Aperture

Definition

In optics, the aperture of an optical system is the hole that controls how much light passes through. It decides which rays of light form an image plane.

An optical system, like a lens or mirror, has parts that can limit light. These can be the edges of a lens or mirror, or special pieces such as a diaphragm. The aperture stop is the most important part because it mainly decides the angle and brightness of the light.

Use in Different Fields

In astronomy and photography, aperture often means the size of this opening. For example, a telescope might be described as having a 100-centimetre (39 in) aperture, meaning the size of its main lens or mirror. In photography, the size of the aperture is given as a ratio related to the focal length.

The term aperture can also describe any opening that blocks off light. In photometric studies of stars, an aperture might be a window around a star’s image where the light is measured.

Application

The aperture stop is a key part of many optical systems. It controls how much light reaches the image or film plane. Sometimes this is because of practical limits, and sometimes it is done on purpose to avoid too much light.

The size of the aperture stop affects several things. It influences how much of the scene stays in focus, called depth of field. A smaller opening keeps more objects in focus at once. The stop also helps reduce image distortions by blocking light that could cause problems at the edges of the lens.

The stop can also change how the image looks at the edges, and its position can change how sizes are measured in images. In telescopes, the size of the aperture is very important because a larger opening lets in more light from distant objects, though there are practical limits to how large it can be. Apertures are also used in lasers and microscopes to control light.

In photography

The aperture stop of a photographic lens can be changed to control how much light reaches the film or image sensor. Together with the shutter speed, the aperture size helps decide how much light the film or sensor gets. A fast shutter needs a larger aperture to let in enough light, while a slow shutter needs a smaller aperture.

A device called a diaphragm usually acts as the aperture stop. It works like the iris of the eye — it changes the diameter of the lens opening. Making the aperture smaller lets in less light and keeps more of the scene in focus.

The lens aperture is often shown as an f-number. Lenses usually have a set of marked "f-stops" that can be chosen. A lower f-number means a larger aperture, letting in more light.

Aperture priority is a semi-automatic mode in cameras that lets the photographer choose the aperture setting, and the camera picks the shutter speed and sometimes the ISO sensitivity for the right exposure.

Typical aperture ranges in photography are about f/2.8 to f/22 or f/2 to f/16.

Maximum and minimum apertures

Further information: Lens speed

Lens specs usually include the maximum and minimum aperture sizes, like f/0.95 to f/22. Lenses with apertures of f/2.8 or wider are called "fast" lenses.

The minimum aperture doesn’t depend on the focal length but is chosen based on practicality. Very small apertures can make the image less sharp, so there’s usually little benefit in using them.

Aperture area

The amount of light captured by an optical system depends on the area of the entrance pupil.

When comparing lenses of the same focal length, the focal length value isn’t needed.

If two cameras have the same angle of view and the same aperture area, they gather the same amount of light.

Aperture control

Although an automatic aperture control was invented as early as 1933, not all early cameras had this feature. From 1956, camera makers developed automatic aperture control, letting viewers see at the lens’s maximum aperture and stopping down at exposure time.

For some lenses, automatic aperture control wasn’t provided. Many of these lenses had a “preset” aperture feature.

Canon EF lenses, introduced in 1987, use electromagnetic diaphragms. This allows automatic aperture control even with special lenses.

Optimal aperture

The best aperture depends on both optics and lens performance.

When a lens is stopped down, the blur from defocus decreases, but diffraction blur increases. There’s usually a “sweet spot” around f/4 to f/8, but this can vary by lens. How much sharpness matters depends on how the image will be used.

In biology

Main articles: Pupil, Iris, and Pupillary response

In animals, including humans, the eye works like a camera. The iris, a colored part of the eye, changes the size of the pupil. The pupil is the opening that lets light in. This helps control how much light enters the eye. When it’s bright outside, the pupil gets smaller. When it’s dark, the pupil gets larger to let in more light. The size of the pupil can change depending on lighting and other factors, but it usually stays between 2 mm and 8 mm.

The iris has special muscles that help change the pupil’s size. These muscles are controlled by different parts of the nervous system. The pupil size can also be influenced by emotions and interest in what you’re looking at.

Equivalent aperture range

See also: Image sensor format

In digital photography, people often look at the 35mm-equivalent aperture range instead of the actual f-number. This equivalent aperture adjusts the f-number to match what it would be for a lens with a 35mm equivalent focal length. Smaller equivalent f-numbers are thought to give better image quality because they let in more light and can make the background less blurry.

For example, a Sony Cyber-shot DSC-RX10 uses a small 1" sensor and has a zoom range of 24 – 200 mm. Its maximum aperture stays at f/2.8, but this translates to an equivalent aperture of f/7.6. This is a lower number than some other f/2.8 cameras with even smaller sensors.

However, recent studies show that sensor size does not really affect how blurry the background appears. The f-number of an aperture does not change based on the camera’s sensor size because it is a ratio related only to the lens. Instead, smaller sensors mean you have to stand farther away to get the same picture, which naturally makes the background less blurry. Also, smaller sensors can make images look darker because of how the light-sensitive parts of the sensor are arranged. These changes are not caused by the aperture itself, but equivalent aperture can help guess how different sensor sizes might change a photo.

In scanning or sampling

In devices that take pictures, such as scanners, cameras, or televisions, "aperture" can mean a small opening or a short time used to capture the image. This opening helps decide how clear the picture will be.

For example, the tiny spots in film, called film grain, are measured by looking at the film through a very small opening, about the size of a grain of sand.

In popular culture

Aperture Science is a pretend company in the game series Portal. The company’s name is based on the idea of an opening in optics. The logo of Aperture Science shows an opening that lets light through. It stands for both the company and the Aperture Science Computer-Aided Enrichment Center where the games take place.