Photographic print is made from dots. Whether a press uses conventional AM screening, stochastic screening or a digital-press process, it converts continuous-looking image tones into printable marks. Your eye blends those marks at normal viewing distance; under a loupe, the structure becomes visible.
What a halftone does
Offset printing cannot lay down a continuously variable layer of each ink at every pixel. A halftone screen represents light and dark areas with dot patterns. Less printed area reflects more paper and appears lighter; more printed area absorbs more light and appears darker.
In traditional AM (amplitude-modulated) screening, dot size changes while the screen grid remains regular. The cyan, magenta, yellow and black screens use different angles so their patterns are less likely to interfere. Poorly matched repeating patterns can create moiré, a distracting ripple or rosette effect.
LPI, DPI and PPI are different measurements
These terms are often mixed up, but they describe different parts of production:
| Term | Meaning | Controlled by |
|---|---|---|
| PPI | Pixels per inch in a placed raster image | Designer or image source |
| LPI | Lines per inch in a traditional halftone screen | Printer or output workflow |
| DPI | Dots per inch produced by an output device | Imagesetter, platesetter or printer |
LPI does not tell you the pixel resolution of your photograph, and a high-DPI printer does not mean every image needs an enormous PPI value. The printing workflow turns image pixels into screened dots.
How much image resolution is enough?
For conventional screen printing, a long-standing starting point is image resolution at roughly 1.5–2 times the halftone screen frequency at the final placed size. A 150 LPI process might therefore receive a 225–300 ppi image. This is guidance, not a rule to apply without context: modern RIPs, screening technology, image subject matter and the intended viewing distance all influence the result.
For standard commercial print, 300 ppi at final size remains a sensible general target for photographic images. It will not rescue a soft original, and increasing the PPI number without adding source detail does not improve the image. Let the printer specify anything unusual, including large-format, textile, newsprint or fine-art workflows.
Paper changes the screen you can use
Coated paper holds small dots more cleanly than absorbent newsprint or rough uncoated stock. The latter may need a coarser screen or a gentler tone curve to preserve shadow detail. That is why a photograph can look different on two papers even when the CMYK numbers are unchanged.
Before retouching for a difficult stock, ask for the output condition. It defines the dot gain, ink limit and separation behaviour expected by the press.
Avoid common halftone problems
- Do not resample images repeatedly during layout; keep one high-quality source at the required final size.
- Avoid scanning or photographing already printed images where possible—the existing screen can conflict with the new one.
- Check very fine patterns, textured fabrics and repeated graphics at the intended output size.
- Keep important shadow detail out of extremely dark CMYK builds that exceed the printer’s total ink limit.
- Request a proof when image detail, gradients or patterns are critical to the job.