ISO film speed point relative to in-camera exposure

Discussion in 'B&W: Film, Paper, Chemistry' started by Anon Ymous, Dec 29, 2018.

  1. Stephen Benskin

    Stephen Benskin Member

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    I'd like to clarify something so we are all on the same page. There is only one film speed under a set developer condition. Shooting under varying conditions can result in a different spectral response of the film. Exposure meters also differ in spectral response depending on which materials the photo-cell is made. This will create a difference in exposure placement, not film speed.

    I make a distinction between exposure and film speed theory and practice. My point is that there are so many variables involved that it's better not to do a film speed test than to do an inherently sloppy one. Find the right development time, then bracket a few frames and evaluate what works best for you.

    Don't know where you saw this unless you are confusing gradient with speed.

    By 18%, do you mean the meter's exposure point because the relative reflectance is 12%. That's introducing 1/2 stop error into the test. And stop down three stops comes from where? Unless you are saying that within +/- 1/3 or so stop, you have found 0.10 over Fb+f three stops down from the metered exposure?

    Why ZS and ISO Speeds are different.jpg
     
    Last edited: Jan 9, 2019
  2. Nodda Duma

    Nodda Duma Subscriber

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    Interestingly enough, here are the developer formulas per the old ASA standard. This is out of Practical Photographic Chemistry by O’Hara & Osterburg.

    7CCDACDE-1D2E-4150-B682-EDB4352BBA4F.jpeg

    Of course, I said f’ it and just base my dry plate speed estimate on developing in HC-110 dil B for 5 minutes.
     
  3. Photo Engineer

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    The best speed value is determined by using the product and in such a way as it gives you optimum results under perfect conditions and with your preferred workflow. From that point on, you can make corrections which make sense.

    PE
     
  4. Nodda Duma

    Nodda Duma Subscriber

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    In my profession, that would fall into the bucket of “politically correct responses”. :smile:

    (Having some fun...I’m coming out of a two-day meeting where it was almost entirely political).
     
  5. ic-racer

    ic-racer Member

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    Low zones are not very sensitive to changes in development conditions. This makes low zones good candidates for tests of exposure when trying to hold development conditions constant.

    For example two situations that yield zone VII densities of, for example 1.35 and 1.65 shown an error in exposure...or no...an error in development...or both... or....well you get the point.

    Two situation that yield zone I densities of, for example 0.08 and 0.38 will more likely result from an error in exposure rather than an error in development.
     
  6. Bill Burk

    Bill Burk Subscriber
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    Kodiak,

    Your colorful language is refreshing, and you have an interesting idea. I'd welcome you to create a thread of your own to give it a place to grow.

    But I can't 'unlearn' what I've grown fond of. The graphs I draw swiftly on paper are beautiful to me so I won't be giving them up anytime soon.

    As beautiful as I think they are, I'd rather look at prints I made from my worst negatives. And I know what those look like because I just printed two of my worst negatives.

    I use two decimal places of precision. There are a few good reasons. Two decimal places handles 0.15 without rounding errors (That number appears a lot because it is equivalent to a half f/stop). My graph paper has 0.02 spacing. When I graph the point of a coordinate that is an odd number (like 1.03), I touch the pencil in between lines. For even numbers I touch the lines. It's not hard to "write" a point this way, and when double-checking it's possible to "read" the graphs back to the original two places.

    I have a "calibrated" grayscale in my sensitometer. On my graphs, I plot the x-axis with the calibrated values instead of the nominal values of the grayscale. All that means is that while the grayscale is created in "half f/stop" steps, these steps aren't perfect. Somebody wrote down the actual gray values. I drew green lines on my personal sensitometry graph for each value of the step wedge: http://beefalobill.com/images/sensitometrymarkvi-5.pdf

    The advantage of the green lines are smoother curves, because the x-axis is "right" and it's a lot faster only having to find a green line and draw a mark at the density that I read off the test film.

    This is the densitometer I use these days. I have a photoelectric one but I prefer the simplicity of this one. It works off the principle of inverse squares...

    https://www.photrio.com/forum/attachments/24bed12b-e662-47af-8cc1-214e2639651b-jpeg.202916/
     
  7. Craig75

    Craig75 Member
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    Thank you for that very clear summary of the test. The chapter in Mees is far too dense for me to understand so this summary is much appreciated.
     
  8. Stephen Benskin

    Stephen Benskin Member

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    The first excellent print papers (The Evaluation of Negative Film Speeds in Terms of Print Quality, parts 1 &2) are only part of the process toward finding an accurate film speed. The test concluded the minimal useful point of exposure that will create a quality print is not based on density, but on the shadow gradient compared to the film's overall gradient. This is something any printer already intuitively knows. Good separation of tonal values in make for a higher quality print, especially in the shadows.

    It's the next paper (A Study of Various Sensitometric Criteria of Negative Film Speeds) that determined the film speed method. The first excellent print test gave Jones sensitometric correlation with the perception of quality in a photographic print. Next he needed to determine a simpler method (pure sensitometric approach) that would most closely match the results of the "judged" speeds. While the print judgement test is the most accurate, it is prohibitively laborious. A Study of Various Sensitometric Criteria tests a number of known and new methods ti determine which methodology comes closest to the judged speeds under the greatest number of situations and film types. The fixed density method of 0.10 over Fb+f was one of the methods tested and rejected. From this series of tests, the Fractional Gradient Method was chosen.

    The difficulty of finding the shadow gradient that was 0.3x the films overall gradient kept the method from achieving universal acceptance. In the late 1950s, C.N Nelson and J.L. Simonds came up with an equation that uses the easy to find fixed density method to determine the fractional gradient speed point (Simple Methods for Approximating the Fractional Gradient Speeds of Photographic Materials). The ISO contrast parameter is the equation in physical form. At least it produces the same answer for Delta-X (ΔX). You can see this illustrated in a earlier post. So, if the parameters are met, the fractional gradient speed point will fall at the same point for any film tested. It compares apples to apples. Anything outside the parameters needs to use the actual Delta-X equation. I wrote a paper on the Delta-X Criteria. If it's not in Bill's link, I can post it here...It's not, so please see below.
     

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    Last edited: Jan 10, 2019
  9. DREW WILEY

    DREW WILEY Member
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    To make actual sense of this, you have to meter the shadow values. A one degree handheld spotmeter really helps in this respect. There is simply no automatic formula about where crisp shadow separation occurs relative to middle (gray card) gray. For one thing, scene contrast varies from circumstance to circumstance, shot to shot. For another thing, different films vary with respect to how far you can dig down into the shadows in a high contrast scene and still remain on the relatively straight line section of the film curve, versus the toe. All the calculations in the world won't help you if you don't understand this fundamental fact first, or really, how to interpret film curves with respect to toe, straight line, and shoulder. I personally consider the specific characteristics of the toe to line transition far more important than any ISO conventionality. And even this kind thing is affected by developer variables. I have certainly done more than my fair share of densitometer plotting etc, as well as working with a very broad range of both color and black and white films. In color work, determining middle gray is important. With black and white film, it's reading shadow versus highlight range that counts. If it's a low contrast scene, just taking the average mid-gray reading will generally work if you've established your personal ASA for a particular film using a particular development regimen. But if you encounter a serious luminance range, you need to know the two endpoints you hope to successfully show texture or value gradation in. It's also important to remember that film curves are plotted on a logarithmic scale, so what seem like minor foibles along the toe can actually have a significant impact if you're relying on that part of the toe.... And sorry to start a brawl, but developer choice and concentration can very much affect the toe shape of
    certain films. I base entire technical lab processes on that fact.
     
  10. Photo Engineer

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    Fellow members and subscribers, I doubt if there is another member here that has had to do this on a real basis for film or paper. I have had to do both. No single method is best, but the one that approaches best is one that works making good negatives (or slides) and prints. It does it over and over. Mees and Haist show it. I have done it and it really works.

    You can use the zone system (a dumbed down system of 21 step sensitometry) and other methods, but use is what works. First acceptable print works. Whatever you use though, remember that what works for you is OK. It is just that it may not work for the rest of us due to your conditions.

    PE
     
  11. Stephen Benskin

    Stephen Benskin Member

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    I agree to use whatever works for your process. Even a broken clock is correct twice a day. Personally, I tend to recommend not to use any film speed method, other than taking actual photographs, for all but irregular conditions such as use of specialty developers, reciprocity situations, and extreme pulls; however, I'm also an advocate for understanding theory in order to make informed decisions.

    For example, while The Zone System does use simplified sensitometry, it also uses it's own interpretation. How many times, through how many years have there been conspiracy stories floating around on how manufacturers lie about the true film speed, or that the scientific testing doesn't represent real world use, or that somehow an amateur with a hand held meter can obtain the most accurate film speeds, when the reason Zone System speeds tended to differ from ISO speeds by 1/2 to one stop, is because they use a different range between the metered exposure point and the fixed density point of 0.10 over Fb+f which will produce different speed values with identical conditions?
     
  12. Bill Burk

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    To get an 0.1 density on the film, move the camera exposure towards less exposure by a factor of 10.
    You can do this by multiplying the arithmetic film speed by 10.

    For example, if the film speed is 100, set the meter or camera to 1000.
     
  13. Stephen Benskin

    Stephen Benskin Member

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    Good trick, but density unrelated to contrast has little significance.
     
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  15. jnantz

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    i always love reading these threads and posts
    because it reminds me about how much i don't know :smile:
     
  16. Bill Burk

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    Haa yes. I just wanted to explain how to place an exposure at the 0.1 speed point.

    I have a few other tests for contrast that I like to offer (shoot one shot at ASA and another with two stops more exposure). That’s the least expensive test I know. But really once the effort exceeds the cost of a 21 step Stouffer scale, the test gets to be more trouble than I think it’s worth (unless it teaches a really good lesson).
     
  17. Bill Burk

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    That’s because it’s in your bones...
     
  18. Stephen Benskin

    Stephen Benskin Member

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    Last edited: Jan 12, 2019
  19. jnantz

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    :smile:
    i know !
    im always so amazed at how complicated photography can be
    and so many of us are ignorant of the stuff that makes
    the stuff we take for granted happen ! :smile:
     
  20. Bill Burk

    Bill Burk Subscriber
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    I was talking to my dad tonight reminiscing about when I was in high school. One day I climbed out my window on the second floor and pulled myself onto the roof where I could get a flat surface with even lighting for a few still life’s. I knew it all then.
     
  21. Bill Burk

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    85C81EE4-AA8D-4507-AA95-322E28AE5C76.jpeg
    One of the pictures I took up on the roof when I was a kid.
     
  22. Photo Engineer

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    Teenagers. Always trying to get a date! :D
     
  23. Tim Stapp

    Tim Stapp Member
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    That was me that you are describing :smile:.

    I'm amazed at how much better my photographs were when I knew absolutely nothing and just went out shooting. Now, I seem to think way to much and shoot way too little.
     
  24. RalphLambrecht

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    Stephen,I'm glad to see that our results match so closely.
     
  25. jnantz

    jnantz Advertiser Advertiser
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    i think this is why picasso wanted to paint like a child
    because as a kid/beginner &c you don't get entrapped
    by all the rules and conventions that we all end up with later.
    those dates look good, im kind of getting hungry :smile:
    yeah, what he said ^^^
     
  26. Photo Engineer

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    Tim, he posted a picture of dates!!!
     
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