Any reason to use my compensating timer with a Dichro head?

Discussion in 'Enlarging' started by PKM-25, Jan 14, 2013.

  1. PKM-25

    PKM-25 Member

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    I now have a Beseler 45S color head that seems to be working OK. I am using a Stopclock Pro / Vario that was just paired with it's sensor in my Aristo D2-HI cold light head, worked great in terms of timing consistency. Since I am moving that head onto my CB7 bellows assembly for a soon to be mural enlarger, I have a spare sensor and was thinking if it might work with interesting results if put into the 4x5 mixing chamber of the color head.

    It would seem by popular opinion that Ilford MG filters maintain exposure in the mid tone while according to Richard at RH designs, the Vario paired with a Zone VI VC head pegs the highlight for consistency, the method that Ralph Lembrecht says is more desirable when calibrating a Dichro Color Head for VC papers.

    So I am wondering what might the effect be in putting the sensor in a logical place in the mixing chamber, like above the small square hole one finds in the styrofoam when taking the chamber apart?

    I can always just try it but I figured if anyone knew right away reasons for or against it, that is always better than drilling needless holes in things..
     
  2. ic-racer

    ic-racer Member

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    Your tungsten lamp should have a practically constant light output during the exposure, so no need to 'compensate.' You would not want to have the sensor reading any colored light from the head if you are using the dichroic filters in the head. I will really mess up your exposures.

    If you were thinking you'd have the sensor read the color of the filtered light and predict the exposure you would need to do some modifications. For example you would probably need 3 sensors. One for each of the emulsions in Ilford MG paper. Then you'd need to filter each one for the corresponding sensitivity. Then you'd need to integrate the readings of the 3 probes with some sort of summing op amp. Then you'd need to calibrate the whole ordeal. You'd be forging new territory as no commercial system has been produced to do that. Usually one would measure the white light only and then apply the filter. That is how the computerized color system works on the Omega D5500.

    I'm actually posting based on experience. I tried to calibrate the baseboard color sensor of the D5500 Translator to the sensitivity of the green and blue layers of Ilford MG paper using a stack of empirically derived filters. Such that it would maintain the middle gray exposure for Ilford MG paper during a yellow/magenta filter change. I spent a lot of time on it but the human eye is so sensitive to such small changes in gray level, calibrating the system was futile.
     
    Last edited by a moderator: Jan 14, 2013
  3. PKM-25

    PKM-25 Member

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    This is what I thought, I have a Zonemaster II meter coming and that will play nice with a constant light source. I'll just save the spare sensor for my 3rd enlarging station which will be a student / workshop one with the regular D2 head.