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At very low light levels, film is less responsive.
Light can be considered to be a stream of discrete photons, and a light-sensitive emulsion is composed of discrete light-sensitive grains, usually silver halide crystals.
Each grain must absorb a certain number of photons in order for the light-driven reaction to occur and the latent image to form.
In particular, if the surface of the silver halide crystal has a cluster of approximately four or more reduced silver atoms, resulting from absorption of a sufficient number of photons ( usually a few dozen photons are required ), it is rendered developable.
At low light levels, i. e. few photons per unit time, photons impinge upon each grain relatively infrequently ; if the four photons required arrive over a long enough interval, the partial change due to the first one or two is not stable enough to survive before enough photons arrive to make a permanent latent image center.

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