Re: Exposure times in tricolor gum
I haven't followed this entire discussion, but if the question is about differential exposure times and curves for tricolor gum layers, my informal experience from years of printing tricolor gum in an intuitive rather than calibrated fashion was that the exposure times were essentially the same for all layers, and my observations from a recent year or more spent carefully studying the calibration of digital negatives for gum have yielded the same conclusion, that there's not that much difference between them. When determining exposure times very precisely for printing PY97, PV19 and Prussian blue on Arches bright white, gelatin-glyoxal, the exposure times were Prussian, 3:00; PV19, 2:45; and PY97, 2:15. In other words, not that much difference, but the yellow was shortest and the cyan longest, not the other way around, and that's always been my experience; when there's a difference it tends in that direction. If anyone is interested in a somewhat longer explication of my thoughts and observations on the subject: http://www.pacifier.com/~kthayer/html/mythfilter.html The most recent tricolor print I've done is the test print I made on masa which I showed here a few weeks ago; my purpose in the testing was just to try to understand the requirements and peculariaties of printing on the masa paper; I didn't care too much about the tricolor print itself, so I just used the same exposure time for all three without bothering to determine them separately, and it seems to have worked just fine. (And by the way, I've decided that masa just isn't worth my time; even though I finally got the hang of it I just don't like the paper http://www.pacifier.com/~kthayer/html/masa.html As for curves, the curves I've calculated for the different layers are so similar that they could easily be superimposed on each other; they are all very deep, including the yellow. During the time I was studying calibration for gum, I generated dozens of curves for all different colors and pigment concentrations; I found differences in curves as a result of differences in saturation of the pigment (note, this isn't the same as pigment concentration in any absolute sense); the less deep the pigment mix, the shallower the curve, and vice versa, but not differences in curves simply as a result of their hue range. And of course exposure times vary similarly; a deeper pigment mix will take longer and a lighter pigment mix will take less time, but this is true across hue ranges rather than sorting out differentially for different hue ranges. Since I have the separate curves, I use them when printing tricolor, but when I've used the same curve for all, it doesn't make any difference in the final print, which makes sense given how similar the curves are. Katharine On Nov 18, 2008, at 5:24 AM, Henry Rattle wrote: Yes, the Y curve is quite a lot closer to the 45 degree line - less
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