The primary element for light capture in photography is the silver halide crystal. When exposed to light, this crystal forms a small, stable "latent image" that is highly amplified during photographic development. In black-and-white photography the reduced metallic silver forms the image. In color photography, the oxidized developer is used to create a colored image in register with the developed silver. By common usage, the term "emulsion" denotes what is actually a dispersion of tiny silver halide crystals (grains) in gelatin. Although the structure of the AgBr and AgCl lattice is face-centered cubic, an enormous variety of crystal shapes can be obtained, depending on the number and orientation of twin planes and the conditions during growth. To add to this complexity, the crystals in commercial emulsions usually contain mixed halide phases. Films suitable for a hand-held camera generally contain silver bromoiodide, in which iodide ions are incorporated into the AgBr lattice during crystal growth. Practical photography is possible from conditions of bright sunlight to night street lighting. These conditions span a factor of about 10⁵ in illuminance, and must be accommodated by the combination of camera shutter speed, lens aperture, and film speed.

Sensitivity to light, or photographic speed, is one of the most important attributes of the emulsion. Intrinsic sensitivity is typically enhanced during manufacture by a heat treatment in the presence of tiny amounts of sulfur and gold compounds (chemical sensitization). Organic dyes, usually cyanine dyes, are then applied to the crystal surface to extend the basic UV and blue sensitivity to other colors in the visible spectrum (spectral sensitization). Different layers in a color film contain emulsions that have been dyed to respond selectively to blue, green, and red light, thus making color photography possible.

The silver halide emulsion literally "sets the scene" for the subsequent complex chemical processes that lead to the formation of a colorful image. The perennial challenge to Kodak’s emulsion scientists is to increase film speed while maintaining image quality and keeping performance.