F-Number

F-Number, also known as a focal ratio, is the ratio of the diameter of the lens aperture to the length of the lens. On a camera, a photographer can alter the f-number by changing the f-stop setting on the camera’s lens. Each increment of an f-stop (and, thus, and f-number) represents a decrease of light entering the lens by half the previous intensity.

The more light the lens allows into the camera, the faster the shutter speed should be set, so as not to overexpose the film. Therefore the smaller the f-number, the more light the camera lets in through the lens. So, moving to a higher f-number will halve the amount of light.

Most lenses have f-stops that are represented numerically by integral powers of the square root of two. Common f-numbers on a lens are f/1, f/2, f/4, f/8, etc.