Aperture

Aperture

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In optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

, an aperture is a hole or an opening through which light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays
Ray (optics)
In optics, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing. This allows even very...

 that come to a focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

 in the image plane
Image plane
In 3D computer graphics, the image plane is that plane in the world which is identified with the plane of the monitor. If one makes the analogy of taking a photograph to rendering a 3D image, the surface of the film is the image plane. In this case, the viewing transformation is a projection that...

. The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If an aperture is narrow, then highly collimated rays are admitted, resulting in a sharp focus at the image plane. If an aperture is wide, then uncollimated rays are admitted, resulting in a sharp focus only for rays with a certain focal length. This means that a wide aperture results in an image that is sharp around what the lens is focusing on and blurred otherwise. The aperture also determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane (the narrower the aperture, the darker the image for a given exposure time).

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
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 or mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm
Diaphragm (optics)
In optics, a diaphragm is a thin opaque structure with an opening at its center. The role of the diaphragm is to stop the passage of light, except for the light passing through the aperture...

 placed in the optical path 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.

In some contexts, especially in photography
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...

 and astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. For example, in a telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). One then speaks of a telescope as having, for example, a 100 centimeter aperture. Note that the aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system.

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

The word aperture is also used in other contexts to indicate a system which blocks off light outside a certain region. In astronomy for example, a photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...

 aperture around a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 usually corresponds to a circular window around the image of a star within which the light intensity is summed.

In photography


The aperture stop of a photographic lens
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...

 can be adjusted to control the amount of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 reaching the film
Photographic film
Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...

 or image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...

. In combination with variation of shutter speed
Shutter speed
In photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....

, the aperture size will regulate the film's or image sensor's degree of exposure
Exposure (photography)
In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an 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
Diaphragm (optics)
In optics, a diaphragm is a thin opaque structure with an opening at its center. The role of the diaphragm is to stop the passage of light, except for the light passing through the aperture...

 usually serves as the aperture stop, and controls the aperture. The diaphragm functions much like the iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 – it controls the effective diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 of the lens opening. Reducing the aperture size increases the depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...

, 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
F-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...

, the ratio of focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 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. The photography term "one f-stop" refers to a factor of √2 (approx. 1.41) change in f-number, which in turn corresponds to a factor of 2 change in light intensity.

Aperture priority
Aperture priority
Aperture priority, often abbreviated A or Av on a camera mode dial, is a setting on some cameras that allows the user to choose a specific aperture value while the camera selects a shutter speed to match. The camera will ensure proper exposure...

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

Typical ranges of apertures used in photography are about 2.8–22 or 2–16, covering 6 stops, which may be divided into wide, middle, and narrow of 2 stops each, roughly (using round numbers) 2–4, 4–8, and 8–16 or (for a slower lens) 2.8–5.6, 5.6–11, and 11–22. These are not sharp divisions, and ranges for specific lenses vary.

Aperture area


The amount of light captured by a lens is proportional to the area of the aperture, equal to:


Where f is focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 and N is the f-number
F-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...

.

The focal length value is not required when comparing two lenses of the same focal length; a value of 1 can be used instead, and the other factors can be dropped as well, leaving area proportion to the reciprocal square of the f-number N.

If two cameras of different format sizes and focal lengths have the same angle of view
Angle of view
In photography, angle of view describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term field of view....

, and the same aperture area, they gather the same amount of light from the scene. The relative focal-plane illuminance
Illuminance
In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per...

, however, depends only on the f-number N, independent of the focal length, so is less in the camera with the larger format, longer focal length, and higher f-number. This assumes both lenses have identical transmissivity.

Aperture control


Most SLR cameras provide automatic aperture control, which allows viewing and metering at the lens’s maximum aperture, stops the lens down to the working aperture during exposure, and returns the lens to maximum aperture after exposure.

The first SLR cameras with internal (“through-the-lens” or “TTL”
Through-the-lens metering
Through-the-lens metering is a photographic term describing a feature of cameras capable of measuring light levels in a scene through their taking lenses, as opposed to a separate metering window...

)
meters (e.g., the Pentax Spotmatic
Pentax Spotmatic
The Pentax Spotmatic is a range of 35mm single-lens reflex cameras manufactured by the Asahi Optical Co. Ltd., later known as Pentax Corporation, between 1964 and 1976. The original 1964 Spotmatic was one of the first SLRs to offer a through-the-lens exposure metering system, initially using...

) required that the lens be stopped down to the working aperture when taking a meter reading. With a small aperture, this darkened the
viewfinder, making viewing and composition difficult.
Subsequent models
soon incorporated mechanical coupling between the lens and the camera body,
indicating the working aperture to the camera while allowing the lens to be at
its maximum aperture for composition and focusing;
this feature became known as automatic aperture control or automatic diaphragm control.

For some lenses, including a few long telephotos
Telephoto lens
In photography and cinematography, a telephoto lens is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a telephoto group that extends the light path to create a long-focus...

, lenses mounted on
bellows
Bellows (photography)
In photography, a bellows is the pleated expandable part of a camera, usually a large or medium format camera, to allow the lens to be moved with respect to the focal plane for focusing....

, and perspective-control and tilt/shift lenses, the mechanical
linkage was impractical,
and automatic aperture control was not
provided. Many such lenses incorporated a feature known as a “preset”
aperture,
which allows the lens to be set to working aperture and then
quickly switched between working aperture and full aperture without looking
at the aperture control. Typical operation might be to establish rough
composition, set the working aperture for metering, return to full aperture
for a final check of focus and composition, and focusing, and finally,
return to working aperture just before exposure. Although slightly easier
than stopped-down metering, operation is less convenient
than automatic operation. Preset aperture controls have taken several
forms; the most common has been the use of essentially two lens aperture
rings, with one ring setting the aperture and the other serving as a limit
stop when switching to working aperture. Examples of lenses with this type
of preset aperture control are the Nikon PC Nikkor 28 mm 3.5
and the SMC Pentax Shift 6×7 75 mm
4.5. The Nikon PC Micro-Nikkor 85 mm 2.8D lens
incorporates a mechanical pushbutton that sets working aperture when pressed
and restores full aperture when pressed a second time.

Canon EF lenses, introduced in 1987,
have electromagnetic diaphragms,
eliminating the need for a mechanical linkage between the camera and the lens, and allowing automatic aperture control with the Canon TS-E tilt/shift lenses.
Nikon PC-E perspective-control lenses,
introduced in 2008, also have
electromagnetic diaphragms.
Automatic aperture control is provided with
the newer Nikon digital SLR cameras; with some earlier cameras, the
lenses offer preset aperture control by means of a pushbutton that
controls the electromagnetic diaphragm.

In scanning or sampling


The terms scanning aperture and sampling aperture are often used to refer to the opening through which an image is sampled, or scanned, for example in a Drum scanner, an image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...

, or a television pickup apparatus. The sampling aperture can be a literal optical aperture, that is, a small opening in space, or it can be a time-domain aperture for sampling
Sampling (signal processing)
In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave to a sequence of samples ....

 a signal waveform.

For example, film grain
Film grain
Film grain or granularity is the random optical texture of processed photographic film due to the presence of small particles of a metallic silver, or dye clouds, developed from silver halide that have received enough photons. While film grain is a function of such particles it is not the same...

 is quantified as graininess via a measurement of film density fluctuations as seen through a 0.048 mm sampling aperture.

See also

  • Numerical aperture
    Numerical aperture
    In optics, the numerical aperture of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one...

  • Antenna aperture
    Antenna aperture
    In electromagnetics and antenna theory, antenna aperture or effective area is a measure of how effective an antenna is at receiving the power of radio waves. The aperture is defined as the area, oriented perpendicular to the direction of an incoming radio wave, which would intercept the same...

  • Angular resolution
    Angular resolution
    Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...

  • Diaphragm (optics)
    Diaphragm (optics)
    In optics, a diaphragm is a thin opaque structure with an opening at its center. The role of the diaphragm is to stop the passage of light, except for the light passing through the aperture...

  • Bokeh
    Bokeh
    In photography, bokeh is the blur, or the aesthetic quality of the blur, in out-of-focus areas of an image, or "the way the lens renders out-of-focus points of light."...

  • Shallow focus
    Shallow focus
    Shallow focus is a photographic and cinematographic technique incorporating a small depth of field. In shallow focus one plane of the image is in focus while the rest is out of focus. Shallow focus is typically used to emphasize one part of the image over another...

  • Deep focus
    Deep focus
    Deep focus is a photographic and cinematographic technique using a large depth of field. Depth of field is the front-to-back range of focus in an image — that is, how much of it appears sharp and clear. Consequently, in deep focus the foreground, middle-ground and background are all in focus...

  • Entrance pupil
    Entrance pupil
    In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil...

  • Exit pupil
    Exit pupil
    In optics, the exit pupil is a virtual aperture in an optical system. Only rays which pass through this virtual aperture can exit the system. The exit pupil is the image of the aperture stop in the optics that follow it. In a telescope or compound microscope, this image is the image of the...

  • Lyot stop
    Lyot stop
    A Lyot stop is an optic stop, invented by French astronomer Bernard Lyot, that reduces the amount of flare caused by diffraction of other stops and baffles in optical systems.-External links:** by Bass et al....