One key to black and white photography is understanding film. Black and white film is made up of three layers: a light-sensitive emulsion layer, a plastic strip to support the emulsion, and an anti-halation layer designed to capture light that has come through the emulsion and plastic and prevent it from bouncing back. The emulsion layer contains grains of silver salt that, once exposed by light, react with a developer and break down into pure silver. These grains of salt are the key to understanding fast or slow film. Larger salt grains require less light to create an image and is therefore a faster film as it can be exposed quicker. Smaller salt grains, conversely, require lighter to create an image and is therefore a slower film. These grains translate to a final print; faster films have larger salt grains and result in a grainier print than slower film.