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A reader of the PIE Blog posted a comment recently describing problems that have occurred in his transition of some titles from offset printing to digital printing, mostly insofar as his digitally printed halftones (and presumably any area screens) printed lighter than expected.
For my response to this reader, I assumed (from the wording of his question) that he was referring to print books produced on uncoated paper using laser printing technology (also referred to as xerography or electrophotography).
Here’s the reader’s question as well as my answer:
Question:
We have run into problems when moving titles from offset to digital. All grayscale images. Contrary to my expectations, digital print images look washed out, losing in my estimation 10% or more (so a 20% screen looks like 10%). Is this your experience? What is the best strategy for converting existing pdf file to digital print (do we have to go back into InDesign and reprocess all the images?).
Answer:
Thank you for your comment. I can usually tell the difference between digitally printed text, screens, and halftones, and offset printed text, screens, and halftones. To me digital screens and halftones just don’t seem as crisp. Also, the transitions in digital screens and halftones don’t seem as consistent (or even) as those printed via offset lithography.
That said, I have seen a lot of really good inkjet work on coated stock (usually posters). So it is probable that the uncoated vs. coated substrate (print books vs. posters) makes a difference, as does the specific digital technology (laser printing vs. inkjet).
In your case it sounds like you are digitally producing print books (which as I note may be of a lesser quality than offset printed books). With this in mind I would suggest that you request samples from your printer (to help you evaluate the technology), check into the HP Indigo electrophotographic press (a laser-based printer of very high quality that uses toner particles suspended in fuser oil), and ask the printer to gang up a number of photos you have darkened a bit in Photoshop to mitigate the problem.
Ask for suggestions as to whether to darken highlights, midtones, and shadows or just the shadows (there may be a less than even transition, and you don’t want to go from overly light to overly dark images). Making some test sheets (and even trying this test with different commercial printing shops) may help you.
I wish I could say this was simple. For print books you are already producing, your printer may be able to make simple changes to the PDF files directly in PitStop (a preflight program) without your needing to go all the way back to the Photoshop files and then reimport them into InDesign before creating the press-ready PDF files.
So the short answer is to set up a test with the print provider to measure just how much lighter the halftones and screens are than expected (i.e., as created) and then to adjust halftones globally (going forward) using Photoshop. Plus, it may be possible to adjust current halftones slightly at the printer using preflight software.
More Thoughts / More Research
After I sent my reply to the reader and posted it online, I did some more research, which I’d like to share with him and with you.
First of all, a lot of the problems reflect the difference between laser printing and inkjet, and between uncoated paper and gloss coated paper.
But first I’ll start with the reader’s probable motivation for choosing digital printing over offset printing, and his resulting need to control the density of halftones, area screens, and gradations. Most probably it was a print book with a limited press run. Such a book is more economical to print digitally because of the absence of extensive make-ready work needed for offset printing.
As another example, I have a client who needs 50 copies of a 184-page book at present. This is ideal for digital laser printing. While the unit cost of the copies will be high, the overall cost of the entire 50-copy print run will be far less than a comparable print job produced via offset lithography.
(Of course the other reason to choose digital technology is for variable-data printing, which is ideal if there must be a level of personalization from copy to copy.)
The Technology
The technology makes a difference as well. Laser printing, particularly dry laser printing (in which flecks of dry toner are attracted to the printer drum with an electrostatic charge, and then transferred to paper, and then bonded to the paper with heat and pressure) can (in my experience) become problematic because the toner flecks may not be as precisely placed with this digital technology as are halftone dots of printing ink in offset lithography. This can cause a fuzzy or ragged look to laser-printed type and images.
Plus, occasionally there are artifacts in digital printing. These are unintended specs or marks that mar an otherwise pristine halftone, area screen, or gradation. Some digital presses seem to be more prone to this than others. And some values (darker or lighter) in black and white images, or some colors in color images, seem more prone to this.
On the positive side, many high-end digital toner presses are available now. I personally like the HP Indigo, which uses toner particles suspended in fuser oil rather than dry toner particles. I have seen excellent results from this digital press, and I believe a number of other vendors have manufactured similar presses as well.
In addition, there’s always inkjet digital printing. When you look at laser printed samples, you’ll see halftone dots on a regular grid (very similar to the halftone dots of offset lithography). In both offset and laser digital halftones, the halftone dots are larger or smaller depending on the amount of a particular color needed (cyan, magenta, yellow, and black).
In contrast, inkjet-printed halftones are dithered. The halftone dots are not larger or smaller. They are all the same size (minuscule). There are just more of them in areas of denser color. So the overall gradations between colors and between values look almost like a continuous tone image (i.e., they are smoother than laser-printed images).
In short, the choice between digital laser technology and digital inkjet technology could help mitigate this PIE Blog reader’s problem with his print books.
The Paper
Paper makes a difference, too. I’ve seen inkjet printing on gloss coated stock that looks as good as offset commercial printing (just as I have seen toner-based digital printing on uncoated offset paper that is lighter than desired, with uneven gradations and occasional artifacts). The PIE Blog reader might want to experiment with paper substrates (depending on his client’s needs).
Photoshop Controls
I had encouraged the reader to select test images and print them digitally after discussing his concerns with his printer. While a simple solution may involve just darkening the images a certain amount on a consistent basis, this may not be enough. The reader may need a more nuanced solution.
For instance, it is possible to selectively increase or decrease the tones of an image in Photoshop using the “Curves” tool. This tool displays a grid with a diagonal line running from the bottom left to the top right (showing initial tones and the new tones to which you will lighten or darken them). This pertains to both black and white images and color images.
By adding control points (or points) to this diagonal line (you can find out how to do this online) and then pulling them up or down (to change the diagonal line on the grid into an “S” curve), you can selectively lighten or darken the quarter tones, halftones, and three-quarter tones of a photo. That is, you can individually tweak the various levels in a halftone image from the lightest lights through the middle tones up to the darkest darks.
The key here is to run tests (physical printouts) with your commercial printing supplier and then apply what you have learned to all subsequent halftone (or area screen and gradation) work.
One of the articles I found also showed a gray scale that transitioned in equal steps from white through grey to black in 10 percent increments. The PIE Blog reader could create a scale like this and then have his custom printing vendor print the scale on both uncoated paper and coated paper, using both laser digital technology and inkjet digital technology. Then the PIE Blog reader could make decisions for future work based on an analysis of the resulting test sheets.
The Takeaway
Here are my suggestions for those of you with a similar problem to that of the PIE Blog reader:
- Involve your commercial printing vendor early. You may even want to find an ally in the printer’s prepress department. No one will be more knowledgeable (both in general and in terms of that printer’s specific equipment).
- Create tests (inkjet and/or digital laser on various paper substrates). Based on these tests, develop a standard operating procedure to lighten or darken images in Photoshop.
- Don’t assume that such problems (with lightness or darkness of images) will be evident across all values of a halftone. You may want to selectively change highlights, midtones, and shadows using such Photoshop tools as Curves.
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on Sunday, April 23rd, 2023 at 12:36 pm and is filed under Digital Printing, Photos.
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