Making a multigrade calibration table for color head

Discussion in 'Enlarging' started by ic-racer, Oct 17, 2008.

  1. ic-racer

    ic-racer Member

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    I got a bunch of Forte multigrade paper on sale at the local shop. Here is how I calibrated my color head to allow me to keep the same middle gray exposure while changing contrast grades with the color head. It involves adding in RED to the Magenta or Yellow filtration as neutral density, so each "Grade" has about the same exposure to print a middle gray. So, for example, 80cc of Yellow gives ISO R (contrast) of about 120. This will eventually become M 32 and Y 112 after the neutral density (32cc RED) is added, so that the printing time matches all the others.

    This is not a complete tutorial (the complete version is here: http://www.butzi.net/articles/vcce.htm)

    First I needed to find out how many CCs of neutral density on my color head equalled an exposure change of one stop. The Butzi article mentions using an enlarger exposure meter, but I used a standard exposure meter and it worked fine.

    I plotted change in neutral density (CCs of combined Cyan, Yellow and Magenta) versus change in exposure meter reading.

    [​IMG]
     
  2. ic-racer

    ic-racer Member

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    So the straight line through the points shows that one stop equals 30cc of neutral density with a 99% correlation.

    Pretty cool, that means 30cc = 0.3 log = 1 stop :tongue:

    Then I looked at the package insert for the paper to get an idea of which color was going be the LEAST sensitive. I would choose this one and then dim all the other colors (with ND or RED) to match.

    I chose 130cc Yellow as the baseline. I made a contact print of a 21 step wedge under 130cc Yellow to compare to all the others.

    Then I used the values in the package insert to guess at some contrast combinations to try. For example I choose 97 Yellow and 17 Magenta and made a test strip with the step wedge. I counted the number of gray segments and got 8. So, on a 21 step wedge each block is 1/2 stop or .15 log. Multiplying .15 x 8 x 100 gives an estimate of the contrast as an "ISO R" value. I got 120 for that combination (about grade 1).

    Next I held the processed strip up against the original strip from 130 cc Yellow and shifted them back and forth until the middle grays lined up and the gray values straddled each other. This showed me I was one step of the step wedge off. One step is .15 log which for my enlarger is 15cc of neutral density. So I added 15cc of Red (leave out the Cyan as the paper doesn't see it) and that made the filter pack 112 Yellow and 32 Magenta.

    I repeated that process for a number of other values to fill out the chart.

    For the two extremes (199 Y, 0 M and 0 Y, 199 M) I just calculated the factor needed to match, again by comparing to the 130 Y test strip and sliding it back and forth until the middle values matched. This gave me 0.5 stops extra exposure needed for 199 Y (factor X 1.4) and 2.5 stops less exposure needed for 199 M (factor 5.6).

    I put it all together in a chart and printed it out for reference.


    [​IMG]
     
    Last edited by a moderator: Oct 17, 2008
  3. ic-racer

    ic-racer Member

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    Typical figures might be Grade 5 = ISO(R) 40 to 45; Grade 4 = ISO(R) 60 to 70; Grade 3 = ISO(R) 80 to 90; Grade 2 = ISO(R) 100 to 110; Grade 1 = ISO(R) 120 to 130; Grade 0 = ISO(R) 140 to 150; Grade 00 = ISO(R) 160 to 180.

    So I get about grade 4 to 00. Again, I'm not using a reflection sensitometer. I'm just counting the number of gray stripes printed by the 21-step scale (leaving out the one that is 'just off white' and 'just off black') then multiplying by 15 to get an estimate of my ISO R contrast grade.
     
  4. Kirk Keyes

    Kirk Keyes Member

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    Good work. I've done this as well. The graph for my Saunders 4500 looks very similar to yours.

    I've also made a graph by using a cheapo color enlarging meter (I bought a Minolta PM2L) and simply measured the filters I have from Ilford and then adjusted the dials on the enlarger to give the same readings. Easy to do, no test prints required, no real math involved, and those color meters are cheap these days.
     
  5. ic-racer

    ic-racer Member

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    Sounds like a great idea!
     
  6. Curt

    Curt Subscriber

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    Do you mean Beseler PM2L? I have a Beseler model. I'm a bit thick today but do I understand it right, you are using an Ilford filter set and using each of the filters to find the equivalent filters on the color head?

    Do you put an Ilford filter under the lens with the color head set to white light and read. . . Geeze I'm brain dead here. . .

    Maybe you can explain it better than I can ask it? I have a color head and I want to use VC paper, I have a color analyzer but no Ilford filter set, yet, I do have a Stouffer calibrated scale. It would be great to make new dial read outs too. I take it that the calibration needs to be done for each brand of VC paper, maybe stick on's or just write it down?
     
  7. ic-racer

    ic-racer Member

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    Curt,

    My explanation was sketchy because that link (and the few books out there) have the detailed explaination, but I'll give a quick overview to help understand.

    With your color head just imagine a series like this

    Max Yellow, 90Y, 60Y, 30Y, 15Y, 0, 15M, 30M, 60M, 90M, Max M

    That will do everything for you.

    The tricky part is making them all print with the same exposure. I make them all print grey, but you can base them all on black or white.

    You use the step wedge and find out which of the ones above is the 'fastest' (probably leaving out the max yellow and max magenta for now) and which is the 'slowest.' Then you need to figure out how much red (Y&M) to add to each so that they all print the same.

    So, once you get your step wedge contact prints you can line them up so the blacks all match, or so the middle gray matches or so the whites all match. You can calculate how far each is off because you know each step of the step wedge is one-half stop.

    In my case I know every 30cc of red gives me one stop of neutral density, so I calculated, and added red where needed, based on that. (I then re-checked the step wedge contact print and fine tuned from there).
     
    Last edited by a moderator: Oct 24, 2008
  8. Curt

    Curt Subscriber

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    I get it now, I was out in the shop stripping the black paint off of a Seneca 5x7 back and was tired and had a headache from the fumes, after a nap and clear air I can see how it works. It looks like that step wedge is going to get some new use. Thanks for the information and post.
     
  9. Kirk Keyes

    Kirk Keyes Member

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    Yes, sorry, Beseler, not Minolta.

    You've got it right, you put the Ilford filter in the enlarger (or simply lay it over the sensor probe), turn off the room and safelights, turn on the enlarger, and "zero" the meter. Then remove the filter, and adjust the dials in the enlarger until your get the same reading as you got with the Ilford filter.

    Actually, after all is said and done, I don't really use the chart I made, as I don't really care what "filter" setting I used to make a print. I just use whatever magenta or yellow setting it takes to get a good print.

    And while you can do things like speed-match the filter settings like IC racer did, I find I like to use the fastest print time I can get ( I usually print 11x14 or bigger, so shorter exposures are desired). So I simply use yellow or magenta, but never both at the same time.
     
  10. ic-racer

    ic-racer Member

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    Kirks post highlight the versatility of using a color head. Another thing one can do is to make a 'grid of exposure factors' for each single color setting. One can then use calculated exposure factors, instead of adding red neutral density.

    So, on the grid you would have Max YELLOW to Max MAGENTA on the X axis and the same on the Y axis.

    Each block in the grid would have the number of stops difference between the two step wedges in the grid. ie 60y to 60m might have an exposure difference of 1.5 stops. Then your exposure factor for that change would be:
    FACTOR = 2^(stops differnence)
    So, in that box on the grid you would put a factor of 2.8, and so on...
     
  11. Kirk Keyes

    Kirk Keyes Member

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    Ic - the problem there is with which print density are you calibrating your table for? The contrast will change and you will not be able to make a single table that would cover all densities in the print. That is, to you calibrate it to a specific dark tone, middle grey, or a specific highlight? Each one would need a separate table.
     
  12. ic-racer

    ic-racer Member

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    For me it is middlle gray. Basically I 'straddle' the step wedge exposure strips when I line them up. When I work I want the blacks blacker and whites whiter when I increase contrast, and visa versa.

    Ilford has the exact same type of table to locate the exposure factors when switching MG filters when using a W45 Aristo. The Ilford one is also based on a middle gray, but the advantage of making one's own table is that one could base it on the blacks or the whites depending on how one lines up the step wedge contact prints.

    [​IMG]
     
  13. Kirk Keyes

    Kirk Keyes Member

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    Right, but you do have to pick the tone you are calibrating for when you make a chart like that.