Glossary: what is an aperture?

In optics, an aperture is a hole or an opening through which light is admitted. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane.

An optical system typically has many openings, or structures that limit the ray bundles (ray bundles are also known as pencils of light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path deliberately to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop is the stop that determines the ray cone angle, or equivalently the brightness, at an image point.

Sometimes stops and diaphragms are called apertures, even when they are not the aperture stop of the system.

The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with variation of shutter speed, the aperture size will regulate the film's degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.

A device called a diaphragm usually serves as the aperture stop, and controls the aperture. The diaphragm functions much like the iris of the eye—it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (the larger the number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.

The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor.

Aperture priority refers to a shooting mode used in semi-automatic cameras. It allows the photographer to choose an aperture setting and allow the camera to decide the correct shutter speed. This is sometimes referred to as Aperture Priority Auto Exposure, A mode, Av mode, or semi-auto mode.


Maximum and minimum apertures

The specifications for a given lens typically include the minimum and maximum apertures. These refer to the maximum and minimum f-numbers the lens can be set at to achieve, respectively. For example, the Canon EF 70-200mm lens has a maximum aperture of f/2.8 and a minimum aperture of f/32.

The maximum aperture tends to be of most interest; it is known as the lens speed and is always included when describing a lens (e.g., 100-400mm f/5.6, or 70-200mm f/2.8).

A typical lens will have an f-number range from f/16 (small aperture) to f/2 (large aperture) (these values vary). Professional lenses for 35mm cameras can have f-numbers as low as f/1.0, while professional lenses for some movie cameras can have f-numbers as low as f/0.75 (very large relative aperture). These are known as "fast" lenses because they allow much more light to reach the film and therefore reduce the required exposure time. Stanley Kubrick's film Barry Lyndon is notable for having scenes shot with the largest relative aperture in film history: f/0.7.

Large aperture prime lenses (lenses which have a fixed focal length) are favored especially by photojournalists who often work in dim light, have no opportunity to introduce supplementary lighting, and need to capture fast breaking events.

Zoom lenses typically have a maximum aperture (minimum f-number) of f/2.8 to f/6.3 through their range. A very fast zoom lens will be constant f/2.8 or f/2, which means the relative aperture will stay the same throughout the zoom range. A more typical consumer zoom will have a variable relative aperture, since it is harder to keep the effective aperture proportional to focal length at long focal lengths; f/3.5 to f/6.3 would be typical.