Film grain or granularity is the random optical texture of processed photographic film that derives from the fact that film derives tone from a collection of small grains of metallic silver developed from grains of silver halide that have received enough photons.

Granularity is a numerical quantification of film grain, equal to the root-mean-square (rms) fluctuations in optical density, measured with a microdensitometer with a 0.048 mm (48-micrometre) diameter circular aperture, on a film area that has been exposed and normally developed to a mean density of 1.0 (that is, it transmits 10% of light incident on it). Granularity is often quoted "times 1000", so that a film with granularity 10 means an rms density fluctuation of 0.010 in the standard aperture area.

When the grains are small, the standard aperture area measures an average of many grains, so the granularity is small. When the grains are large, fewer are averaged in the standard area, so there is a larger random fluctuation, and a higher granularity number.

The standard 0.048 mm aperture size derives from a drill bit that some guy at Kodak had lying around.