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Thread: Yet another BTZS film testing/PSP/Effective film speed question

  1. #1

    Join Date
    Mar 2023

    Yet another BTZS film testing/PSP/Effective film speed question

    I just finished developing my three sheets of Delta 100 in D76 1:1, as per the "Your Personal Reference Speed Point" article on, and the tip of the ISO triangle falls right at 2.25 on the X-axis. The exposure was setup exactly as described in the article, EV 4.0 as metered off a white card in the center of the light circle, digital timer set for 0.5s. And the 4, 8 and 16min curves come in with EFS 40+, 64+ and 80 for SBRs of 8.3, 5.8, and 3.7 respectively with the Plotter default PSP of 2.4.

    According to the article, what I *should* do is increase the exposure by a half-stop to EV 4.5 so that the tip of the ISO triangle will land right at 2.4 (or set my PSP to 2.25), which will give the three curves EFS of 64--, 100--, and 100+. Then use that as the reference exposure for all future film tests. But that feels like fudging the data to match expectations, not following the data to its logical conclusion.

    Assuming my meter is metering accurately and I'm really getting EV 4.0 worth of light out of the enlarger. And, assuming my timer is working correctly and really giving me 0.5s. Shouldn't I conclude that my workflow is yielding the lower film speeds rather than trying to fudge the test exposure to give me what the box says? Even if my meter isn't reading correctly, it's the meter I'm using out in the world, so it should be off by a consistent amount out there too...

    Or, given that it's only half a stop and some of my shutters probably have at least that much slop in them, am I over thinking this? Should I just pick one, go point the camera at stuff, and make photographs?


    Oh, btw. Not actually brand new at this, it's just been 20 years some of those brain cells are pretty dusty...

  2. #2

    Join Date
    Sep 2014
    Montreal, Canada

    Re: Yet another BTZS film testing/PSP/Effective film speed question

    Over-complicating. Delta 100 is an ISO 100 film and has that speed in D-76. The Zone System EI is 2/3 stop lower than the ISO speed, which would be 64 for Delta 100. You can skip the testing and go directly to that result. If the test gives you a materially different answer, it is generally because there is something in the test (and/or measurement) itself that caused it. Contrary to what is usually written about this stuff, these Zone System EI deviations have little to do with “your own personal etc.” and are more misleading than anything else unless the person doing the test is at least somewhat familiar with exposure theory and the variables involved - in which case the test doesn’t “reveal” anything photographically useful.

    The best course of action is to start with either the ISO speed or an EI one stop less (if you wish to use a Zone System EI and/or give yourself a safety factor), and adjust based on printing. If you are competent or better at printing/editing, and find you are consistently struggling with muddy dark tones, perhaps a decrease in EI is in order, or if you find yourself consistently printing down needlessly dense negatives, maybe you can increase your EI. That’s how you really find a “personal EI”.

  3. #3

    Join Date
    Mar 2002
    now in Tucson, AZ

    Re: Yet another BTZS film testing/PSP/Effective film speed question

    Well said, Michael R. That very precise testing is all well and good... but you have to realize that running the tests just once won't give you the answer. That level of accuracy requires repeated testing and careful measurement every time... and eventually averaging out the answers. That's how all those speed ratings and development times were tested at Kodak, Ilford, Fuji, and every other film manufacturer. (I was there).
    Luckily you can make good photographs just by getting close... 1/3 stop in B/W or 30 seconds in development time won't make that much difference.
    I will point you to the well-known Ansel Adams book "Exposures". There he describes his exposure, development, and printing methods of forty of his photographs. And the lesson I got from that was... despite all the care he took in making his negative, he still worked like a sonofabitch when making a print, in order to get the result he wanted.
    lf_dean, just set your meter at 64 and go make pictures.
    I once took a workshop with Paul Caponigro, whose comment was "those people are living inside a test tube. You can't make pictures there".

  4. #4

    Join Date
    Mar 2020
    San Antonio, Texas

    Re: Yet another BTZS film testing/PSP/Effective film speed question

    I taught a few workshops with Phil Davis and as others have noted, close may actually be good enough. Most people were off by at least a stop (in film speed rating) and more importantly, the development times were way off in may cases. Personal techniques, temperature and other factors went into play for most. In the end, you have many, many variables and the idea is to get as many pinned down and identified as possible. Check your shutters as well since that was what we found to be an enormous area for errors (timing) in many LF applications. Metering errors were the second most common issue. People would get their film speed and developing times worked out and compensate for bellows factors and then blow the metering by a stop (zone) or not adjust for filters properly. So many variables.....

  5. #5

    Join Date
    Mar 2023

    Re: Yet another BTZS film testing/PSP/Effective film speed question

    Thanks guys. I figured “over thinking it” was the answer. Sometimes it’s useful to hear someone else say it though.

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