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In
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
, an aperture is a hole or an opening through which
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
travels. More specifically, the aperture and
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative fo ...
of an
optical system Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
determine the cone angle of a bundle of rays that come to a
focus Focus, or its plural form foci may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film *''Focus'', a 1962 TV film starring James Whitmore * ''Focus'' (2001 film), a 2001 film based ...
in the
image plane In 3D computer graphics, the image plane is that plane in the world which is identified with the plane of the display monitor used to view the image that is being rendered. It is also referred to as screen space. If one makes the analogy of taki ...
. 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 A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
or
mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
, 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 to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop is the stop that primarily determines the ray cone angle and brightness at the image point. In some contexts, especially in
photography Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed i ...
and
astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, gala ...
, ''aperture'' refers to the diameter of the aperture stop rather than the physical stop or the opening itself. For example, in a
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
, the aperture stop is typically the edges of the
objective lens In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be a single lens or mirror, or combinations of several optical elem ...
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. In
astrophotography Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1840, but it was no ...
, the aperture may be given as a linear measure (for example in inches or mm) or as the dimensionless ratio between that measure and the
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative fo ...
. In other photography, it is usually given as a ratio. 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 aperture around a
star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth ma ...
usually corresponds to a circular window around the image of a star within which the light intensity is assumed. The word "aperture" is also used as a small hole, similar to a peek-hole. For example, in military terms, a bunker's aperture means a small peeking hole made artificially or by natural means. A bunker's aperture can be used for preserving the body from enemy fire while achieving a clear line of sight. (Infantry Combat/The Rifle Platoon/John F. Antal p.91)


Application

The aperture stop is an important element in most optical designs. Its most obvious feature is that it limits the amount of light that can reach the image/ film plane. This can be either unavoidable, as in a telescope where one wants to collect as much light as possible; or deliberate, to prevent saturation of a detector or overexposure of film. In both cases, the size of the aperture stop is constrained by things other than the amount of light admitted; however: *The size of the stop is one factor that affects
depth of field The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera. Factors affecting depth of field For cameras that can only focus on one object dis ...
. Smaller stops (larger
f number In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...
s) produce a longer
depth of field The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera. Factors affecting depth of field For cameras that can only focus on one object dis ...
, allowing objects at a wide range of distances from the viewer to all be in focus at the same time. *The stop limits the effect of optical aberrations. If the stop is too large, the image will be distorted. More sophisticated optical system designs can mitigate the effect of aberrations, allowing a larger stop and therefore greater light collecting ability. *The stop determines whether the image will be vignetted. Larger stops can cause the intensity reaching the film or detector to fall off toward the edges of the picture, especially when, for off-axis points, a different stop becomes the aperture stop by virtue of cutting off more light than did the stop that was the aperture stop on the optic axis. *A larger aperture stop requires larger diameter optics, which are heavier and more expensive. In addition to an aperture stop, a photographic lens may have one or more ''field stops'', which limit the system's
field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Humans ...
. When the field of view is limited by a field stop in the lens (rather than at the film or sensor)
vignetting In photography and optics, vignetting is a reduction of an image's brightness or saturation toward the periphery compared to the image center. The word '' vignette'', from the same root as ''vine'', originally referred to a decorative borde ...
results; this is only a problem if the resulting field of view is less than was desired. The biological pupil of the
eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and con ...
is its aperture in optics nomenclature; the iris is the diaphragm that serves as the aperture stop. Refraction in the
cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical po ...
causes the effective aperture (the
entrance pupil In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front (the object side) of the lens system. The corresponding image of the aperture as seen through the back of the lens system ...
in optics parlance) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4 mm in diameter, although it can range from 2 mm () in a brightly lit place to 8 mm () in the dark. In astronomy, the diameter of the aperture stop (called the ''aperture'') is a critical parameter in the design of a
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
. Generally, one would want the ''aperture'' to be as large as possible, to collect the maximum amount of light from the distant objects being imaged. The size of the aperture is limited, however, in practice by considerations of cost and weight, as well as prevention of aberrations (as mentioned above). Apertures are also used in laser energy control, close aperture z-scan technique, diffractions/patterns, and beam cleaning. Laser applications include spatial filters, Q-switching, high intensity x-ray control. In light microscopy, the word aperture may be used with reference to either the condenser (changes angle of light onto specimen field), field iris (changes area of illumination) or possibly objective lens (forms primary image). ''See''
Optical microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsc ...
.


In photography

The aperture stop of a
photographic lens A camera lens (also known as photographic lens or photographic objective) 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 capa ...
can be adjusted to control the amount of
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
reaching the
film A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...
or
image sensor An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of cu ...
. In combination with variation of
shutter speed In photography, shutter speed or exposure time is the length of time that the film or digital sensor inside the camera is exposed to light (that is, when the camera's shutter (photography), shutter is open) when taking a photograph. The am ...
, the aperture size will regulate the film's or image sensor's degree of exposure to light. Typically, a fast shutter will require a larger aperture to ensure sufficient light exposure, and a slow shutter 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 Iris most often refers to: *Iris (anatomy), part of the eye *Iris (mythology), a Greek goddess * ''Iris'' (plant), a genus of flowering plants * Iris (color), an ambiguous color term Iris or IRIS may also refer to: Arts and media Fictional ent ...
of the
eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and con ...
 – it controls the effective
diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid ...
of the lens opening. Reducing the aperture size (increasing the f-number) provides less light to sensor and also increases the
depth of field The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera. Factors affecting depth of field For cameras that can only focus on one object dis ...
, 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 f-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 In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...
, the ratio of
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative fo ...
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 (approx. 1.41) change in f-number, which in turn corresponds to a factor of 2 change in light intensity.
Aperture priority Aperture priority, often abbreviated ''A'' or ''Av'' (for aperture value) on a camera mode dial, is a mode on some cameras that allows the user to set a specific aperture value (f-number) while the camera selects a shutter speed to match it that ...
is a semi-automatic shooting mode used in cameras. It permits the photographer to select an aperture setting and let the camera decide the shutter speed and sometimes also ISO sensitivity for the correct exposure. This is also referred to as Aperture Priority Auto Exposure, A mode, AV mode (aperture-value mode), or semi-auto mode. Typical ranges of apertures used in photography are about – or –, covering six stops, which may be divided into wide, middle, and narrow of two stops each, roughly (using round numbers) –, –, and – or (for a slower lens) –, –, and –. These are not sharp divisions, and ranges for specific lenses vary.


Maximum and minimum apertures

The specifications for a given lens typically include the maximum and minimum aperture sizes, for example, –. In this case, is currently the maximum aperture (the widest opening on a full-frame format for practical use), and is the minimum aperture (the smallest opening). The maximum aperture opening tends to be of most interest and is always included when describing a lens. This value is also known as the lens "speed", as it affects the exposure time. The aperture is proportional to the square root of the light admitted, and thus inversely proportional to the square root of required exposure time, such that an aperture of allows for exposure times one quarter that of . Lenses with apertures opening or wider are referred to as "fast" lenses, although the specific point has changed over time (for example, in the early 20th century aperture openings wider than were considered fast. The fastest lenses for the common 35 mm film format in general production have apertures of or , with more at and , and many at or slower; is unusual, though sees some use. When comparing "fast" lenses, the
image format An Image file format is a file format for a digital image. There are many formats that can be used, such as JPEG, PNG, and GIF. Most formats up until 2022 were for storing 2D images, not 3D ones. The data stored in an image file format may be ...
used must be considered. Lenses designed for a small format such as half frame or
APS-C Advanced Photo System type-C (APS-C) is an image sensor format approximately equivalent in size to the Advanced Photo System film negative in its C ("Classic") format, of 25.1×16.7 mm, an aspect ratio of 3:2 and Ø 31.15 mm field ...
need to project a much smaller
image circle The image circle is the cross section of the cone of light transmitted by a lens or series of lenses onto the image plane. When this light strikes a perpendicular target such as photographic film or a digital camera sensor, it forms a circle of ...
than a lens used for
large format Large format refers to any imaging format of or larger. Large format is larger than "medium format", the or size of Hasselblad, Mamiya, Rollei, Kowa, and Pentax cameras (using 120- and 220-roll film), and much larger than the frame o ...
photography. Thus the optical elements built into the lens can be far smaller and cheaper. In exceptional circumstances lenses can have even wider apertures with f-numbers smaller than 1.0; see lens speed: fast lenses for a detailed list. For instance, both the current Leica Noctilux-M 50mm ASPH and a 1960s-era Canon 50mm rangefinder lens have a maximum aperture of . Cheaper alternatives have appeared in recent years, such as the Cosina Voigtländer 17.5mm , 25mm and 42.5mm manual focus lenses for the Micro Four-Thirds System. For a long time, the f/0.95 fast f-number for full-frame stopped around 50mm or longer focal length. Until 2021, the lens manufacturer Venus Optics ( Laowa) announced the Argus 35mm f/0.95 FF. This is currently the fastest lens with a focal length of 35mm and the widest lens for f/0.95. Professional lenses for some movie cameras have f-numbers as small as .
Stanley Kubrick Stanley Kubrick (; July 26, 1928 – March 7, 1999) was an American film director, producer, screenwriter, and photographer. Widely considered one of the greatest filmmakers of all time, his films, almost all of which are adaptations of nove ...
's film ''
Barry Lyndon ''Barry Lyndon'' is a 1975 period drama film written, directed, and produced by Stanley Kubrick, based on the 1844 novel '' The Luck of Barry Lyndon'' by William Makepeace Thackeray. Starring Ryan O'Neal, Marisa Berenson, Patrick Magee, L ...
'' has scenes shot by candlelight with a NASA/Zeiss 50mm f/0.7, the fastest lens in film history. Beyond the expense, these lenses have limited application due to the correspondingly shallower depth of field – the scene must either be shallow, shot from a distance, or will be significantly defocused, though this may be the desired effect. Zoom lenses typically have a maximum relative aperture (minimum f-number) of to through their range. High-end lenses will have a constant aperture, such as or , which means that the relative aperture will stay the same throughout the zoom range. A more typical consumer zoom will have a variable maximum relative aperture since it is harder and more expensive to keep the maximum relative aperture proportional to the focal length at long focal lengths; to is an example of a common variable aperture range in a consumer zoom lens. By contrast, the minimum aperture does not depend on the focal length – it is limited by how narrowly the aperture closes, not the lens design – and is instead generally chosen based on practicality: very small apertures have lower sharpness due to diffraction, while the added depth of field is not generally useful, and thus there is generally little benefit in using such apertures. Accordingly, DSLR lens typically have minimum aperture of , , or , while
large format Large format refers to any imaging format of or larger. Large format is larger than "medium format", the or size of Hasselblad, Mamiya, Rollei, Kowa, and Pentax cameras (using 120- and 220-roll film), and much larger than the frame o ...
may go down to , as reflected in the name of
Group f/64 Group 64 or f.64 was a group founded by seven 20th-century San Francisco Bay Area photographers who shared a common photographic style characterized by sharply focused and carefully framed images seen through a particularly Western (U.S.) viewp ...
. Depth of field is a significant concern in
macro photography Macro photography (or photomacrography or macrography, and sometimes macrophotography) is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is grea ...
, however, and there one sees smaller apertures. For example, the Canon MP-E 65mm can have effective aperture (due to magnification) as small as . The
pinhole A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
optic for Lensbaby creative lenses has an aperture of just . Image:Jonquil flowers at f32.jpg, – small aperture and slow shutter Image:Jonquil flowers at f5.jpg, – large aperture and fast shutter Image:Aperture Example Wall.jpg, – small aperture and slower shutter (Exposure time: 1/80) Image:Aperture Example Wall 2.jpg, – large aperture and faster shutter (Exposure time: 1/2500) Image:Povray focal blur animation.gif, Changing a camera's aperture value in half-stops, beginning with and ending with Image:Povray focal blur animation mode tan.gif, Changing a camera's aperture diameter from zero to infinity


Aperture area

The amount of light captured by a lens is proportional to the area of the aperture, equal to: :\mathrm = \pi \left(\right)^2 = \pi \left(\right)^2 Where the two equivalent forms are related via the
f-number In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...
''N = f / D'', with
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative fo ...
''f'' and aperture diameter ''D''. 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 The angle of view is the decisive variable for the visual perception of the size or projection of the size of an object. Angle of view and perception of size The perceived size of an object depends on the size of the image projected onto the ...
, and the same aperture area, they gather the same amount of light from the scene. In that case, the relative focal-plane
illuminance In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness ...
, however, would depend only on the f-number ''N'', so it 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

Though as early as 1933 Torkel Korling had invented and patented for the
Graflex Graflex was a manufacturer that gave its brand name to several models of camera. The company was founded as the ''Folmer and Schwing Manufacturing Company'' in New York City in 1887 by William F. Folmer and William E. Schwing as a metal working ...
large format reflex camera an automatic aperture control, not all early 35mm single lens reflex cameras had the feature. With a small aperture, this darkened the viewfinder, making viewing, focusing, and composition difficult. Korling's design enabled full-aperture viewing for accurate focus, closing to the pre-selected aperture opening when the shutter was fired and simultaneously synchronising the firing of a flash unit. From 1956
SLR camera A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. With twin l ...
manufacturers separately developed ''automatic aperture control'' (the Miranda T 'Pressure Automatic Diaphragm', and other solutions on the Exakta Varex IIa and Praktica FX2) allowing viewing at the lens's maximum aperture, stopping the lens down to the working aperture at the moment of exposure, and returning the lens to maximum aperture afterward.Sidney F. Ray. The geometry of image formation. In ''The Manual of Photography: Photographic and Digital Imaging'', 9th ed, pp. 136–137. Ed. Ralph E. Jacobson, Sidney F. Ray, Geoffrey G. Atteridge, and Norman R. Axford. Oxford: Focal Press, 2000. The first SLR cameras with internal ( "through-the-lens" or "TTL") meters (e.g., the
Pentax Spotmatic The Pentax Spotmatic refers to a family of 35mm single-lens reflex cameras manufactured by the Asahi Optical Co. Ltd., later known as Pentax Corporation, between 1964 and 1976. All Pentax Spotmatics used the M42 screw-thread lens mount which wa ...
) required that the lens be stopped down to the working aperture when taking a meter reading. Subsequent models soon incorporated mechanical coupling between the lens and the camera body, indicating the working aperture to the camera for exposure while allowing the lens to be at its maximum aperture for composition and focusing; this feature became known as open-aperture metering. For some lenses, including a few long telephotos, lenses mounted on
bellows A bellows or pair of bellows is a device constructed to furnish a strong blast of air. The simplest type consists of a flexible bag comprising a pair of rigid boards with handles joined by flexible leather sides enclosing an approximately airtigh ...
, 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. A 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 and the SMC Pentax Shift 6×7 75 mm . The Nikon PC Micro-Nikkor 85 mm 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, a feature extended to their E-type range in 2013.


Optimal aperture

Optimal aperture depends both on optics (the depth of the scene versus diffraction), and on the performance of the lens. Optically, as a lens is stopped down, the defocus blur at the Depth of Field (DOF) limits decreases but diffraction blur increases. The presence of these two opposing factors implies a point at which the combined blur spot is minimized ( Gibson 1975, 64); at that point, the f-number is optimal for image sharpness, for this given depth of field – a wider aperture (lower ''f''-number) causes more defocus, while a narrower aperture (higher ''f''-number) causes more diffraction. As a matter of performance, lenses often do not perform optimally when fully opened, and thus generally have better sharpness when stopped down some – note that this is sharpness in the plane of
critical focus In a photograph, the area of critical focus is the portion of the picture that is optically in focus. This does not relate to depth of field which describes apparent sharpness. Reducing the size of the aperture will increase the depth of field b ...
, setting aside issues of depth of field. Beyond a certain point, there is no further sharpness benefit to stopping down, and the diffraction begins to become significant. There is accordingly a sweet spot, generally in the – range, depending on lens, where sharpness is optimal, though some lenses are designed to perform optimally when wide open. How significant this varies between lenses, and opinions differ on how much practical impact this has. While optimal aperture can be determined mechanically, how much sharpness is ''required'' depends on how the image will be used – if the final image is viewed under normal conditions (e.g., an 8″×10″ image viewed at 10″), it may suffice to determine the f-number using criteria for minimum required sharpness, and there may be no practical benefit from further reducing the size of the blur spot. But this may not be true if the final image is viewed under more demanding conditions, e.g., a very large final image viewed at normal distance, or a portion of an image enlarged to normal size ( Hansma 1996). Hansma also suggests that the final-image size may not be known when a photograph is taken, and obtaining the maximum practicable sharpness allows the decision to make a large final image to be made at a later time; see also critical sharpness.


Equivalent aperture range

In digital photography, the 35mm-equivalent aperture range is sometimes considered to be more important than the actual f-number. Equivalent aperture is the f-number adjusted to correspond to the f-number of the same size absolute aperture diameter on a lens with a 35mm equivalent focal length. Smaller equivalent f-numbers are expected to lead to higher image quality based on more total light from the subject, as well as lead to reduced depth of field. For example, a Sony Cyber-shot DSC-RX10 uses a 1" sensor, 24–200 mm with maximum aperture constant along the zoom range; has equivalent aperture range , which is a lower equivalent f-number than some other cameras with smaller sensors.


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 scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting or an object and converts it to a digital image. Commonly used in offices are variations of the desktop ''flatbed scanner'' w ...
, an
image sensor An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of cu ...
, 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 a signal waveform. For example,
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 i ...
is quantified as ''graininess'' via a measurement of film density fluctuations as seen through a 0.048 mm sampling aperture.


See also

*
Numerical aperture In optics, the numerical aperture (NA) 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 propert ...
*
Antenna aperture In electromagnetics and antenna theory, the aperture of an antenna is defined as "A surface, near or on an antenna, on which it is convenient to make assumptions regarding the field values for the purpose of computing fields at external points. T ...
*
Angular resolution Angular 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, thereby making it a major determinant of image resolution ...
*
Diaphragm (optics) In optics, a diaphragm is a thin opaque structure with an opening (aperture) at its center. The role of the diaphragm is to ''stop'' the passage of light, except for the light passing through the ''aperture''. Thus it is also called a stop (an ...
*
Waterhouse stop The Waterhouse stop or Waterhouse diaphragm is an interchangeable diaphragm with an aperture (hole) for controlling the entry of light into a camera. A thin piece of metal (the diaphragm) is drilled with a hole (the aperture); a set of these wi ...
*
Bokeh In photography, bokeh ( or ; ) is the aesthetic quality of the blur produced in out-of-focus parts of an image. Bokeh has also been defined as "the way the lens renders out-of-focus points of light". Differences in lens aberrations and ...
*
Shallow focus Shallow focus is a photographic and cinematographic technique incorporating a small depth of field. In shallow focus, one plane of the scene is in focus while the rest is out of focus. Shallow focus is typically used to emphasize one part of th ...
*
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, or how much of it appears sharp and clear. In deep focus, the foreground, middle ground, and ...
*
Entrance pupil In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front (the object side) of the lens system. The corresponding image of the aperture as seen through the back of the lens system ...
*
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 comp ...
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Lyot stop A Lyot stop (also called a glare stop) is an optical stop, invented by French astronomer Bernard Lyot, that reduces the amount of flare caused by diffraction of other stops and baffles in optical systems. Lyot stops are located at images of the sy ...


References

* Gibson, H. Lou. 1975. ''Close-Up Photography and Photomacrography''. 2nd combined ed. Kodak Publication No. N-16. Rochester, NY: Eastman Kodak Company, Vol II: Photomacrography. * Hansma, Paul K. 1996. View Camera Focusing in Practice. ''Photo Techniques'', March/April 1996, 54–57. Available as GIF images on th
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{{Authority control Science of photography Geometrical optics Physical optics Observational astronomy