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  1. #1

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    Puzzled about Kodachrome processing

    Here's a question for PE or anyone else who understands the K-14 process - there's one thing that's puzzling me:

    Since the first (B&W) development is obviously not taken to completion, there must be developable silver halide left in all layers after that step. So, after the red re-exposure, in the cyan development stage, how come cyan dye is only formed in the bottom (red-sensitive) layer, and not in the two others?
    Last edited by Fredrik Sandstrom; 11-04-2010 at 02:40 AM. Click to view previous post history. Reason: typo

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    Presumably because only the bottom layer is sensitive to red light? (Which is perhaps why 3 separate re-exposures (or chemical fogging for the final layer) are needed?). With E6, there is just one re-exposure or fogging stage (in older reversal processes, color or B&W, this was, of course,, done with exposure to a white light).

    I'm sure it's more complicated than that, so look forward to responses from PE and others more expert than I.

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    Quote Originally Posted by railwayman3 View Post
    Presumably because only the bottom layer is sensitive to red light?
    No no, I'm talking about the little bit of silver halide in the blue and green sensitive layers that is already developable, regardless of any red re-exposure. Why is cyan dye not formed there? Or perhaps it is, but it's compensated for in some way..?

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    I understand what you are saying, and the potential problem of developable halide in the "wrong" layers.

    There is a Wikipedia entry on K-14 processing,
    ( http://en.wikipedia.org/wiki/K-14_process ), which states:
    "First Developer -
    All (my italics) exposed silver halide crystals are developed to metallic silver via a PQ developer."

    This seems to indicate that first development is therefore carried to completion? (The Wiki article is quite interesting, would be useful if PE or our other experts have any comments as to its accuracy.)

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    Quote Originally Posted by railwayman3 View Post
    This seems to indicate that first development is therefore carried to completion?
    Interesting. How, if that is indeed the case, is push-processing (which is offered for K-14) carried out? Increased time in the first developer (which I thought was the way) would make no difference...

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    Quote Originally Posted by Fredrik Sandstrom View Post
    Interesting. How, if that is indeed the case, is push-processing (which is offered for K-14) carried out? Increased time in the first developer (which I thought was the way) would make no difference...
    I'm not sure of ther answer, but maybe the "development to completion" is for a correctly dense "negative" image to be produced from a film exposed at box speed, i.e. a correct scale of tones, with a highlight almost fully exposing the halide.
    For push-processing, an image taken at a higher film speed setting (less exposure) would then appear underexposed if the first developer were used for the standard time. Increased development in the first developer would bring such an image to a correct scale of tones, so that effectively the film speed was increased.

    I don't think that "development to completion" means that more time in the developer would have no further effect, only that development is completed to produce a correct range of tones on a "correctly exposed" picture? Remember that, if we push-process a B&W film, a negative exposed at the box speed will become too dark, i.e. "over-exposed".

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    Hmm. If all exposed silver halide has been converted to silver (as the Wikipedia article implies) I don't see what more time in the developer could possibly accomplish. Please enlighten me if this is a misconception!

    If there is (as I still believe must be the case) some amount of developable silver halide in all layers after the first developer, it seems to me this must turn into a very thin cyan colored negative image in the cyan development step. If this is somehow prevented, I'd be curious to understand how.

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    The first developer produces a negative image, leaving a positive 'image' in unexposed and undevelopable (if you don't leave it in the developer for too long) in silver halide.

    The next steps gives a selective colour exposure to the three layers, with development after each exposure only forming dye in the layer that is made developable by the selective colour exposure.

  9. #9

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    Quote Originally Posted by Q.G. View Post
    The first developer produces a negative image, leaving a positive 'image' in unexposed and undevelopable (if you don't leave it in the developer for too long) in silver halide.
    Yes, but what about the exposed and developable halide that the first developer leaves behind when it does not go to completion? That's what I'm asking about.

    The next steps gives a selective colour exposure to the three layers, with development after each exposure only forming dye in the layer that is made developable by the selective colour exposure.
    Yes, that's the very basic principle by which Kodachrome works. I'm familiar with all that. My initial question remains.

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    Quote Originally Posted by Fredrik Sandstrom View Post
    Yes, but what about the exposed and developable halide that the first developer leaves behind when it does not go to completion? That's what I'm asking about.
    As I said in the previous post, I'm sure that "completion" in this context means the production of a negative with a balanced set of tones for a scene correctly exposed at the box speed of the film. I don't think that this necessarily infers that there is exposed and developable halide left at that point?

    If a shot is nominally "underexposed" (i.e. exposed at a higher film speed rating), normal development will not give a full range of tones. So, as with any push-processing, we develop longer to give a more dense silver image, i.e. the nucleus of exposed halide is "developed" to produce more silver.

    I agree that there still seems to be some weakness in that explanation....paging PE to help us out here?

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