What is the f figure on camera lenses?

f/numbers show the relationship between the diameter of the aperture and the focal length of the lens. Each is a reciprocal of d/fl, diameter over focal length, so f/8 means the diameter is 1/8 the focal length. The numbers in the f-number series 1-1.4-2-2.8-4-4.5-5.6-8-11-16-22 each represent a change in admitted light intensity by a factor of two, with larger numbers allowing less light in, and smaller numbers allowing more. Doubling the light intensity does not follow a doubling of f-numbers since the intensity is a function of the area of the opening, while the f-number represents its relative diameter.