How does photographic film give such good images with so little exposure to light? The answer, in a word, is amplification. Light causes clusters of silver atoms to form on silver halide microcrystals in a photographic emulsion. These clusters, which may contain as few as three or four silver atoms, catalyze the reduction ofallthe silver ions in the microcrystals to which they are attached, giving a “gain” of over a billion. This is the basis for the high sensitivity and image quality of silver halide photographic film. As I will explain in this article, much is known about the physical mechanisms involved, and by using this knowledge one can improve the sensitivity and efficiency of film.