ƒ-numbers explained

What is an ƒ-number?

An ƒ-number is the ratio of the lens's focal length to the diameter of the apperture and a measure of lens speed — an important concept in photography. It is commonly indicated using a hooked ƒ with the format ƒ/n, where n is the ƒ-number, such as ƒ/1.8 or ƒ/2.8

What is lens speed?

You've probably heard of fast and slow lenses but maybe you're not sure exactly what that means. What is 'fast' and 'slow' when we are talking about photographic lenses?

Lens speed isn't about motion or shutter speed, it refers to how much light passes through the lens when the shutter is open. Lens speed is controlled by the lens' diaphragm, which determines the aperture size. The faster a lens, the larger the maximum aperture and the more light will hit the sensor (or film) at the same shutter speed. Fast lenses with a wide-open aperture can highlight a subject by separating it from the background. This shallow depth-of-field leads to endless creative opportunities and can add emotion to an otherwise uninteresting photo.

What is a fast lens??

A lens' speed is usually referred to by the size of the maximum aperture of the lens. The smaller the ƒ-number the larger the apperture and the more light through when the shutter is open. High ƒ-numbers refer to the smaller apertures. Typical ƒ-numbers halve or double the amount of light passed to the sensor. These are known as ƒ-stops. Going from ƒ/5.6 to ƒ/8 means only half the amount of light is used to expose an image. In this case, we've stopped down one f-stop. To get the same exposure with an aperture of ƒ/8 you need a slower shutter speed than you would at ƒ/5.6 to let in the same amount of light. An ƒ/1.4 lens is twice as fast as an ƒ/2.0 lens.

The following image shows how Apperture (lens speed), Shutter Speed and Iso affect an photo: