Color Negative: Mask questions and confusions

Discussion in 'Film and Processing' started by friedemann_wachsmuth, Jan 22, 2007.

  1. I have googled a lot here and elsewhere, but still have not find a good explanation why C-41 color negative film actually has its mask and why in that particular color -- inverted a turquoise-blue. Can anybody explain?
    More related questions, in case anybody knows:
    • The mask color is in the film base, not created during development, right?
    • Why does C-41 film processed in E-6 then get a greenish or blueish mask instead of
    • I digitize my negatives with a bellow on a 5D, using inverted CN-Mask as filter in front of the flash to compensate the mask. Seems to work fine. However, people here say "a mas is not a filter!" -- I don't understand that difference.
    • Anybody sheding light on my questions will help me to shed more appropriate light on my next films :)
     
  2. I wish I had the slides that I was shown when I took the photo technology training class when I started at Kodak. Dr. Hansen was director of Research at the time. He was the original inventor of the colored coupler mask. I will try to describe it in words.

    Dyes are not perfect. They have some unwanted absorptions. The cyan dye absorbs mostly red light, but it also absorbs some green light. To compensate for this unwanted absorption, a colored coupler is added to the red sensitive layer. This colored coupler is magenta to start with, but it loses the magenta color and forms cyan dye during development. This means that while the red layer has a negative dye image in cyan dye it has a positive dye image in the unused magenta colored coupler. This magenta image offsets the unwanted green light absorption of the cyan dye.

    The magenta dye in the green sensitive layers absorbs mostly green light but also has some unwanted absorption of blue light. A yellow colored magenta dye forming coupler is added to the green layer.

    The yellow dye in the blue sensitive layers absorbs mostly blue light, but also absorbs some UV energy. Since we can't see UV energy, we don't have to compensate for it.

    With magenta colored and yellow colored coupler added to the film, the minimum density (Dmin) areas are orange colored. This color is in the emulsion layers, not in the plastic support.

    I've never tried cross processing so I can't comment on it.

    A mask is different from a filter because a mask has lighter areas and darker areas. A filter has uniform lightness across the whole image (except as defined in US patent 5972585, but that is another story).
     
  3. Also, the very first Kodacolor film had no mask. Then they added a silver positive mask, which reduced contrast. But the current orange mask didn't take too long to evolve.
     
  4. Thanks guys, this is very illustrative.
    I gotta dig deeper wrt the mask (vs. a filter) -- if I understand the definition right, it means the orange is not equally spread over the frame, but e.g. blue-colored parts of the negative are less orange? Is my approach to "blue-flash" for compensating mask color then wrong? Indeed the inverted repros look pretty soft and colorshifted, but are easily correctable in PS.
    Check this example.
    How do film scanners compensate the mask? Do the use blueish light or compensate through software? That would require a highly dynamic ccd (since e.g. slides have no mask...)
     
  5. As Ron explained so well, there are two masks, not one.

    Your method of filtering the flash is similar to the technique used when transferring film to video using a 'film chain' - a projector and camera. We used a blue-ish filter when transferring negative to limit the amount of colour correction that had to be done. It is a long time since I did that, and I can't remember which particular filter was used but I could dig around a bit - it shouldn't be difficult to work it out. It would be something like a blue colour balancing filter because they affect red and green, with the green absorption being less than the red. What filter are you using?

    The effect of the masks is to shift the characteristic curves of the three layers upwards - if you look at the characteristic curves for colour negative film you should see that they are almost parallel (important), and offset vertically. Blue at the top, green in the middle and red at the bottom - that's the effect of the two masks. When scanning you can set the black point and white point at the extremities of the histogram for each channel. This practice corrects for the masks.

    Best, Helen
     
  6. Sorry, I forgot to answer another part of your question.

    Your use of a filter is correct. The whole purpose of the masks is to make filtration possible. The combination of each mask and the corresponding dye 'impurity' in the image is designed to be uniform across the film: it appears as a combination of a uniform filter and a pure dye image.
     
  7. Thanks a lot Helen -- I think I totally got it now. The curves gave me the hint.
    The filter in front of my flash is just xerographic film printed on a color laser printer. I
    inverted an unedited shot of a blank color negative to get the compensating color,
    reduced saturation a little bit and printed the results out. Depending on density of the
    mask, I put one, two or three of these foils in front of my flash -- camera set to 5500K
    WB, I try out until the perforation edge looks kinda neutral gray (and checking the
    camera's RGB histogram on a blank frame)

    Thanks everybody -- the fact that C-41 in E-6 makes different mask colors should have
    also told me that the mask is not in the base but the emulsion. You never stop learning...
     
  8. I posted some hand drawn spectrophotometric curves and some sensitometric curves to illustrate the use of masking here on PN in this forum about a year or 2 years ago.

    You may want to look these curves up, as they illustrate what Ron Andrews describes so well above.

    Ron Mowrey
     
  9. One must remember that scanning wasn't heard of when the color negative dyes were being selected years ago. The purpose of the color film negtive is to print on color negative paper. There were dye constraints in the paper as well so the trick was to get a system where the paper (where most of the dyes are) was easy to manufacture and didn't use more expensive dyes or coupled dyes that weren't invented yet and then design a film with ( errors from what would seem ideal ) equal and opposite to the paper. Optical printing is a system with the goal of getting the best print.

    Even in B&W printing the print naturally has the highest contrast since the light goes through it twice (on the way to the paper base and then back out agian) and so negative is designed to be lower contrast to compensate.

    I remember reading Sakura? (not Fuji) invented a completely different dye set for color photo paper and then invented a different negative to go with it with dyes and coupled dyes that were equal and opposite in their crossover errors to come up with a unique system that worked well*. But marketing it was a problem, different chemistry and process was required complared to the then C22 ( I think it was about the same time Kodak was changing to C41). So their film could only be printed on their paper. In the world they were a small company and didn't have the marketing mass to pull it off. And they knew it so in the end they didn't really try. Also there were rumored enviromental problems.

    (* note since this was all a secret at the time - we only have their leaked world that it worked well.)

    I wouldn't be a bit surprised if Fuji was trying the same thing but with yet another dye set. It just didn't make sence to market it - corporate suicide i.e. Kodak is bigger and basically set the industry standards.

    ____________________________________

    So now as we scan negatives we have to put up with these errors :-/ When scanners scan negatives and they are set up for negatives they apply and nonlinear set of crossover errors before it ever gets to your PC software in an attempt to get the best result possible.

    note:
    The above is totally based on what I have read, not on personal involvement with the industry.

    From time to time I've wanted to get more involved with the chemistry on a hobby basis -- maybe when I retire I will have the time. When I worked for Kodak it was just a college summer job in the stockroom for a couple of sumers. One summer I worked at the Elmgroove plant and would spend my lunch time often in the technical library reading the latest published scientific papers on photo sensitivity or related matters. At the time Kodak was selling their new Kodacolor emulsion in the employee stores and also Ektachrome for E6. I remember trying to process the Ektachrome in E4 at home and concluding that -yes there were big changes. I think that was just before I did a lot of reading.
     
  10. There must be journal references but the only one I know in book form is Lloyd Varden's appendix to the 1960s reprint of Joseph S. Friedman's History of Color Photography. Varden is ironic and vague and he writes more about the industrial spying angle.

    Here goes. As your first response pointed out, dyes are impure, failing to absorb light frequenecs they should for good reproduction. The easiest way to understand this is to imagine ideal dyes for C, M and Y, call them C*, M*, Y*.

    Then the actual dyes could be represented by

    C (actual) = C* + 3%M* + 1%Y*. This would be for some actual dye where there is an equivalent of 3% magenta and 1% yellow unwanted contamination. (A perfect C dye would not absorb any M or Y.)

    M (actual) = M + 2%Y*

    Y (actual) = is relatively good.

    Now to take just the cyan dye. In an area of the image where there is only cyan (which is rare) nothing can be fixed by masking. But imagine an area when the color is supposed to be 100C* + 30M* + 10Y*. With acutal cyna and magenta dyes the color becomes 100C* (+ 3M* + 1Y*) + 30M* (.30x2Y*) + Y* which equals 100C* + 33M* + 10.6Y*. So there is now too much magenta and a little too much yellow.

    Here is where masking comes in. What we want to do is to subtract from the magenta densities an amount equal to the unwanted magenta in the cyan dye. (And also, to subtract from the yellow an amount equal to the unwanted yellow contamination in both the cyan and magenta dyes. So what we want is an image that is 100C(actual) + approximately 27M(actual) + approximately 9.4M(actual).

    In the dye transfer process, which uses color separation negatives, you would make the three separation negatives. Then from the magenta negative you would make a thin positive (a mask). Then you sandwich this mask with the cyan negative when you expose the cyan negative. This subtracts magenta values equal to the cyan dye impurities.

    And you do the same for the magenta negative. This masking leads to all kinds of issues. Since you are sandwiching a weak positive with a negative, the negative has to be processed more contrasty than if you did not use the mask. The mask must be exposure and processed to represent the dye impurity of the dye you are trying to correct.

    For color negatives, Thomas Hanson (I am not sure of his name) realized that you could do this by using a color dye that starts not clear but colored. So for the cyan dye the original dye is a pale red-pink and during development it is converted to a cyan. After development there are two images related to the cyan exposure -- a negative cyan image and in exact register a positive red-pink image. Similarly for the impure magenta dye there are two images. A negative magenta image and a positive yellow image.

    The mask in color negative film only looks like it covers the whole film. It is thinner in areas of the image where the cyan and magenta densities are the strongest. So where the cyan layer goes from 0 to 100% the mask runs from (say) 30 magenta and 10 yellow (when Cyan is 0) to zero (where the cyan is 100C).


    Okay, so you can completely correct for dye errors in the negative film. Using the same reasoning you can also correct for the presumed dye errors in the color print material you will be printing on. Its cyan also will have unwanted magenta and yellow so you make the negative to subtract those unwanted impurities.

    This means, to answer your question, that a mask is not a filter. Its use in printing cross-processed negative would be to make the filter pack closer to what you would normally use with a regular color negative. Otherwise you would have to add red and yellow to the filter pack. Since a real color negative material has this positive superimposed onto the camera image, the contrast of color negative materials is usually lower than cross developed film.

    If you are digitizing your film, the scanner expects to see a masked image. You could try scanning the images as if they were slides and flipping them in Photoshop. A guess is that the best way to go would be to do the flip in LAB color.

    If you are using a camera and flash to digitize, the CN-mask should not be necessary. No harm but not gain.

    To answer your other questions, the mask is in the film base before development. But the point is that some of it is destroyed in the areas where there is actual image, so that in some places there is no original dye left after development.

    In cross-developed film the "bluish mask" is probably not a mask but just and overall stain on the film. Just a guess.

    One thing you could do is shot a color swatch card in both regular color negative material and cross-processed film (and maybe color slide film normally processed). Then in Photoshop you could find a curve layer that converts between them (best done, again, in LAB) and save the adjustment layer. Or save the curve in CURVES and reuse it.

    PS the Joseph Friedman book explains all about masking color separation negatives, but not the integral color-coupled masks of color negative film.
     
  11. Wow, this is an excellent and really interesting response. Especially surprising after so many
    months -- while the topic is still relevant to me :)
    Thanks so much for all your insight on this. I will definitely do some tests with LAB-color
    repros -- at least.
     

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