Choosing film stock based on contrast curve and spectral-sensitivity

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  • #169879
    samdavidzhang
    Participant

      Hi Roger and community,

      I am currently doing a research paper for my school, and I am tasked with answering the question: “Reading the film stock curve and selecting a stock based on their color & contrast representation.”

      I’ve done some preliminary research into the subject, reading Kodak’s Sensitometry Workbook along with a handful of film manuals at the library, and have interpreted the essay prompt as “How can the characteristic curve and spectral-sensitivity chart inform a decision on how a film stock will perform?”

      I decently understand the concepts of contrast/gamma, how to feel out latitude of a stock from a quick glance at its D logE cuve, and the differences in sensitometry between reversal and negative stocks.

      However, I wanted to reach out to the community and ask any cinematographer who has worked a lot with film – or is currently working with film on a show – how reading the spec sheet has informed your decision on a film stock. What did you do with the information? How has it led to your testing phase? Is it even important in the end versus real life testing?

      In addition, I had one technical question that deals with the RGB curves in the D logE chart when it comes to normal process vs push/pull process.

      As an example, let’s take a look at Kodak 200T listed at this pdf link below:

      https://www.kodak.com/content/products-brochures/Film/VISION-200T-Sellsheet_US_4PG-EN.pdf

      In the contrast curve (named “sensitometric curve”), it appears that Blue and Green curves are slightly steeper when compared to the Red curve. I want to confirm that this means the gamma of the individual channels for blue and green are higher, meaning there is “higher contrast in those colors.” Practically, this means that I can expect more Blues and Greens as exposure go up into the highlights, versus the shadows?

      Furthermore, say I push this stock by 1-stop. If I charted the resulting sensitometric curve of the developed negative, the toe would remain the same… but the shoulder would be lifted by that 1-stop compensation. Does this 1-stop push get evenly spread across the RGB channels? If so, does that means the steeper gammas of Blue/Green compared with the Red channel becomes even more exaggerated? In theory, I would expect some more blue/green in the highlights?

      Please let me know if I have this concept wrong. Also, would love to hear any of your personal experiences or thoughts on this topic as a whole.

       

      Thank you

      Sam

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    • #169881
      samdavidzhang
      Participant

        Oh also forgot to include, spectral-sensitivity charts: what are the most practical applications of interpreting this information? Is it solely limited to blue/green screen work? I understand different color responses favoring blue could affect lighting a blue screen for a shoot, but why would it lead me to favor one stock over the other in a  regular shoot?

        #169892
        gabj3
        Participant

          I don’t work a lot with film! I have worked with film, but its organic nature is harder to quantify.

          “resulting sensitometric curve of the developed negative” I’m just going to call the SRC (Spectral Response Curve) of the Negative.

          The SRC dictates a few things; its amplification of G and B channels showcases its tungsten balanced stock (higher gain in the blue channels to compensate for the tungsten illuminant (and I believe the aim is a D65 illuminant).

          If you get very tech’y’ you can look at the SRC concerning the CIE standard for a tungsten illuminant, and you’ll be able to gauge its apparent colour shift and such in comparison to other illuminants.

          However, that is a; to quote a Cambridge professor here, ‘shit ton of math, and it’s easier to shoot it.

          With spiky discontinuous illuminants like RGB LEDs (or spiky illuminants such as fluorescents, HMI’s and so forth), you can ensure the SPD of the illuminant aligns with the SRC of the negative in terms of producing white and mitigating unwanted colour shifts.

          Unless a photographic push defies my fundamental understanding of applying gain to channels, it amplifies all RGB values linearly, and the separation in channels becomes greater linearly in a radiometric sense.

          Let’s say you have an RGB triplet of 100, 75 and 50 in a linear space. If you push it by a stop, you should have an RGB triplet of 200, 150 and 100 in a linear space. As such, the difference between the three triplets has doubled accordingly. In a logarithmic camera container, the math becomes trickier and is dictated by linear bias and encoding.

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