How can black dots suggest shades of gray?

A black-and-white printed photograph can show pale skin, a gray sky and a dark coat even when the printer uses only black ink. Under magnification, many such tones resolve into patterns of black marks separated by unprinted paper. Halftoning varies these patterns to represent tonal differences rather than mixing a separate gray ink for every part of the image.

At a suitable viewing distance, individual fine dots may no longer be easy to resolve. Their combined black-and-white pattern can appear more like a continuous tone. Moving closer or enlarging the pattern exposes the separate marks again. The printed structure has not changed; the scale of its detail relative to the view has.

The ideal example repeats non-overlapping circular dots inside square cells. Coverage measures the fraction of a cell occupied by black. At 25 percent coverage, one quarter is black and three quarters white. At 50 percent, the two areas are equal. Increasing coverage changes the geometric mixture while the individual dots remain black.

Cell size and coverage are separate choices. Smaller cells can fit more, smaller dots into the same patch while preserving the combined black area. This changes texture and the ease of resolving dots without necessarily changing coverage. The enlarged cell explains this area relationship rather than showing an actual printing scale.

Coverage is not a universal equation for perceived brightness. Ink spreading, paper, illumination, display behavior and viewing distance affect the result. Even exactly 50 percent black area need not look like a perceptual midpoint gray. Halftoning joins an exact geometric pattern with a physical and perceptual viewing process: one ink can carry much more tonal information than its single color first suggests.