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  1. #11
    keithwms's Avatar
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    Here's the ISO explanation of how to determine b&w film speed.... if you want to pay for it..

    http://www.iso.org/iso/iso_catalogue...?csnumber=3580

    and here's something for image sensors from Kodak...

    http://www.kodak.com/ek/uploadedFile...rements(1).pdf

    Note that many things have to be standardized and stipulated before you can measure the speed. You'd need to know the colour temp of the lamp, because different films have different wavelength sensitivities. And bear in mind that if there is no grain standard, you could arguably call any b&w film any speed you want just by dev'ing it to the standard density you want under a given exposure... knee and toe and gamma be damned! And what about gamma.... who decides the standard gamma? Many things to consider....

    So... everybody has to come to their own film speed- the box speeds are just (very good) suggestions.
    Last edited by keithwms; 12-31-2011 at 03:11 PM. Click to view previous post history.
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  2. #12
    Stephen Benskin's Avatar
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    Quote Originally Posted by keithwms View Post
    Note that many things have to be standardized and stipulated before you can measure the speed. You'd need to know the colour temp of the lamp, because different films have different wavelength sensitivities. And bear in mind that if there is no grain standard, you could arguably call any b&w film any speed you want just by dev'ing it to the standard density you want under a given exposure... knee and toe and gamma be damned! And what about gamma.... who decides the standard gamma? Many things to consider....

    So... everybody has to come to their own film speed- the box speeds are just (very good) suggestions.
    Don't forget hold time. The testing parameters are all part of the important information associated with film speeds. And this is a topic which I love to discuss in great detail. But determining the film speed, isn't what this thread is about. It's not the final goal, but a place to start in how to link the sensitivity of the film to the exposure meter. I'm currently putting together the next part to be discussed. In the mean time consider this, B&W film speed and color reversal film speed are determined at different points, B&W film speed in the toe and color reversal in the middle of the curve, yet they can have the same film speed and work with a the same meter. You don't need a meter for color reversal and a meter for B&W.

    The reason why it's important to understand the film speed equations is because this is how the sensitivity of the film is translated into a number that then links to the exposure meter that then connects back onto the film with exposure placement.

    Side note: the standard gamma of the B&W film speed standard is to create a good correlation between the speed point of the fixed density method of the standard and the fractional gradient speed point. In actuality, the fixed density method of the standard is just a proxy for the fractional gradient method. To nit-pick, it can't be called Gamma or even contrast index because the standard's parameters don't adhere to the testing methodology associated with either method.
    Last edited by Stephen Benskin; 12-31-2011 at 04:47 PM. Click to view previous post history.

  3. #13
    Bill Burk's Avatar
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    Your PDF implied that the needs of black and white negatives took a back seat to color (hey it has lots of latitude and people already know how to expose it well).

    It's interesting that Super-8 was deliberately overexposed (this was done in camera, meters and ISO weren't changed) so that amateur projections would be bright enough to see.

    And wasn't color slide film standard exposure chosen to be suitable for projection? I think this inspired photographers to give less exposure when intended for print production (Galen Rowell underexposed Velvia).

  4. #14
    Stephen Benskin's Avatar
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    Quote Originally Posted by Bill Burk View Post
    Your PDF implied that the needs of black and white negatives took a back seat to color (hey it has lots of latitude and people already know how to expose it well).

    It's interesting that Super-8 was deliberately overexposed (this was done in camera, meters and ISO weren't changed) so that amateur projections would be bright enough to see.

    And wasn't color slide film standard exposure chosen to be suitable for projection? I think this inspired photographers to give less exposure when intended for print production (Galen Rowell underexposed Velvia).
    I love looking behind the curtain. With slide film, the preferred density range changes depending on the way it is viewed. Projected requires a greater range than viewing on a light table.

  5. #15
    keithwms's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    Determining the film speed, isn't what this thread is about. It's not the final goal, but a place to start in how to link the sensitivity of the film to the exposure meter.
    This may be useful, then....

    http://bythom.com/graycards.htm
    "Only dead fish follow the stream"

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  6. #16

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    I really do enjoy all this technical information explaining what I've taken for granted over the years. Perhpas I once knew what all this was but some things just become a blur with experiance. I've found I can explain it very simply boiled down to this....

    If you were to fill a standarized sized glass with water, how long will it take at a certain flow rate? The glass can be the various ISOs, flow is the brightness vs fill time, exposure.

    But of course I have leared so much from you guys, so please don't stop, just give me a little gigle.


    Happy New year everyone.

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  7. #17
    Stephen Benskin's Avatar
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    Quote Originally Posted by keithwms View Post
    This may be useful, then....

    http://bythom.com/graycards.htm
    I've written rather extensively on this. In fact, there's a current thread on this very issue "Comparison of Reflectance from 18% and 12% Zone Models." If you happen to read the Thom piece, you will find very little there. Where's his proof? I'm not saying he's wrong about the average reflectance, he just doesn't explain how or why. This is what I'm wanting to do with this thread. We might think we are on sound ground with certain concepts, but are we. We've all been there. "I read it in an article by Thom. He says Exposure meters don't see 18%, they see 12%." Great, what's the next sentence? How well can this concept be explained to someone else based on Thom's article?

    I used to have a lot of fun with the articles from the early years of View Camera Magazine. The authors would present a new technique or some evaluation of a new film or something like that then they would come to that point where they didn't have enough information or theory to continue and the articles would quickly devolve into a meandering series of music analogies.
    Last edited by Stephen Benskin; 12-31-2011 at 10:15 PM. Click to view previous post history.

  8. #18
    keithwms's Avatar
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    I'd have to get further into it to figure out the argument for 1; at some point it's probably an issue of QE, which depends on the spectrum, so I am not sure how the math shakes out. Anyway I just thought it interesting that something this basic is still contested.
    "Only dead fish follow the stream"

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  9. #19
    Stephen Benskin's Avatar
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    Quote Originally Posted by keithwms View Post
    Anyway I just thought it interesting that something this basic is still contested.
    Totally agree. Although it's not contested in any scientific capacity, just misunderstood at the average user's level. I believe this is because of a lack of access to solid information. And by that I mean "the maths." Have you had a chance to look through the Connelly paper? Admittedly, the paper is not something that can be understood at a single reading. I always read these types of papers with Mathcad running on the computer.

  10. #20
    keithwms's Avatar
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    Nope, haven't read it in detail. I get the gist, but frankly it isn't written in the first-principles style that I'd prefer. Relationships are simply presented and that's that. An annoying style, frankly. I am a photons and electrons kind of guy
    "Only dead fish follow the stream"

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