imagesHalftones are printed by breaking the photo down into a series of dots. The larger the dot, the more ink it’ll print, so the darker that bit of the picture will be. This extreme version of Marilyn illustrates the point (from Casey Klebba’s 2011 Halftone Calendar, via MoCo Design).

Dot gain is caused by ink spreading out beyond the inked area (the dot) on the plate. You can think of it as a sort of squeezing out of the ink under the pressure of the press — just as the white cream in a warm Oreo can be squished beyond the edges of the cookie. It is an unavoidable part of printing, and if left uncontrolled will lead to “plugging” where the shadow areas lose detail and become almost solid. A coated sheet will incur less dot gain, because its surface is less absorbent than an uncoated sheet’s. More ink and more viscosity will tend to mean more dot gain. The greatest dot gain will be in the mid-tone areas (40-60%). The finer the screen the more noticeable dot gain will be. Dot gain can also be incurred at prepress in processing the negatives or in platemaking. However modern prepress systems allow for automatic compensation for dot gain, making the dots on the plate smaller by the required percentage, so that they’ll swell up to full size on the press. Each press will have to be benchmarked for different paper grades in order to figure out the amount of dot gain to be expected.

Letterpress printing has its own version of dot gain, ink squash, a sort of tangible build-up of ink around the edges of the type or image. This is often extolled by the nostalgic as a hallmark of the excellence of the letterpress process, as they run their fingers across the printed page, feeling the indentations of the type and the little ink ridges.

Ink squash from a woodblock

Letterpress ink squash from a woodblock










The useful Offset Pressman article on dot gain can be found here.