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James, you'll find a lot of information here: http://printinthemix.com/, the printing technology website of Rochester Institute of Technology.
Not much that's earth-shaking has happened since the advent of Level III PostScript, but there isn't a whole lot left to be addressed. While the offset print universe was shrinking, it was also getting better and better, largely through the efforts of companies such as Heidelberg, who develops their own RIPs or RIP variants for things such as FM (frequency modulated) screening. Agfa and a few others had done a lot, too, having come up with some good ideas that have barely been implemented.
Keep in mind that print involves variables that can only be addressed by experience: the performance of specific papers and specific ink formulations. There are also maintenance issues, such as the condition of blankets and a myriad of mechanical adjustments. After all these years of progress--and there has indeed been great progress--printing is still something of an art that depends on acuity of vision and, most importantly, a clear sense of intent on the part of all who are involved.
As you know, I design Hebrew-English prayerbooks that are printed on very light stock (e.g. 42gsm). I adjust the weights and details of every type I make or use to work optimally on the specific paper and the inks I print on them. Because we print hundreds of thousands of copies of these books, which are usually around 1000 pages, the investment is fully worthwhile. But similar issues come up in my image-driven books, too, which are on much heavier coated stock, and in which the black ink used for text is usually Process Black rather than a richer black, so type weights are a sensitive issue there, as well, and often require compensation. If there's no time or budget to make specific types, I look for something that meets the requirements. There are types I like that I simply can't use because they won't perform well under the specific typesetting/printing circumstances of a project. Digital printing has yet another set of concerns, often the opposite of offset.
It's a shame that most younger type designers have little experience with offset printing, which is not going away anytime soon.
Raphael, it’s been a long time—good to hear from you. There are now 372,000 copies in print of the machzor (High Holiday prayerbook, for those who are unfamiliar) Mahzor Lev Shalem, which I designed, composed, and produced for the Conservative Jewish congregations in 2010. The print run for the follow-up volume for Sabbath and Festivals, Siddur Lev Shalem, which first appeared in January of this year, is now at 78,000 copies. Both books are nearly 1000 pages. The machzor I produced for the Reform congregations, Mishkan HaNefesh, which was published in 2015 in two volumes, had an initial print run of 250,000 sets (a half million books); an additional 20,000 sets were printed this year. These numbers are a reflection of the size of the liberal, American-Jewish community. (Some of the books are used in Israel, England, Australia, and South Africa.) I’m curious to know how many of the Koren siddurim you produced have been printed.
There are a lot of misconceptions here about offset printing! Some of you seem to be confusing dpi (dots per inch), which is the overall imaging resolution, with lpi (lines per inch), which refers to the screen resolution of bitmap images (i.e., halftone images). In offset printing, dpi refers to the resolution at which the plates are imaged. By and large, this is 2400 dpi or greater. The need for any finer resolution is rare in printing on paper, especially as PostScript dots are so fine and well-controlled. For the printing of type, ±2400 dpi is optimal. For halftone images, the screen resolution is sometimes as low as 133 lpi (120 lpi used to be common) for ordinary web-printed work on uncoated paper, though 150 lpi or even 175 lpi will produce much finer results. In sheet-fed printing of art-quality books, 200 lpi is the minimum and this applies to both 4-color process (CMYK) and single-color halftones. Since the 1940s, a small number of the best-quality printers were printing images at 300 dpi, producing a result in which the dot is not visible to the unassisted eye. As this high-resolution work was slower to print, it was used only for elite work. This kind of traditional screening, regardless of resolution and whether it was imaged to film (now completely outmoded) or direct to plate, is called “AM screening” (amplitude modulation).
Since the 1970s, printers began experimenting with non-linear screening, known as “stochastic” screening or FM screening (frequency modulation), in which the density and size of dots changed according to the desired color level. (A stochastic “dot” [it is more of a block] can be as small as 10 microns.) In the 1990s and 2000s, RIP and press manufacturers began offering hybrid screening that combined the virtues of both AM and FM. With this AM/FM screening, the resolution of a single image can range from, say, 175 to 300 (and more) lines per inch, depending on what is required by the image. As often happens during major technology shifts, the old technology (printing) became better than ever, especially at the low end. Whenever you see poorly reproduced pictures, it’s nowadays seldom the fault of the printing (unless the ink is poorly matched to the paper surface), but more often the fault of incompetent pre-press.
I am not aware of a good guide book to current offset printing techniques, though one can find a lot of good articles and research papers on the RIT (Rochester Institute of Technology) website. I’m sure there are some, somewhere, but as someone who, in printing, is the product of old-fashioned apprenticeships and forty years of hands-on experience, not of formal schooling, I never paid much attention to basic instruction books.