Aperture

In optics, the aperture of an optical system is the hole or opening that mainly controls how much light passes through the system. It decides which rays of light from an object will meet at a point to form an image plane.

An optical system, like a single lens or a complex setup, often has many parts that can limit the light rays. These parts can be the edges of a lens or mirror, or special pieces placed in the light path, such as a diaphragm. Among these, the aperture stop is the most important because it mainly decides the angle and brightness of the light rays that reach the image.

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

The term aperture can also be used more generally to describe any opening that blocks off light outside a certain area. In photometric studies of stars, for example, an aperture might be a circular window around a star’s image where the light is measured.

Application

The aperture stop is an important part of most optical systems. It controls how much light can reach 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 creates a larger depth of field, keeping more objects in focus at once. The stop also helps reduce image distortions by blocking light that would otherwise cause problems at the edges of the lens.

The stop can also affect 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 to avoid too much light.

A device called a diaphragm usually acts as the aperture stop and controls the aperture. It works like the iris of the eye — it changes the diameter of the lens opening. Making the aperture smaller (using a higher f-number) lets in less light and increases the depth of field, meaning more of the scene stays in focus. Generally, a smaller aperture (higher f-number) keeps more of the scene in focus.

The lens aperture is often shown as an f-number, which is the ratio of the focal length to the effective aperture diameter. 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. In photography, "one f-stop" means changing the f-number by a factor of √2, which changes the light intensity by a factor of 2.

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. This is also called Aperture Priority Auto Exposure, A mode, AV mode, or semi-auto mode.

Typical aperture ranges in photography are about f/2.8 to f/22 or f/2 to f/16, which covers six stops. These can be split into wide, middle, and narrow groups of two stops each.

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. Here, f/0.95 is the widest opening, and f/22 is the smallest. The maximum aperture is often the most interesting because it affects how long the exposure takes. The aperture area is linked to the light allowed through the lens, and the aperture diameter relates to the square root of the light allowed.

Lenses with apertures of f/2.8 or wider are called "fast" lenses. The fastest lenses for 35 mm film can go down to f/1.2 or f/1.4, with many at f/1.8 and f/2.0. Some special lenses can be even wider, like f/0.95. For movies, some lenses can go as low as f/0.75. Zoom lenses usually have a maximum aperture between f/2.8 and f/6.3, but high-end ones keep the same aperture throughout the zoom.

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 due to diffraction, so there’s usually little benefit in using them. DSLR lenses often have a minimum aperture of f/16, f/22, or f/32, while large format lenses can go down to f/64. In macro photography, smaller apertures are sometimes needed, like f/96 for the Canon MP-E 65mm. The pinhole optic for Lensbaby creative lenses has an aperture of just f/177.

• f/32 – small aperture and slow shutter

• f/5.6 – large aperture and fast shutter

• f/22 – small aperture and slower shutter (Exposure time: 1/80)

• f/3.5 – large aperture and faster shutter (Exposure time: 1/2500)

• Changing a camera's aperture value in half-stops, beginning with f/256 and ending with f/1

• Changing a camera's aperture diameter from zero to infinity

Aperture area

The amount of light captured by an optical system depends on the area of the entrance pupil, which is the image of the aperture on the object side. The formula is:

A r e a = π ( D 2 ) 2 = π ( f 2 N ) 2 {\displaystyle \mathrm {Area} =\pi \left({D \over 2}\right)^{2}=\pi \left({f \over 2N}\right)^{2}} !{\displaystyle \mathrm {Area} =\pi \left({D \over 2}\right)^{2}=\pi \left({f \over 2N}\right)^{2}}

Where the two forms are connected through the f-number N = f / D, with focal length f and entrance pupil diameter D.

When comparing lenses of the same focal length, the focal length value isn’t needed. The area is proportional to the reciprocal square of the f-number N.

If two cameras have the same angle of view and the same aperture area, they gather the same amount of light. But the relative focal-plane illuminance depends only on the f-number N, so it’s less in the camera with the larger format, longer focal length, and higher f-number. This assumes both lenses have the same transmissivity.

Aperture control

Although an automatic aperture control was invented as early as 1933 by Torkel Korling for the Graflex large format reflex camera, not all early 35mm single lens reflex cameras had this feature. Korling’s design allowed full-aperture viewing for accurate focus, closing to the chosen aperture when the shutter fired and syncing with a flash unit. From 1956, SLR camera makers developed automatic aperture control, letting viewers see at the lens’s maximum aperture and stopping down at exposure time.

For some lenses, like long telephotos, lenses on bellows, and perspective-control and tilt/shift lenses, mechanical links were impractical, so automatic aperture control wasn’t provided. Many of these lenses had a “preset” aperture feature, letting the lens be set to working aperture and quickly switched between working and full aperture.

Canon EF lenses, introduced in 1987, use electromagnetic diaphragms, removing the need for a mechanical link between camera and lens. This allows automatic aperture control even with Canon TS-E tilt/shift lenses. Nikon PC-E perspective-control lenses, introduced in 2008, also use electromagnetic diaphragms.

Optimal aperture

The best aperture depends on both optics (depth of scene vs. diffraction) and lens performance.

When a lens is stopped down, the blur from defocus at the Depth of Field limits decreases, but diffraction blur increases. These two effects oppose each other, leading to a point where the combined blur spot is smallest. At this point, the f-number is best for image sharpness for that depth of field — a wider aperture causes more defocus, while a narrower aperture causes more diffraction.

Lenses often don’t perform best when fully open, so they’re usually sharper when stopped down a bit. But beyond a certain point, stopping down further doesn’t help much, and diffraction at the aperture edges starts to affect image quality. There’s usually a “sweet spot” around f/4 to f/8, but this can vary by lens. Some lenses are designed to work best when wide open. How much sharpness matters depends on how the image will be used. If the final image is viewed normally, you might not need extreme sharpness, but if it’s going to be very large, you might want the sharpest possible image.

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, which 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, and 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 it 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, interest in what you’re looking at, and other body signals. Some people can even control their pupil size on purpose, but this is very rare.

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 capture images, like drum scanners, image sensors, or televisions, the word "aperture" can describe the small opening or time period used to take in the image. This opening helps decide how detailed the picture will be.

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

In popular culture

Aperture Science is a made-up company in the game series Portal. The company’s name comes from the idea of an aperture in optics. The logo of Aperture Science shows an opening that lets light through, and it represents both the company and the Aperture Science Computer-Aided Enrichment Center where the games happen.