“Grain” and “noise” are often used interchangeably to describe the fine texture that breaks up a smooth tone, but the two arise from unrelated physical processes. Silver-halide grain is a permanent structure built into a developed negative; sensor noise is a statistical fluctuation in the count of photons and electrons at the moment of capture. The distinction matters because it predicts how each behaves across the tonal scale, how it scales with enlargement, and ultimately how it reads on a monochrome print.

How Silver Grain Forms

A black-and-white emulsion is a suspension of light-sensitive silver-halide crystals in gelatin. Exposure creates a latent image, and development reduces exposed crystals to tangled filaments of metallic silver. The visible texture is not a single crystal but a clump: developed silver grains overlap and cluster, and the eye integrates these clumps into the irregular pattern seen at magnification. Crucially, this structure is fixed once the negative is processed. It does not change with how the print is later exposed.

Manufacturers quantify it as diffuse RMS granularity, the root-mean-square fluctuation in optical density measured through a 48-micrometre circular aperture at a net density of 1.0 and read at 12x magnification. Kodak’s data for conventional cubic-grain Tri-X 400 lists an RMS granularity of 17, while the tabular-grain T-Max 400 reaches 10 and T-Max 100 reaches roughly 8 at the same standardized conditions. The tabular, or “T-grain,” emulsions Kodak introduced in its black-and-white films in 1986 — and which Ilford parallels in its Delta line — use flat plate-like crystals only about 0.1 to 0.3 micrometres thick. Their larger surface-area-to-volume ratio captures light more efficiently for a given silver mass, yielding finer apparent grain at equal speed.

How Sensor Noise Forms

A digital sensor counts photons. Light arrives as discrete quanta, and the number landing on a photosite during an exposure follows Poisson statistics. The defining property of a Poisson process is that the variance equals the mean, so the standard deviation of the count — the photon shot noise — equals the square root of the signal. This noise is not a flaw in the sensor; it is a property of light itself, present even in a theoretically perfect detector.

Two consequences follow. First, shot noise is signal-dependent: brighter regions carry more absolute noise but a higher signal-to-noise ratio, which rises with the square root of the photon count. A highlight is therefore cleaner in relative terms than a shadow. Second, a separate component — read noise — is added by the electronics that amplify and digitize the charge. Read noise is essentially independent of exposure level, so it dominates only in deep shadows where the photon signal is small. The familiar mottle in underexposed digital files is largely this read-noise floor becoming visible once the shot noise drops below it.

Why They Look Different on a Print

The two textures behave in opposite ways across the tonal scale. Film grain is most conspicuous in mid-tones and lighter areas, where developed silver is dense enough to clump and modulate density; clear film base in deep shadows carries little visible structure. Sensor noise does the reverse, growing most intrusive in the shadows, where the signal is weakest and read noise is unmasked.

Their geometry differs as well. Grain clumps are organic and randomly placed, with no relationship to the rectangular pixel array, so enlargement magnifies an irregular, somewhat soft pattern. Sensor noise is sampled onto a fixed grid; even when the fluctuation itself is random, demosaicing and per-channel processing impose a structure that can read as finer or more regular, occasionally with a faint cross-hatched or chroma-tinged character before conversion to monochrome. Film grain also has a built-in scale set by the emulsion and the degree of enlargement, while digital noise can be smoothed, sharpened, or otherwise redistributed after capture. The result is that grain tends to sit within the image as a material part of it, whereas noise tends to sit on top of the tone — a difference rooted entirely in their separate physics.