Editorial Feature

What is an Aperture?

Oct 6 2014

Image Credits: Vladimir Arndt/shutterstock.com

An aperture is an opening or a hole through which light passes by. In an optical system, the aperture is the opening that defines the cone angle of a set of light rays that converges at a focal point in an image plane

Apertures evaluate the collimation of incoming light rays that is critical for the appearance of an image plane. The size of the apertures is limited by the size of a diaphragm placed along the path of light or a mount holding the optical component. For narrow apertures, highly collimated light rays are introduced thereby forming a sharp focus for light at the image plane. For wide apertures, uncollimated rays can be introduced to obtain a sharp focus for light with certain focal length.

Aperture Stop

In general, the optical system has several structures known as stops that limit the light rays. An aperture stop is a component that determines the cone angle of light rays at an image point.

The depth of field is one factor that is affected by the size of the aperture stop. Smaller stops will generate longer depth of field thereby focusing all objects at a wide distance simultaneously. The stop also affects the optical aberration effect. Larger stops produce distorted images.

Aperture area

The area of the aperture is proportional to the quantity of light captured by a lens of the optical system.

Area = π (ƒ/2N)²

where ƒ is a focal length and N is the ƒ-number. The focal length value can be considered as 1 while comparing lenses of same focal length.

Maximum and Minimum Apertures

Every lens has a specific aperture size. The lens specifications typically include the maximum and minimum aperture sizes. For instance, if the specification is ƒ/1.4-ƒ/22, ƒ/1.4 is the maximum aperture and ƒ/22 is the minimum aperture.

The maximum aperture of the lens will have the lowest ƒ-number and indicates the lens speed. By contrast, minimum apertures have the highest ƒ-number. A lens having an aperture of ƒ/1.2 or ƒ/1.4 as maximum is said to be a fast lens due to its ability to allow more amount of light to pass through than, for instance, a lens with a maximum aperture of ƒ/4.0.