#F/ Number or F/ Stop

An f/ stop is a number used to indicate the size of a lens's aperture opening. Aperture blades open and close like the human eye's iris to let in more or less light. A large opening, which lets in more light and is used in darker situations, is described with a small f/ number like f/1.8, and a small opening, which lets in less light and is used in brightly lit scenarios, is described with a large f/ number like f/22.