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I spoke and did 20 or so one-on-one design consultations at a conference this past weekend, and by coincidence (I hope) several people complained that their printed pieces came out much darker than they appeared on the screen or on their laser printer proof. Although press operators have been known to create bad printing from good artwork, I think the culprit in every case I looked at this weekend was a failure to take "dot gain" into account.

Every printing press and paper combination spreads (darkens) the dots that make up halftone images (aka "pictures). The 2-3% dot gain on a typical commercial offset press printing on coated stock is hardly noticeable. However many older, smaller presses (particularly those favored by "quick print" shops) can have dot gain of 8%-12%, often even more on uncoated paper, along with an insufficient ability to adjust ink flow to different areas of the paper. Web presses, especially newspaper web presses (which typically do not heatset the ink immediately after printing), are a different story altogether. Dot gains of 25-30% are common.

Since dot gain darkens photos, it makes normal photos muddy. Photos that are heavily screened back so they can appear behind text become too dark. For example, a photo that has been lightened to 20% of normal will appear as if the screen is 30-40% on a web press. (There isn't a direct correlation between dot gain and darkness because there are many other variables, especially with color printing.)