About twenty years ago I designed a 6” x 9” 4-color print catalog. I was an art director at a local non-profit organization. I had just received the color proof of the catalog, which was about to be printed via web-offset lithography. I was horrified. Everything was too light: text, images, everything.
Keep in mind that this was twenty years ago. I was looking at a proof created from litho films. If I recall correctly, it was a 3M Matchprint proof.
What had happened was that the printer had adjusted everything to compensate for the dot gain that would occur on the heatset web press during the custom printing process. As its name implies, dot gain means that halftone dots print darker on press than expected. Therefore, in this case, the film had to be adjusted to compensate for this darkening. Had the printer not reduced the size of the halftone dots in the film from which the plates would be burned, the final printed product would have been too dark, and everyone who received a print catalog in the mail would have been horrified.
(As an aside, you might ask why the commercial printing supplier didn’t produce one set of films for the platemaking process—again, it was twenty years ago—and another for the proof. This would have made the proof useless as a technical diagnostic tool. I would have been looking at a picture-perfect proof that bore no resemblance to the final film, plates, or printed product.)
What Is Dot Gain?
In each successive process in platemaking and custom printing, the size of halftone dots increases. Now that printers produce plates directly from digital files, there is no dot gain in the initial step of making films (from which plates used to be made) because there is no longer any film. However, dot gain does occur in the making of plates and in the printing of the final job. Therefore, the printer must adjust the digital file to reduce the size of the dots before proceeding.
The amount of dot gain that occurs depends on the printing substrate and the printing process. For instance, a gloss coated press sheet has a hard surface. Therefore the ink sits up on top of the sheet. For this reason, there is less dot gain on a gloss coated sheet than on an uncoated sheet. In contrast, newsprint is very absorbent, so the ink spreads into the paper fibers, and the halftone dots expand more than on a coated sheet.
Sheetfed offset printing yields a certain amount of dot gain, but web-offset printing yields even more dot gain (this was the process used for my print catalog twenty years ago). This is due to the increased pressure between the press blankets, plates, and rollers needed to keep an almost endless ribbon of commercial printing paper in place as it travels through the press much faster than cut press sheets travel through a sheetfed press. This pressure increases the dot gain.
The other determinant of dot gain is the screen frequency of the halftones. Finer halftone screens produce higher dot gain, while coarser screens (with a lower number of dots per inch) produce less dot gain.
All of this is physical dot gain. Interestingly enough, there is one more type of dot gain: optical dot gain. As small as they may be, halftone dots on a page do have a certain thickness, and it is possible for light to hit the printing dots and cast shadows. This makes the dots appear larger, creating what is called “optical dot gain.”
What Can You Do?
All is not lost. Since the dots get larger in predictable ways, depending on the printing paper and the printing technique, a printer can intentionally make them smaller, by a predetermined amount, prior to burning custom printing plates.
How Do You Predict/Measure Dot Gain?
In most cases, dot gain is measured at the 40 percent and 80 percent tones using a densitometer. It is measured in absolute terms. For instance, if your printer tells you there is 20 percent dot gain in the 40 percent tones of an image, this means that a 40 percent halftone dot will print as a 60 percent (40 + 20) halftone dot. Therefore, using the Photoshop levels or curves tools, you or your printer must reduce the size of the halftone dots to compensate.
Should You Compensate for Dot Gain, or Should Your Let Your Printer Do This?
At the very least, discuss this with your printer. When I produced the print catalog twenty years ago, the printer adjusted the files for the dot gain. Now I usually do this myself. It depends on your level of confidence and expertise. After all, you will see a proof, but you don’t want to have all images in a proof be too light or too dark, requiring more work and money to correct the problem.
So talk to your printer, find out what the dot gain will be based on the paper and the press, and then ask how he wants to compensate for dot gain in the halftones and 4-color images.
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