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  1. #1
    galyons's Avatar
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    Split Grade Printing Theory Check

    I have had several questions regarding controlling contrast, split grade printing and a simple LED VC head. I'm looking for some input and consensus.

    So, the basics, VC paper uses blue versus green sensitized emulsion to achieve the overall contrast range. The papers were designed to respond to, the then available, tungsten light sources. VC filters and color dichroic filters are designed primarily for tungsten light. That is a light spectrum heavily weight on the red end and lot's of heat, (heat = infra red!).

    In every dichroic head, that I have examined, the range of M or Y filtration is determined by how far into the light beam the single value dichroic filter is pushed or pulled. That is, the dichroic filters are not graduated as one might assume from the numbers on the dials.

    So my theory, and the basis for my LED head, is that the variation of exposure time at green and blue is the exact same relationship as how much of the tungsten light beam is passed through the Y or M dichroic filter. If one spins the Y-M dials to max, in split grade printing, then the relationship of hard & soft contrast is exposure time based, just as in the simple LED G-B configuration. Varying the ratio of green to blue light exposure time can provide any contrast range between the two extreme produced by the LED spectrum. This theory is confirmed in Lambrecht-Woodhouse Way Beyond Monochrome, (page 79, figure 3).

    So why are some folks telling me that they MUST have the ability to vary the G-B light intensity as a control function on the LED head? I can understand if they want to achieve a certain "equivalent contrast grade" overall or localized in the more traditional VC printing concept. But they are insistent that the variable density, (luminosity), of the B-G light is critical in split grade printing as well!

    Am I missing something?

    You thoughts would be greatly appreciated!

    Cheers,
    Geary
    But your flag decal won't get you into Heaven any more. They're already overcrowded from your dirty little war.
    Now Jesus don't like killin' no matter what the reason's for, and your flag decal won't get you into Heaven any more. – John Prine

  2. #2
    Jon King's Avatar
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    Geary,

    You have it right.

    Jon
    Jonathan
    -----------------------------------------------

  3. #3
    jeroldharter's Avatar
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    I think some people would want to vary the intensity of the LED output to keep exposure times relatively constant to keep the lens at a particular f-stop that might be sharper than others.

    I understand that with LEDs, dimming them is more complicated than with other light sources.

    I think that having separate blue, green, and white (for focusing) channels would suffice. I would prefer white to red for focusing because I focus on a dummy sheet of paper anyway and find the white light much easier on my eyes than the red.

    Rather than dimming the LED's, it might be easier and cheaper to have a filter drawer for neutral density filters.
    Jerold Harter MD

  4. #4
    Dan Henderson's Avatar
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    I split grade print with green and blue gel filters placed in the filter tray of an Aristo cold light head. I first determine the exposure time needed through the green filter for the highlights, then determine the exposure time needed through the blue filter to set the shadows where I want them, after the green filter exposure has been made. So for me, anyway, split grade printing is at maximum green and blue; no variability in density, just time. Maybe others split grade print at less than grade 5 or more than grade 0 or 00.


    web site: Dan Henderson, Photographer.com

    blog: https://danhendersonphotographer.wordpress.com/

    I am not anti-digital. I am pro-film.

  5. #5
    Curt's Avatar
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    In every dichroic head, that I have examined, the range of M or Y filtration is determined by how far into the light beam the single value dichroic filter is pushed or pulled. That is, the dichroic filters are not graduated as one might assume from the numbers on the dials.
    On one Beseler head I have the filters move up and down into the light to give the mix. However on the Beseler Universal 45 I have the three Dichroic filters, Red, Green, Blue, not Cyan, Magenta, Yellow, are each fixed in a housing in front of the Halogen bulb. A controller makes the filter pack and adjusts the light intensity. Color temperature feedback is part of the mix making it very complicated design wise.
    Everytime I find a film or paper that I like, they discontinue it. - Paul Strand - Aperture monograph on Strand

  6. #6

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    For split-grade printing, based on my understanding of the theory, I don't think there'd be any difference between dimming the LED and reducing the time it's on. Dimming would be necessary for traditional single-exposure printing, though, and as such, the ability to dim an LED light source would be important if you were to market one or if you expect to ever do single-exposure printing. Also, if the exposure times ever got very low, a dimmer could be handy. For instance, if you wind up with 1-second exposures on some super-fast paper, that's just not enough time to do dodging or burning, and print-to-print timing accuracy might not be good enough.

    FWIW, when I was researching LED light sources a while back, I ran across several circuit diagrams for LED dimmers. Unfortunately, I lost them all in a disk crash. I'm sure a Google search will turn them up. They weren't that complex, as I recall.

    One final point: According to an article that Pat Gainer wrote, focusing with green light alone is about as good as it gets, because of the way the human eye focuses light. I use green light alone on my Philips PCS130/PCS150, which has halogen bulbs with red, green, and blue filters. According to Gainer, white light is also good, but given the odd spectral characteristics of "white" LED light, I'm not sure I'd trust a white LED for focusing.

  7. #7

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    Quote Originally Posted by Curt View Post
    on the Beseler Universal 45 I have the three Dichroic filters, Red, Green, Blue, not Cyan, Magenta, Yellow, are each fixed in a housing in front of the Halogen bulb. A controller makes the filter pack and adjusts the light intensity. Color temperature feedback is part of the mix making it very complicated design wise.
    This is the same design as the Philips PCS150 and PCS2000. This is the first I've heard of another manufacturer using this system, although I have heard of people retro-fitting the Philips light source and controller into other enlargers. Do you know if this is the original design, or could this be a retrofit? (A separate control box with "Philips" printed on it would be a dead giveaway! )

  8. #8

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    Quote Originally Posted by galyons View Post
    So why are some folks telling me that they MUST have the ability to vary the G-B light intensity as a control function on the LED head? I can understand if they want to achieve a certain "equivalent contrast grade" overall or localized in the more traditional VC printing concept. But they are insistent that the variable density, (luminosity), of the B-G light is critical in split grade printing as well!
    Am I missing something?
    In another current thread on split grade printing, it has been recommended that (See http://www.lesmcleanphotography.com/...ull&article=21) the negative should be of higher contrast for split grade printing otherwise printing at G0 or max green light can produce muddy highlights. i.e. highlights without enough separation. It may be that these people want control over the level of green light so as to increase highlight contrast. But I would of thought that just reducing intensity would just increase required print time. If they require a harder (G1 or G2) low contrast exposure it would require a mix of green and blue.
    Not knowing how your head works but assuming you are working on just two exposures, one green and one blue, then you have no control over highlight contrast. Highlight contrast would all have to be done in the negative which kind of negates the need for VC paper. The same applies to the high contrast exposure as not all people use G5 for that. Some use G4 or G3.

    So you may require a mix of green and blue for the low contrast exposure and a mix of green and blue for the high contrast exposure. Just using G0 and G5 won't necessarily do it unless the negative is of perfect contrast for the required print.
    That obviously creates a major problem in designing an LED light source for split grade printing because the mix of green and blue for any intermediate grade has to be calibrated to the specific paper and developer combination. That appears to be what the the heiland split grade head does although it is not an LED source, but it has to know how the paper will respond to any combination of Y+M or G+B to be able to work out intermediate values for starting point or burning and dodging.

  9. #9

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    Also being able to set time for soft contrast exposure and hard contrast exposure to be equal for a "normal" print would be useful as a starting point.
    And being able to adjust times to be relatively equal for different magnifications would be useful. And has been mentioned, being able to set a standard print time to allow burning in and dodging times to be long enough would be useful.

  10. #10
    Mick Fagan's Avatar
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    Geary, I agree as well, you have it correct.

    Regarding the Lambrecht Woodhouse book, if you read a bit further and carefully, you will note that figures 4, 5, and 6 tell a very interesting story about the accuracy of printing either way first.

    That is, highlight or shadow exposure first, will not make a difference in the final product. I actually concur with this, after doing the tests in my own darkroom.

    I think the compelling piece of information I gained from that very detailed and concise chapter, is that exposing in f/stops precisely is virtually required to gain the maximum repeatable and logical benefit, of a split grade printing technique.

    I myself have been using f/stop printing for over 25 years and have never seen the need to get an f/stop timer.

    However with split grade printing, I can see how much easier it could make life in the darkroom. If I was a commercial printer, I would certainly get an f/stop timer.

    Regarding the Lambrecht / Woodhouse book. I think it is possibly one of the best written books about the technical side of B&W photography I have had the pleasure to read. The two authors really do compliment each other with their differing but similar backgrounds.

    A Masters Degree in Manufacturing Engineering (Lambrecht) and a Masters Degree in Electronic Engineering (Woodhouse). Combining those two disciplines with a personal love of photography by both of them, they ended up with a no nonsense book which covers the technical side of photography in a practical way that the lay person can quite easily understand.

    Mick.

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