Aperture Calculator

Aperture Calculator


Aperture
Focal Length
fstop value

Definition

The aperture of a lens is an opening through which light enters. Logically the larger the size of the aperture the more light will enter

In lenses, aperture is expressed in f-numbers (for example f/5.6). These f-numbers that are known as “f-stops” are a way of describing the size of the aperture. A smaller f-stop means a larger aperture, while a larger f-stop means a smaller aperture

To calculate the size of the aperture at a given fstop, we divide the focal length of the lens by the fstop number

The aperture of a 40mm lens at f/8 will be a circle with diameter of 40/8 = 5mm

The aperture of a 24mm lens at f/8 will be a circle with diameter of 24/8 = 3mm

Note:the light coming in at a given fstop is constant for all lenses of different focal length. Here is the explanation:

  • The size of aperture at 24mm will be a circle with diameter of 3mm at f/8
  • The size of aperture at 40mm will be a circle with diameter of 5mm at f/8

The lens at 40mm is longer than the lens at 24mm. Light falls in intensity over the distance it travels. So the light at the end of a longer lens is less intense than at the end of a shorter lens. Hence, for a 40mm lens, if the size of the aperture is larger as compared to the 24mm lens, the amount of light coming in will be the same

Formula

To calculate the size of the aperture at a given fstop, we divide the focal length of the lens by the fstop number.

Aperture size(m) = Focal length(m) / f-stop number

For Example:

What is the size of aperture of a 40mm lens at fstop f/8?

Sol. :

Aperture size(m) = Focal length(m) / f-stop number

Aperture size = 40(mm) / 8

Aperture size = 5(mm) = 0.005(m)

Units

S.I unit of aperture is meter.