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  1. #11

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    From bernard_L: So much for my (vague) plans to try an LED lamp in my enlarger.

    VC printing was set up around a tungsten source (I would think), so the ratio of dyes would have been derived accordingly. So if an LED has a different spectral distribution, I'm really not surprised by the result. I think the graded paper shows that an LED source can be a perfectly viable option. Just need to understand the response characteristics. In practice I'm not sure how much the "kink" in the VC curve would be a problem, as long as you understand the relative difference in the shadows and highlights.
    Regards,
    Jim

  2. #12

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    Quote Originally Posted by Nicholas Lindan View Post
    See:

    http://www.darkroomautomation.com/su...vcworkings.pdf

    In general, changes in contrast in the lower grades are in the shadows while the contrast changes in the higher grades are in the highlights.

    The 'bump' in the middle of the lower VC grades is an intrinsic property of VC paper. The difference you are seeing is because the two heads are printing at slightly different contrasts because they have different spectral curves.
    Thanks for the pointer to the application notes. Really helps me understand the results I see. Especially the grade 00 curve.

    So it seems (to net it out) that the kink in the curve is there for VC paper in general and the particular LED source I am using may exaggerate the kink a bit compared to the tungsten source. Your paper describes the impact of this effect. If you want a "kink free" curve, you should be using graded paper.

    Given the benefits of the LED source (economical diffusion head + cool operation) and paper characteristics, I am going to look at switching papers to the graded variety and adjust my upstream processes accordingly.
    Last edited by jlpape; 03-06-2013 at 11:14 AM. Click to view previous post history.
    Regards,
    Jim

  3. #13

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    Thanks for the pointer to the application notes. Really helps me understand the results I see.
    Well, except that as a direct consequence of the hypothesis made in this application note,
    That first point is worth stating again: All emulsions have the same intrinsic contrast - the same
    range of illumination that takes them from white to their DMax. There is a common
    misconception that the paper is made from a blue sensitive high contrast emulsion and low
    contrast green sensitive emulsion - this is not the case.
    there should be a bump in the middle of the D-versus-log(exp) curve (see the graphs there). And what you see is a dip, consistent with the superposition of two emulsions, one soft, one hard.

  4. #14

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    Quote Originally Posted by bernard_L View Post
    Well, except that as a direct consequence of the hypothesis made in this application note,

    there should be a bump in the middle of the D-versus-log(exp) curve (see the graphs there). And what you see is a dip, consistent with the superposition of two emulsions, one soft, one hard.
    Maybe I misunderstood his paper, but for 3 emulsion papers, such as Ilford MGIV, his Theroy section shows a bump in the grade 00 filter plot, which I see as well, or more accurately, show each of the three emulsions kicking in. The rest are harder to tell where each kicks in (grades 2, 3.5 and 5). In the "Real World" section for 3 emulsion papers, he does show a break in the curve (change in slope or "dip" if you will). The position and degree of the change in slope is where the different emulsions kick in and their additive effect given a specific spectral distribution. So the net effect could "look" like a dip or a bump... I think... my brain hurts thinking about this.

    By the way, thanks for the links you provided. The one one on Memorabilia is quite interesting.
    Last edited by jlpape; 03-06-2013 at 03:03 PM. Click to view previous post history.
    Regards,
    Jim

  5. #15
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    Quote Originally Posted by jlpape View Post
    I recently purchased a densitometer (Xrite-810) to get a better handle on the relationship between my exposure, negatives and prints. I also purchased a Stouffer step wedge that I used to make prints on Ilford MGIV Deluxe RC Pearl paper using Ilford VC filters. Development was done in Dektol 1+2 20C and 1m30sec development. When I measured the results I was a little surprised in that the contrast in the hi-light region appeared to be the same in grades 1,2 and 3 with the only separation being in the shadows. Attached is the data for both a LED Diffusion head and Tungsten Condenser (75W bulb). Both are quite similar. The densitometer checks out vs it's calibration material (reflected metal piece and the transmitted step wedge). I also measured the Stouffer step wedge using the densitometer. Do these results seem typical? They do not seem to match the graphs that Phil Davis has in his BTZS book. Then again, I think he was using single graded paper. My concern is that in practice the shadows will print at a higher relative contrast vs the highlights.
    Perhaps you should disregard the densitometer and step wedges and evaluate by eye? As this at the end of the day is what we all do.

    “The contemplation of things as they are, without error or confusion, without substitution or imposture, is in itself a nobler thing than a whole harvest of invention”

    Francis Bacon

  6. #16
    Nicholas Lindan's Avatar
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    Quote Originally Posted by bernard_L View Post
    what you see is a dip
    A dip is the space between two bumps - or a bump is the space between two dips...

    VC low contrast filtration with MGIV produces two areas of lower contrast. Or, if you will, three areas of higher contrast.

    Quote Originally Posted by bernard_L View Post
    with the superposition of two emulsions, one soft, one hard.
    "All parts of the emulsion have the same contrast." Quote from ILFORD CONTRAST CONTROL FOR ILFORD MULTIGRADE VARIABLE CONTRAST PAPERS (http://www.ilfordphoto.com/Webfiles/...8932591755.pdf)

    The earliest Ilford VC papers from the 1930's attempted contrast control with hard and soft emulsions. This approach has all sorts of problems: the two emulsions develop at different rates, one is warm and the other cold tone, they don't tone at the same rate....

    Ilford soon gave up this approach and adopted the Defender system - better known as Dupont Varigam. This system uses emulsions with the same intrinsic contrast and properties and has been the basis for all VC paper for the past 70+ years (at least in the US; Ilford may have kept it's original method longer). The emulsions aren't perfect matches (well, they can't be with the addition of sensitizer to some of them) and VC paper continues to split tone slightly.
    Last edited by Nicholas Lindan; 03-06-2013 at 09:05 PM. Click to view previous post history.
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  7. #17
    Nicholas Lindan's Avatar
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    Quote Originally Posted by cliveh View Post
    Perhaps you should disregard the densitometer and step wedges and evaluate by eye?
    Good advice.

    It is worth knowing how materials work, but it is certainly secondary.

    If knowing how it worked was the key then the chemists at Kodak and Ilford would be the world's best photographers.
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  8. #18

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    Quote Originally Posted by Nicholas Lindan View Post
    Good advice.

    It is worth knowing how materials work, but it is certainly secondary.

    If knowing how it worked was the key then the chemists at Kodak and Ilford would be the world's best photographers.
    I humbally disagree. I won't argue that at the end of the day we use our eyes and our intuition to determine what we want to convey, but why would we dismiss something that can give us some insight into what we do? This little exercise taught me something that I would not have picked up on as easily without some quantitative data. If running some tests to understand the difference between exposure and development a bit better, why would I not take advantage of that. I think that both can live in harmony with each other. Your last quote about Kodak and Ilford engineers could be re-written... Knowing how it works allows Kodak and Ilford engineers to give us some of the best materials in the world for us to convey our vision. Don't confuse knowing technically how photography works with artistic vision and intuition. Both have their place.
    Regards,
    Jim

  9. #19
    Nicholas Lindan's Avatar
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    Quote Originally Posted by jlpape View Post
    I humbly[non sic] disagree ...
    ??????? I'm not sure what you are disagreeing with, as I agree with what you just said.

    My point is that one can make wonderful pictures without any knowledge of how it all works; knowing how it all works isn't a guarantee of wonderful pictures; but for some knowing how it works is a great help to making said wonderful pictures.

    And I am one of the major uber-geeks around here. I once nearly bolted from my dentist's chair when he confessed he flunked calculus in college so many times they gave him a pass just to get rid of him - until it struck me that skill in dentistry really doesn't require calculus.
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  10. #20

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    Quote Originally Posted by Nicholas Lindan View Post
    I once nearly bolted from my dentist's chair when he confessed he flunked calculus in college so many times they gave him a pass just to get rid of him - until it struck me that skill in dentistry really doesn't require calculus.
    Nicholas,
    One definition of calculus: tartar

    One definition of tartar: an incrustation on the teeth consisting of plaque that has become hardened by the deposition of mineral salts (as calcium carbonate)


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