Film speed rating

Discussion in 'Exposure Discussion' started by michael_r, Dec 18, 2013.

  1. michael_r

    michael_r Subscriber

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    For some of the less experienced out there (or experienced - doesn't matter), I thought I'd throw out this diagram I had done for myself back when I started learning about this stuff.

    2 hypothetical films given identical exposure to a calibrated step tablet and processed identically.

    Which is "faster", or which would you give a higher EI, etc.

    Enjoy
     

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  2. pentaxpete

    pentaxpete Subscriber

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    I think the RED one ?
     
  3. pdeeh

    pdeeh Member

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    I've read a lot this year about films and slopes and chemistry and whatnot, and I still know almost as little as when I started. But I'll have a go.

    Am I right in thinking that as the two curves intersect at 0.1 density (which I think = base fog?) then anything to the LHS can be disregarded, and we have to look at the shape/slope of the curves to the RHS of that point?

    In which case the blue line appears to gain density at a greater rate with exposure, which makes me think it denotes the "faster" film.

    Now I shall be happy to wait and be shot down in flames ... well, I hope not internet-style flames ...

    EDIT hmm have I forgotten to take inertia into account? damn
     
  4. markbarendt

    markbarendt Subscriber

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    Looks to me like the red one would gather the most total shadow detail based on the exposure. But my guess is that because of the slope they might be both ISO rated the same.
     
  5. baachitraka

    baachitraka Subscriber

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    Blue: Happy mid-tones.
     
  6. michael_r

    michael_r Subscriber

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    It's sort of a trick question but sort of not, because EI might be a personal preference vs ISO which has a defined set of criteria. So Mark is correct - based on the information on the chart (and assuming they were both given identical processing treatment to ISO standards, these both have the same ISO speed.

    pdeeh: Fog is out of the equation since the y-axis is showing net density (ie density above film base+fog).

    But there is then the matter of "personal EI". Different people might say different things. From a Zone System perspective, someone targeting 0.1 above fb+fog as a speed point 4 stops below metered might find the "red" film faster.

    Regarding mid-tones, one would have to extrapolate since the graph does not extend far enough to the right. But the far right of this graph is only 4 1/3 stops from the exposure where both films have a net density of 0.1. So the red film might actually have "happier" mid tones because contrast is higher in the midtones vs the blue film.

    Just some things to think about.
     
  7. markbarendt

    markbarendt Subscriber

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    It's a good trick question.