Color Bias in characteristic curve in negative films

Discussion in 'Color: Film, Paper, and Chemistry' started by Ted Baker, Nov 28, 2017.

  1. Photo Engineer

    Photo Engineer Subscriber

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    Here is a copy of an old aim curve in my files. Does this help?

    And actually, the speeds are matches but offset by mask density. Draw parallel vertical lines on the curve(s) and you will see that they can actually print neutrally when balanced for the mask density.

    PE
     

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    Ted Baker

    Ted Baker Member

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    Thanks I was hoping for that kind of detail/accuracy for current films. However I am correct that the speed point for blue is typically higher in colour negative? I think I figured out the answer to my riddle BTW.
     
  3. Photo Engineer

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    The threshold speed of the blue layer is slightly higher, but the mid scale speeds are the same.

    PE
     
  4. aphid

    aphid Member

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    Ted Baker

    Ted Baker Member

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    Thanks, yes that is very interesting, I will go through that carefully.

    I think I may have solved the puzzle, well maybe...

    This first image, represents a positive, where the negative has a blue layer that has a higher speed point than red or green. the gamma of each is identical and has been adjusted to 2.4. Filtration has been applied to remove the orange mask for a neutral black,

    Note: X and Y are both linear scales, normalised to 0-1, so become intensity ratios. This 'film and paper' has a negligible toe and shoulder.

    ec_blue_layer_slope_only_g1c.png

    This second image, is identical to the first but the filtration has been applied to remove the orange mask for a neutral white. Note that the slope of each layer should change because of the filtration however the slopes will never be same between all three layers as the blue has higher speed point. (I have not adjusted the slopes for that reason)

    ec_blue_layer_slope_plus_lift_g1c.png

    This final image, is like the second in that filtration has been used to remove the orange mask for a neutral white, but the blue layer of the negative now has a different gamma (2.2), so both a neutral highlight and midtone has been achieved.

    ec_final_g1c.png

    I have done this in more detail tracing the whole process through from scene, to negative, to positive and this seems to be the best explanation thus far.
     
    Last edited: Dec 17, 2017
  6. Photo Engineer

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    Using my curve alone, I have to say that you are missing something.

    PE
     
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    Ted Baker

    Ted Baker Member

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    That's very possible:D Though thus far this seems to me to the most correct answer, the math seems to add up, unless I have made a mistake... At some point I will post a more detailed analysis from which covers scene->negative->print.
     
  8. Photo Engineer

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    Do you see the vertical line drawn measuring the "real" speed? Superimpose all of the 3 curves by moving them to lie atop each other.

    PE
     
  9. alanrockwood

    alanrockwood Member

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    Just a general comment about color reproduction: When you consider the difficulty of the problem, it is amazing that the C41 process reproduces color as well as it does. Consider the problem from the following perspective. There are at least twelve spectral curves that need to line up sufficiently well to give good color reproduction, four in each of the three layers.

    Considering just a single layer, there is what I would call the "activation spectrum". (I don't know the correct name for it.) When the layer absorbs photons it activates silver crystals so they can be developed. The efficiency of activation depends on the wavelength, and this wavelength dependence is what I am calling the "activation spectrum". Once the development and dye coupling takes place there is another spectrum to consider, which is the absorption spectrum of the dye in that layer. The whole picture repeats itself in the paper, i.e. an activation spectrum for activating development of a particular color layer, and the absorption spectrum of the dye in the same layer after then end of processing.

    Multiply this by three to account for the three layers and you have twelve different spectra that all have to combine in a way to give accurate color reproduction. (And I am sure that this is only a small part of all of the factors that need to be aligned, such as characteristic curves and such.) Given this, it seems almost a miracle that color films/prints do as well as they do. All of the scientists and engineers who developed these processes deserve high praise indeed!

    The same goes for color transparencies, although there are only six spectra to align, which probably simplifies things a bit and may be a factor in making it easier;, but there is also no chance to adjust color like there is in the color print process, which would tend to make the problem harder, so again the scientists and engineers deserve high praise.
     
    Last edited: Dec 20, 2017
  10. Photo Engineer

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    Alan, I am quite familiar with this aspect. As a team member, my job was the Magenta layer, ie, coupler, colored coupler and DIR coupler and spectral sensitizer. As the person who might change the spectral sensitizer, I was also the leader of the team for overall color reproduction. So, as I fiddled, each of the others fiddled (Cyan and Yellow), and I had to balance all as I contemplated a change from dyes G2 to G84 in our terminology. I oversaw a similar change in color paper about 10 years earlier.

    This is not an easy task. I made about 10 coatings every other week changing the Magenta while using the latest Yellow and Cyan from my teammates. The intervening weeks, we evaluated and compared data.

    PE
     
  11. MattKing

    MattKing Subscriber

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    Which makes some of your APUG experience kind of ironic, wouldn't you say:whistling:?
     
  12. DREW WILEY

    DREW WILEY Member

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    What's wrong with magenta? Without it, everyone would be Jolly Green Giant!
     
  13. alanrockwood

    alanrockwood Member

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    PE, I thought you might have been involved. It sounds incredibly difficult.
     
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  15. Photo Engineer

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    Yes Matt, very much so. :wink:

    Alan, it was. It was more than a years work by 3 people coating and 3 making and sensitizing the emulsions. That would be 6 people coating every other week amounting to a lot of silver just for tests. Admittedly, about 1/2 of them were what we called single layers or single layer coatouts. The former refers to a true single layer, and the latter to a coating with all components of a layer or typically all 9 emulsions.

    PE