Why your 20% tint printed at 30%

Ink is one density; tone is an illusion

A press lays down ink at full strength or not at all. There is no such thing as 30% cyan ink. A 30% tint is a field of tiny cyan dots covering 30% of the area, and your eye averages them.

That conversion (continuous tone into a pattern of dots) is screening, and the RIP does it.

AM screening, the traditional kind

Amplitude modulated screening puts dots on a regular grid at a fixed spacing and varies their size. The spacing is the line screen, measured in lines per inch:

• Newsprint: 85 lpi
• Uncoated stock, general commercial: 133 lpi
• Coated stock, quality commercial: 150 to 175 lpi
• Fine art and packaging: 200 lpi and above
• Screen printing on textile: 35 to 65 lpi, because the mesh cannot hold finer
• Large format outdoor: as low as 15 to 25 lpi, seen from distance

Each of the four inks is screened at a different angle so the dots do not sit on top of each other. The conventional set is cyan 15°, magenta 75°, yellow 0°, black 45°: black at 45° because a diagonal pattern is least visible to the eye, and yellow at 0° because it is the palest and its pattern shows least.

Get the angles wrong and you get moiré: a coarse interference pattern from two fine patterns overlapping. Get them right and you get a rosette, the tiny flower shape you can see with a loupe on any printed photograph.

Moiré also appears when your artwork contains a fine regular pattern (a scanned halftone, a fabric weave, a pinstripe) that interferes with the press screen. The fix is to blur the artwork very slightly or change its scale, and it must be done before the RIP sees it.

FM screening

Frequency modulated or stochastic screening uses dots of one small size scattered pseudo-randomly, varying their number. No grid, so no angles and no moiré at all, and much finer detail. It demands better press control and it dries differently. Increasingly common on good digital and offset work; ask whether your shop offers it if the job has a scanned pattern in it.

Dot gain, and the number you need

A dot printed on paper comes out bigger than the dot on the plate. Two separate mechanisms:

• Mechanical gain. Ink physically spreads as it is pressed into the paper. Worse on absorbent, uncoated stock.
• Optical gain. Light enters the paper beside a dot, scatters, and emerges under it. The dot appears larger than it is even when it is not.

Together they are substantial. Typical midtone (50%) gain:

• Coated sheetfed offset: 13 to 17%, so a 50% tint reads as about 64%
• Uncoated: 20 to 25%: a 50% tint reads as about 72%
• Newsprint: 25 to 30%
• Screen print on textile: can exceed 30%

The press profile already accounts for this. Convert with FOGRA51 for coated or FOGRA52 for uncoated and the numbers going to the plate are adjusted so the printed result is right. Dot gain becomes a problem when you do not use a profile, or when you use the wrong one: an uncoated job converted with a coated profile prints visibly dark and muddy.

What to do about it in your artwork

Three practical consequences you control directly.

Light tints close up. A 5% tint on uncoated can print at 12% or vanish entirely depending on the press’s minimum dot. If a design depends on a very pale wash, print a test or use a heavier tint of a lighter colour.

Small reversed type fills in. White text on a dark ground loses width as ink spreads into the counters. Below about 8 pt in a light weight on uncoated stock, an e closes up. Use a heavier weight, a larger size, or accept it.

Fine screens in artwork clash. Avoid pinstripes below about 0.3 mm pitch in anything going to litho.

The check that costs nothing

Ask the printer for a printed tint chart on the stock you are using: a grid of 5% steps in each ink and in common builds. Many shops have one and will hand it over. It is the single most useful object in the trade: it tells you exactly what 20% cyan looks like on that paper, on that press, with no screen involved in the decision.