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  1. #11
    Ed Sukach's Avatar
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    Quote Originally Posted by Ed Sukach
    I'll check it out ... I've estimated that the paper itself (Ilfocolor) will attenuate approximately one - one and a half stops. I'll check it with the ColorStar.
    Here I go ... quoting myself again.

    I've found that Ilfocolor paper attenuates light coming through it from the "wrong side" 1.31 LogD ... or approximately 2 1/2 stops.

    I've got to try this on more images with distinct diagonal lines. I'd give exposure times more thought, though, based on the density above.
    Carpe erratum!!

    Ed Sukach, FFP.

  2. #12
    gma
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    Maybe "Carpe erratum" should be the slogan of APUG. I know I have made plenty of mistakes in my sixty years.

  3. #13
    Dave Miller's Avatar
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    Quote Originally Posted by gma
    Maybe "Carpe erratum" should be the slogan of APUG. I know I have made plenty of mistakes in my sixty years.
    I don’t make mistakes; I create learning opportunities!
    Regards Dave.

    An English Eye


  4. #14

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    Quote Originally Posted by titrisol
    I have to try the black background!

    How did you compensate for focus?
    Whenever I open a new box of paper I always write off the first sheet to use the back in the easel to focus on. Then when I take the sheet out I know the sheet I'm going to expose it is focused at the same point. Also if the new batch is of a different thickness my focus is still at the same level not that it should vary by much. The black background stays on the easel

  5. #15
    gma
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    I think there will always be enough depth of focus at the paper plane to cover variations in paper thickness. Why not simply use a piece of extra bright white bond paper for the target? I can't bear the thought of wasting a whole sheet from every fresh box of paper. My Scandinavian frugality, I suppose.

  6. #16
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    I just save a wasted sheet, you know, bad contrast or exposure, and turn it backside up for a focus sheet. If you use a grain focuser it generally recommended to use a sheet under it for accurate results.
    Gary Beasley

  7. #17

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    What bothers me is...

    yes the DOF of the lens is enough to compensate for the thickness of the paper but the image was also filtered through and very much difused by the paper. SO relections or this feedback through the paper is causing nothing more than fog in my image. I really never noticed and always thought I was going blind. My grain scope says it's perfectly focused, it looks good but not razor sharp as I once was able to get. I atributed it to the cold light softening my prints.

    I have noticed my few prints I made last night look cleaner now that I am using a black paper under the print. I think my pix are sharper but that is probably just my imagination working hard to justify my observations.

    I'll be playing with this more during the weekend when I have more time.

    Thanks for the feedback.

  8. #18
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    Quote Originally Posted by titrisol
    I have to try the black background!

    How did you compensate for focus?
    I'm responding from my own experience. Some time ago I wrote the article "Hazards of the Grain Focuser" for Photo Techniques. I tested red, green and blue separation filters to determine if the spectrral sensitivity of the paper or the eye made a difference. I did a number of trials with each color and without any filter to find mean and standard deviations of focusing errors. I focused for each trial by moving the enlarger head until sharpest visual focus was obtained because movements of the lens carrier would have been too small to measure accurately. I measured focus distance from negative carrier to baseboard.

    You might expect only a difference in standard deviation due to change in focusing color, but there was also a different mean error for each color. The exception was that white and green gave the same result: smallest random error as well as smallest mean error.

    The question of which color was absolutely closest to true focus was answered by making greatly enlarged prints from each mean focus distance. This i did by first making a print on ortho lith film, then using that as a negative to square the enlargement from 10 to 100. The grain in the final photos was convincing: the sharpest focus was obtained at the point of least mean error.

    The conclusion I reached was that the ancient reason for using blue filters to focus telescopes was not applicable to modern enlarging lenses. The focusing error was more likely due to chromatic abberation of the human eye, which is easily demonstrated. This effect has been known to astronomers for a long time: the aberration of the eye increases the apparent aberration of a refracting telescope.

    Thus, the sharpest focus of an enlarger will be obtained by either white or green light because the acuity and sensitivity of the eye are both greater there. Modern enlarging lenses are achromatized very well over at least the blue and green, so the best strategy is to favor the optimum color for the eye.

    Now as to thickness of the paper, I used in my tests the optimum aperture for the lens I had, an El Nikor f/2.8. The visible grain was sharpest at the 5.6 aperture. The smallest average error in focusing was considerable more than the thickness of the printing paper, and the mean square deviation was greater yet.
    Gadget Gainer

  9. #19
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    I forgot to mention that one of the effects of halation is a decreas in contrast. It is not necessary to use black on the easel unless you are using panchromatic paper. A yellow easel or one that looks like the VC safelight should do the job and might still be bright enough for composition. The grain focuser won't see the color of the easel anyway.
    Gadget Gainer

  10. #20

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    Paul
    You did the right thing painting your easel black, otherwise there will be reflection in the back of paper. That is what I also did. I don't know until now why manufacturers make them white.
    "The usual white surface is excellent to focus upon, but..... an appreciable fogging effect..... (so) a back of dark cardboard should always be used ..." (AA ; The Print)
    sergio caetano

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