the focal length of a lens divided by the clear aperture (diameter) through which light enters. The adjustment for f-ratio is called the f-stop.

The f-ratio determines the brightness of the image formed by the lens; lower f-ratios produce brighter images. Thus, a camera with an f/1.8 lens requires much less light to take a picture than a camera with an f/8 lens, even with the same film or electronic image sensor.

The brightness of the image is inversely proportional to the square of the f-ratio. That is why f-stops on lenses are often numbered as powers of ??: f/2, 2.8, 4, 5.6, 8, and so on. Each f-stop gives half as bright an image as the next larger (lower-numbered) one.

To increase or decrease exposure n "stops" means to increase it or decrease it by 2ⁿ. Thus a one-stop decrease means to cut the exposure in half, and a two-stop decrease means to cut it to 1/4 its original value.

The rated f-ratio of a lens refers to its widest opening; smaller openings (higher f-ratios) are provided as an automatic or manual adjustment. The actual front glass element is much larger than the opening that the light must pass through. The f-ratio of a zoom lens generally varies as the focal length is changed.