What is dot gain, you might ask? And why do I need to know about it? If you're planning to self-publish a book that includes black and white photo illustrations, it's helpful to understand this aspect of the printing process.
One of the biggest challenges for book printers is reproducing photographs in books that are otherwise mostly text. It's the task of a book designer to prepare these photos so they reproduce as well as possible.
How Photos Are Reproduced for Books
Before you can understand what goes into this preparation, you need to know a little about how photographs are reproduced. After all, black & white photos are mostly composed of gray tones, yet we don’t print with gray ink. Translating the gray tones of a photo into an image that can be printed with just black ink, yet still look like a photo, is accomplished by screening the photographs.
Traditionally we used halftone screens, which would produce a dot pattern. Large dots—where the paper is mostly covered with ink—created dark areas of gray, while small, even tiny dots—leaving mostly paper showing—represented the light areas.
If you make the dots small enough, they seems to disappear because the human eye isn’t sensitive enough to see them, producing what looks like a smooth gray tone. You can see this in the two pictures below.
Dots Are Not Always Dots
The problem is that the perfect dots produced by the halftone screen, or by software that processes images in the printer’s workflow, don’t always look so perfect when they hit the paper. In the real world, books are printed on fairly soft, porous paper. For fine reproductions of black & white photographs, for a museum catalog, for instance, you would use coated paper, a nonporous paper that keeps the dots of ink closer to their ideal size.
But with the uncoated papers we use in books, the little dots spread out. It’s the job of whoever is preparing your photos for reproduction to communicate with the book printer about how much he can expect the dots to spread, what we refer to as dot gain.
Typically, dot gain in book printing is about 20%. This means that if you have an area in your photograph—like a large expanse of sky—that’s about 30% gray in your original file, by the time the ink and paper come together, the dots can spread to 50% gray. This will affect the midtones—the middle grays—of the photograph the most, and is the reason why black & white photos often look darker and muddier than they should.
The Three Principal Causes of Dot Gain:
1 - Ink absorbed into the paper—softer papers will absorb more ink, harder papers will absorb less.
2 - Ink spreading out onto the paper—on coated paper, ink will be squeezed outward creating larger dots. This in turn is affected by the viscosity of the ink used.
3 - “Rimming”—the tendency for halftone dots to be surrounded by a small additional circumference of ink. This is most common in offset printing
The soft, uncoated paper used in books is the biggest cause of poor photo reproduction. It’s just very challenging to get a clear image with a full tonal range on these papers. And many publishers—myself included—prefer the cream or ivory-colored book papers, which are much more restful for reading. But the added color to the paper makes the photos even more challenging, because it darkens the lighter tones of the photos.
A New Kind of Dot Screen
In recent years, a different form of screening for printing has been developed, called stochastic screening. Instead of varying the size of the dots, something that could not be done by digital imaging engines, the dot size is reduced, but the printer lays down a greater density of these same-size dots to cover more of the paper, producing a darker tone.
Stochastic screening is based on randomly distributed dots, using a sort of frequency modulation to make a density of dots according to the gray level desired. New advances are creating stochastic screens that combine the frequency modulation of equal sized dots with the amplitude modulation of the different sized dots produced by traditional halftone screening. You can see examples in the accompanying illustration above.
By the way, in the illustration you can also see an example of a flat tint, a percentage tint of a color, in this case black. These tints are also produced by screening, but for flat tints the dots are uniform.
Flat tints are also affected by dot gain. If you specify a 60% tone in your layout or art software, you will almost certainly be displeased with the results, when you get your book back and the rich gray you saw on your screen has turned to a muddy dark near-black.
How Book Designers Help
There are quite a few tasks your book designer or production artist will address when it comes to preparing photos for use in a book. For instance, each photo has to be:
- Rendered in grayscale
- Scaled and cropped to reproduction size
- Be adjusted to 300 dpi (dots per inch) at the reproduction size
- Have levels or curves adjusted for anticipated dot gain
- Examined for imperfections that can be repaired
- Sharpened for the intended use
Dot Gain and the Indie Author
It’s important to leave room in your schedule and your budget to accommodate this work during the production of your book. Nothing is more critical to having the photographs reproduce well. The more central the photographs are to the main theme of your book, the more crucial it is to prepare them properly.
Preparing black & white photographs for printing in most books is calibrated by the book designer to achieve the best reproduction possible for the given book, printing paper, and its intended use.
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