Focal length
mm
f-number
Aperture diameter
mm
Area
mm²

# Aperture Area Calculator

By Miłosz Panfil, PhD

This Aperture area calculator helps you compute the aperture area of a lens. Try the calculator right now or keep on reading to learn about the aperture diameter, f-number, and the aperture area equation.

## Aperture diameter and f-number

Aperture is a hole, or an opening, in an optical system through which the light enters. The larger the aperture, the more light can come. In the same time, the light is less collimated. Smaller aperture results in more collimated light entering at the cost of lower intensity. If you want to learn about different ways of measuring light intensity check the lumen calculator. The aperture diameter is just the diameter of the opening.

The other characteristics of an optical system, such as a lens, are the f-number and the focal length. The focal length is a distance over which the initially collimated light rays are brought to a focus. The larger the focal length, the more distant objects can be seen sharply. You can check the thin lens equation or lens-maker equation calculators to check how to compute the focal length of a lens.

The f-number `n` is simply a ratio of the focal length `f` and the aperture diameter `D`

`n = f / D`.

In commercially produced lenses we can usually set the f-number to some prescribed values like `1.4, 2, 2.8, 4, 5.6, ...`. They correspond to decreasing the aperture diameter by a factor `√2`. In turn, the aperture area decreases by a factor `2`.

## Aperture area equation

We can write the aperture area equation in two ways:

`A = π (D / 2)^2 = π (f / (2 * n))^2`,

where

• `D` is the aperture diameter,
• `f` is the focal length,
• `n` is the f-number,
• `A` is the aperture area.

For example, if we set the `f-number` to be `1.4` for a common lens of a focal length `f = 70 mm`, then the aperture diameter is `D = 50 mm`. Using the aperture area calculator we find that the aperture area is `A = 1963.3 mm²`.

Miłosz Panfil, PhD