 320 Tri-X at E.I. 250 and TMY at E.I. 320, both only a third of a stop slower than box speed. This with PMK. So not all of us Zonies end up rating our film at half box speed. Even so, I'm aware that I've built maybe 2/3 of a stop cushion into my system. When the chips are down and I need a faster shutter speed to stop wind movement or the like, I won't hesitate to go ahead and "underexpose" my film by a stop (thus undercutting my safety factor). No one likes printing underexposed negs, but sometimes that's the only way to get a shot...

    Best,

    Doremus


    www.DoremusScudder.com

  9. #29
    markbarendt's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    The Zone System doesn't have one.
    I assume you mean "no safety factor" and that makes sense.

    Part of what I see running through ZS thought, discussion, and testing is that everybody's seems to be trying to balance the absolute minimum amount of exposure (so as to minimize grain, exposure time, etc...) against a loss of expected shadow detail. They want to know where the edge of the cliff is.

    Many times though it seems that the generic "1/2 box speed" advice is a lot like a parent telling a child to "step back from the edge of that cliff" even though we know its not really a cliff, the slope is just changing; it's not an absolute point beyond which things are unworkable, it's just starting to require more work.

    It seems to me that over the years also, lenses/coatings had improved and flare was less an issue, at least in part that allowed the switch to ISO from ASA on a technical basis.
    Mark Barendt, Beaverton, OR

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

  10. #30
    Stephen Benskin's Avatar
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    Quote Originally Posted by markbarendt View Post
    I assume you mean "no safety factor" and that makes sense.

    Part of what I see running through ZS thought, discussion, and testing is that everybody's seems to be trying to balance the absolute minimum amount of exposure (so as to minimize grain, exposure time, etc...) against a loss of expected shadow detail. They want to know where the edge of the cliff is.
    Many times though it seems that the generic "1/2 box speed" advice is a lot like a parent telling a child to "step back from the edge of that cliff" even though we know its not really a cliff, the slope is just changing; it's not an absolute point beyond which things are unworkable, it's just starting to require more work.

    It seems to me that over the years also, lenses/coatings had improved and flare was less an issue, at least in part that allowed the switch to ISO from ASA on a technical basis.
    No, it has no exposure constant.

    The edge of the cliff is the fractional gradient point. That is why it was chosen as the speed point. Once again, just because speed is calculated from the speed point doesn't mean that the shadow exposure is designed to fall there. That's where the exposure constant comes in. Post #23 and #25 on the thread ISO Speed Determination Constants begins to discuss this issue. I've also offered to email Simple Method for Approximating the Fractional Gradient Speed of Photographic Materials to anyone interested. Plus in thread #80, I've uploaded a pdf copy of Safety Factors in Camera Exposure if anyone is interested on exposure placement in regards to film speed. Thread #81 has a copy of Calibration Levels for Film and Exposure Devices if you want to know about the actual relationship between the metered exposure and speed point.


    Flare is less of an issue. The illuminance range of the camera image has increased from approximately 1.50 to 1.80. But it is still a factor and many discrepancies between methodologies or confusion over numbers not matching up is do to flare.

    Film speed is based on the first excellent print test. It showed that additional exposure over the first excellent print point showed no significant change in quality over a number of stops (large format). Adding 1/3 to 1 stop, therefore, isn't a big deal. Remember, film speeds were a stop slower before 1960.

    The Zone System doesn't have any great insight into the photographic process. It's just that many people aren't familiar with anything else. The Zone System is just a simplified version of tone reproduction theory. Under tone reproduction the statistically average scene luminance range is 2.20. This is based on diffused highlights and shadow with some detail. That doesn't mean it ignores accent black and specular highlights. These simply print outside the LER range of the paper. Adams isn't much different. Anyone remember the aim density range in the testing section? It's from Zone I to Zone VIII, seven stops.

    Click image for larger version. 

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    And I'm sorry Ian, you really should support such claims.
    Last edited by Stephen Benskin; 03-10-2013 at 12:30 PM. Click to view previous post history.

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