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  1. #11
    df cardwell's Avatar
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    An ISO rating refers to how a film performs under specific laboratory conditions, designed solely for the comparison of film peformance.

    An EI rating is determined by the photographer to describe how a film performs in real world conditions. Since film makers have no idea how you are going to use their product, the best they can do is to test in a certain way and then tell you to only use the data a starting point.

    Use ISO data to compare films, use EI to make pictures.

    .
    "One of the painful things about our time is that those who feel certainty are stupid,
    and those with any imagination and understanding are filled with doubt and indecision"

    -Bertrand Russell

  2. #12
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    Quote Originally Posted by ElectricLadyland
    basically i dont see why having the EI precisely what it is matters or is beneficial, becasue curves vary between films so even same EI films will have differnt tones for the same exposure... but i thik what is important is knowing that at any given EI for a particualr film, this meter reading and developer/time will develop to this tone. what are some thoughts on this?
    Excellent observation. The fixed density method of the ISO standard is really a short cut, and the Zone System fixed density method is conceptually flawed. Most people just want to know just enough to make something work. They usually aren't interested in the reason why.

    Without going into great detail, the most accurate speed method is based on the gradient of the curve and not a specific density. It's call the Fractional Gradient Method. Todays ISO standard is representative of this method except it uses a fix density short cut method. Lyod A. Jones determined, after extensive testing, that consistently high quality negatives are produced when the speed point is at a point on the curve where the gradient is 0.3x the average overall film gradient. In many cases, this isn't at 0.10 over fb+f.

    With this in mind, using any fixed exposure or density method of print determination is also counter indicated since there isn't a specific base density in the negative to key on.

  3. #13

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    Quote Originally Posted by Stephen Benskin
    Excellent observation. The fixed density method of the ISO standard is really a short cut, and the Zone System fixed density method is conceptually flawed. Most people just want to know just enough to make something work. They usually aren't interested in the reason why.

    Without going into great detail, the most accurate speed method is based on the gradient of the curve and not a specific density. It's call the Fractional Gradient Method. Todays ISO standard is representative of this method except it uses a fix density short cut method. Lyod A. Jones determined, after extensive testing, that consistently high quality negatives are produced when the speed point is at a point on the curve where the gradient is 0.3x the average overall film gradient. In many cases, this isn't at 0.10 over fb+f.

    With this in mind, using any fixed exposure or density method of print determination is also counter indicated since there isn't a specific base density in the negative to key on.
    Stephen,

    Thanks for your excellent explanation of the objective reality of this matter.

  4. #14
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    Having been involved extensively in speed calculations for one of the first Gold 400 films back in the 80s, I can say that there is a lot of 'slop' in all of these figures.

    We tried to use the ANSI methodology in such a way that ISO, ASA and EI all gave just about the same result. This involved getting the curve shape just right so that all three of these methods aligned to give similar speeds. If they still adhere to this, I would guess that most Kodak films work the same whether you use any of those three to meter your film.

    However, I've found that an overexposure of about 1/3 stop is usually indicated for most B&W and color negative films. This moves the exposure up the scale and gives you less imaging on the softer portion of the toe. It therefore tends to eliminate differences between films and the measuring methods. The latitude of most negative films is long enough to allow for that little bit of overexposure.

    PE

  5. #15

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    Use ISO data to compare films, use EI to make pictures.


    The film speed, ISO or EI, is only half the story. A film speed is like a fish without water. You need to know both the speed and the contrast. Only then can you start making photographs.

  6. #16
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    Quote Originally Posted by j-fr
    Use ISO data to compare films, use EI to make pictures.


    The film speed, ISO or EI, is only half the story. A film speed is like a fish without water. You need to know both the speed and the contrast. Only then can you start making photographs.
    Although this is correct in theory regarding speed vs contrast, it is actually ony 'correct' for ISO and ASA which are rather constant regardless of mid-scale contrast, and EI which varies very much according to contrast. (this is a relative statement with relation to these 3 values)

    The thing that damps this out is that a good negative film must have a long straight line portion to the curve to have good latitude, and the ANSI standards define that the mid-scale contrast of the best negative films should be between 0.5X and 0.6X or about 0.55 - 0.65 (IIRC - I have forgotten the exact values unfortunately and have no reference here to them).

    Some ANSI definitions seem to do this in terms of latitude and Dmax while others seem to do this in terms of contrast. Since I have usually read much of this in the works of others on this matter, it may be their way of interpreting the standard for some degree of simplicity.

    Now, the thing is that Kodak B&W and color films are designed with a contrast of about 0.6 - 0.63. The professional products are near the lower value and the consumer products are at the higher value (due to lens flare expected in cheaper cameras or single-use cameras). In any event, this coupled with the toe shape allows the ASA, ISO and EI to be close in value.

    In theory, you could have a high contrast, soft toe product which had similar EI, ISO and ASA values, but I wouldn't give the product a long life time. No one would like the results. It would have short latitude and bad tone reproduction along with a number of other faults. By math models, it is possible to come up with an 'ideal' toe and mid scale that yields the best film curve for all, but also brings EI, ISO and ASA into close congruence.

    By that, I refer you to my comment above. I believe that these values are all within about 1/3 stop of being identical on most modern Kodak films.

    PE

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