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  1. #91
    Stephen Benskin's Avatar
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    What's important to remember about speed points is that they don't necessarily represent where the exposure is supposed to fall. For the fractional gradient point, it is the 0.3x average gradient point and can be considered the minimum useful gradient point. The Delta-X criterion's and the ISO standard's use of the 0.10 fixed density point is in it's correlation to the fractional gradient point. The testing establishes the lower limitations of the film. Based on this point, any adjustment would be relative to this known quality limitation. Once the speed point is determined, the exposure required to produce that point is entered into an equation that contains a speed constant. The film speed value is part of the exposure meter's equation to determine exposure placement. There is a known relationship between the exposure at the speed point and the exposure at the metered exposure point. Adjusting the speed constant, changes the film speed and consequently changes the relationship between the speed point and the metered exposure point.

    The speed equation for reversal film used to be 8 / HR, which is the metered exposure point. Now itís 10 / HR. Thatís a 1/3 stop adjustment without changing how the speed point is determined. The same could have been said for b&w film speed. They could have simply adjusted the film speed constant to eliminate the safety factor, but many people didnít like how difficult it was to determine the fractional gradient speed point. The only way to create a universal standard was to simplify the process. Thatís where Delta-X comes in. It has good correlation with the fractional gradient method while using an easy to find fixed density.

    Hereís an example of the math. This is all built into the ISO standard when the ΔD is 0.80 at Δ 1.30 log-H, but if you want effective film speeds for any other development conditions, youíll need to use the equations. Anyone recognize the ISO standard's contrast parameters in this example?

    Click image for larger version. 

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    This chart gives the ΔX value for a given ΔD.

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  2. #92
    Stephen Benskin's Avatar
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    The value of Delta-X is the difference between the fixed density point of 0.10 and the fractional gradient point which is the speed point. Referring to the Delta-X Table in the previous post, as the contrast (ΔD) is reduced, the value of ΔX increases and as the contrast is increased, the value of ΔX is decreased. The further to the left of the speed point, the faster the film is in relation to what it would be using only the fixed density method. What this means is that the film doesn't lose speed as quickly with reduced development and doesn't increase as significantly with expanded development compared to the fixed density method. With the Delta-X method, the speed point will remain at the same 0.3x average gradient at all levels of processing. It's an apples to apples comparison.

  3. #93

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    This is a great, informative thread. I've read it and reread it a couple of times to try and follow it all. Now please bear in mind I'm not very well versed in the development procedures. That said, have I got the (admittedly oversimplified) generalizations correct?

    1) E.I. is a personal choice based upon how the developed images correlate with the desired intent of the photographer whereas the ISO is the standard given as a starting reference by the manucaturer.

    2) The photographer sets the film speed on the camera *at the ISO* and develops as though it were shot *at the E.I.*

    Knowing that the E.I. is a personal choice of the photographer, for those of you who have gone through the process of determining your own E.I., may I ask what specific criteria you found useful in this process? At what point in your photography did you feel you'd learned enough to evaluate those criteria?

    Thanks!

    John

  4. #94
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    John, three criteria.

    Can I get the print I want?

    How easy is it to get that print?

    How big is my safety factor? (How far can I over or under expose and get what I want?)

  5. #95
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    Quote Originally Posted by pharmboycu View Post

    Knowing that the E.I. is a personal choice of the photographer, for those of you who have gone through the process of determining your own E.I., may I ask what specific criteria you found useful in this process? At what point in your photography did you feel you'd learned enough to evaluate those criteria?

    Thanks!

    John
    The work of Jones et al provides me with a good minimum point for the EI and I have no qualms his findings. I found the "W speed" as an estimation of 0.3G easier to calculate with a spreadsheet than either directly finding 0.3G or claculating Delta-X. So I use that [W-speed] when I do the math on my sensitometric data.

    To link my sensitometric findings with a lens/shutter/camera/exposure meter system I use the 0.1log d ASA method with an 'in camera' Zone 1 exposure.

    I also, have been using a 'minimum flare density of 0.1 log d' by placing a box or can with a hole in it in a typical composition. This provides a simple and practacle empiric solution to the issue of flare and its effect on film speed.

    For my large format work I tend to use a generous safety factor to minimize exposure errors in the field. This is possible because there is very little or no image deterioration of the 8x10 negative image due to over exposure. Especially when considering the largest prints I make with that format are only 2.4x magnification ( 20x24").

    I control contrast when I print (dichroic head) so that simplifies negative processing a thousand-fold
    Last edited by ic-racer; 05-08-2014 at 08:30 AM. Click to view previous post history.

  6. #96

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    Is there a place where I can get a pdf of the paper "Study of Various Sensitometric Criteria of Negative Film Speeds" by Jones and Nelson?

  7. #97
    AntiLynd's Avatar
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    If you have internet access via a university network, you can download it directly (http://dx.doi.org/10.1364/JOSA.30.000093), or if you happen to live near an (analog) library, you'll probably be able to get a (digital) copy via their computers.

  8. #98
    Bill Burk's Avatar
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    Quote Originally Posted by alanrockwood View Post
    Is there a place where I can get a pdf of the paper "Study of Various Sensitometric Criteria of Negative Film Speeds" by Jones and Nelson?
    I'd like to read it too, but I see that it takes a bit of cash to read that article...

    What are you curious about? Maybe we've already discussed it or can get answers from someone on APUG (Stephen Benskin or PE may know the answer)...

  9. #99

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    Quote Originally Posted by Bill Burk View Post
    I'd like to read it too, but I see that it takes a bit of cash to read that article...

    What are you curious about? Maybe we've already discussed it or can get answers from someone on APUG (Stephen Benskin or PE may know the answer)...
    It's not so much a single question as a desire to study the article.

    I don't think our local university has the Journal of the American Optical Society, so that makes it harder to get a copy.

  10. #100
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    There is a generic description of curve shape and speed in Haist.

    PE

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