Re: Gum calibration (was: Paper negatives- Ink Selection)
Keith, on that point I defer to Mark; I was just pointing to his comment which seemed to support the idea that these variables aren't likely to have a big effect on exposure time. All I can attest to, myself, is my own experience with a great lot of different kinds of negatives over two decades. With negatives made of prints photocopied onto transparencies, silver film and paper (analog) negatives, negatives printed on laser printers of various resolutions and inkjet printers of various generations on various paper and transparency media, negatives carefully colorized and calibrated to the proper DR and curves generated to match the emulsions and printing protocol, both greyscale and colorized ... with all these different kinds of negatives my exposure times have remained constant except for variations directly related to changes in humidity, pigment concentration and other such printing-related variables. As a result of those years of observation, I have arrived at the (tentative) conclusion that while there are many variables that affect exposure time, negative variables are not a significant factor. I would be happy to be proved wrong by data, but theoretical arguments in the absence of practical dat aren't terribly persuasive to me. I understand what Loris is saying, I think, that theoretically one should expose longer in order to print the highlights if the DR of the negative is too high, so it follows to reason that if you're printing with the right DR negative the exposure time "should" be shorter, but in practice it doesn't really work that way in gum, at least in my experience; if you expose long enough to expose the highlights of a negative that's too dense for the process, you're going to expose so long that you'll block up the shadows, and you're just going to be robbing Peter to pay Paul during the development: if you develop long enough to open the blocked up shadows, you may well lose the highlights in development anyway. So I think in practice most people just settle for multiply printing those negatives to get the highlights and shadows in separate steps. But that's what works best for gum printing anyway, if you want to print a longer print tonal scale than say a log density range of around . 75, which is about the most gum will print in one coat; since you'll get smoother tonal gradations in the highlights using a lighter pigment mix, and of course darker DMax in the shadows using a heavier pigment mix. If you really want to print a nice long print tonal scale with nice gradations throughout the scale, this double printing is the only sensible way to do it IME. As I said, I'll be glad to be convinced by actual data, but I'm certainly not persuaded by the theoretical argument that since Loris and Keith do everything the same except how they produce the negative, then the difference in their exposure times must be because Keith isn't doing his negatives right. There are just too many things that could be different enough to account for the difference, without bringing the negatives into it, and I am just really philosophically opposed to any suggestion that anyone is doing something "wrong" in their gum practice; I will always resist that kind of characterization of one gum printer by another. But I've made the point and will stand by for actual data to the contrary. I think people read too much into what I say sometimes; I'm certainly not against calibrating and developing curves for negatives. All I'm saying is that if you're doing it with the goal of reducing your exposure times, my experience suggests you will be disappointed. At any rate, if you want a really good calibration system, the best system in my opinion is to follow Michael Koch-Schulte's tutorials using his RNP arrays to determine the color and Chart Throb to determine the curves. I don't recommend doing it the way I did, spending months going over and over and over things and calibrating dozens of pigment colors at many different concentrations, until I understood all the ins and outs of what was going on, but that's just how I am; I have to understand things. You can actually get to a very quick and reliable result just by following the directions. http://www.inkjetnegative.com/images/RNP/rnp.htm Katharine On Oct 18, 2008, at 12:39 PM, Keith Gerling wrote: Sorry Katherine (and Mark), but it doesn't make sense to me why ink
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