In an optical system, an aperture is a hole or an opening through which the light enters. The amount of light entering an optical system is determined by the size of the aperture. As the size of the aperture increases, the light entering through it becomes less collimated or more diverged. The diameter of this opening or the aperture is known as the aperture diameter (D).

The focal length of an optical system is the distance from the center of the aperture to the principal focus where the collimated rays entering through the aperture get focused.

The aperture area of an optical system is calculated from the aperture diameter as,

Here D is the aperture diameter.

The f-number is a dimensionless quantity that indicates the amount of light entering an optical system through the aperture. It is the ratio of the focal length and the aperture diameter of an optical system. It is also known as focal-ratio, f-ratio, or f-stop.

Here F is the focal length of the system.

The f-number of an optical system is expressed in a format preceded by ‘f/’. For example, if a lens’ focal length is 20 mm and its aperture diameter is 10 mm, the f-number would be 2. This is expressed as “f/2” in a lens system. The aperture diameter of such a system would be f/2.

A 100 mm focal length f/4 lens has an aperture diameter of 25 mm and a 100 mm focal length f/2 lens would have an aperture diameter of 50 mm.

Relative aperture is another dimensionless parameter that determines the light-gathering power of an optical system. It is the inverse of the f-number. Relative aperture is the ratio of the aperture diameter to the focal length of the optical system. For example, for an f/4 lens, the relative aperture is 1/4.