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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 optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

optics
, an aperture is a hole or an opening through which
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
travels. More specifically, the aperture and
focal length The focal length of an optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

focal length
of an
optical system Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entitie ...

optical system
determine the cone angle of a bundle of rays that come to a
focus Focus, FOCUS, or foci may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe The Adelaide Fringe, formerly Adelaide Fringe Festival, is the world's second-largest annual arts festival (after the Edinburgh Festival ...
in the
image plane In 3D computer graphics 3D computer graphics, sometimes called CGI, 3DCG or three-dimensional computer graphics Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in d ...
. 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 Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, s ...

lens
or
mirror A mirror is an object that reflects an image An image (from la, imago) is an artifact that depicts visual perception Visual perception is the ability to interpret the surrounding environment Environment most often refers t ...

mirror
, or a ring or other fixture that holds an optical element in place, or may be a special element such as a
diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...
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
ray cone angle and brightness
at the image point. In some contexts, especially in
photography Photography is the art Art is a diverse range of (products of) human activities Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, and int ...

photography
and
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
, ''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 an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common ...

telescope
, 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 Focus (optics), focuses the ray (optics), light rays to produce a real image. Objectives can be a single Lens (optics), lens or mirr ...
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 photography Photography is the art Art is a diverse range of (products of) human activities Humans (''Homo sapiens'') are the most populous and widespread species of primat ...

astrophotography
, 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 Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

focal length
. 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
photometricPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electroma ...
aperture around a
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

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


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 A film plane is the surface of an image recording device such as a camera, upon which the lens A lens is a transmissive optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary pa ...
. 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 For many cameras, depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. The depth of field can be calculated based on focal length The focal length of an optical ...

depth of field
. Smaller stops (larger
f number In optics Optics is the branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of natur ...
s) produce a longer
depth of field For many cameras, depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. The depth of field can be calculated based on focal length The focal length of an optical ...

depth of field
, 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 Seen may refer to: * Seen (album), ''Seen'' (album), by Tom Bailey * Seen (artist), graffiti artist * Seen (Winterthur), a district of the city of Winterthur, Switze ...

field of view
. When the field of view is limited by a field stop in the lens (rather than at the film or sensor)
vignetting In photography Photography is the art Art is a diverse range of (products of) human activities Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, ...
results; this is only a problem if the resulting field of view is less than was desired. The
biological pupil
biological pupil
of the
eye Eyes are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is t ...

eye
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 Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * ...

cornea
causes the effective aperture (the
entrance pupil In an optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and t ...
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 an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common ...

telescope
. 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__NOTOC__ Condenser may refer to: Heat transfer * Condenser (heat transfer), a device or unit used to condense vapor into liquid. Specific types include: ** Heat exchanger#HVAC air coils, HVAC air coils ** Condenser (laboratory), a range of laborat ...
(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 A microscope (from grc, μικρός ''mikrós'' 'small' and ''skopeîn'' 'to look (at); examine, inspect') is a used to examine objects that are ...
.


In photography

The aperture stop of a
photographic lens A camera lens (also known as photographic lens or photographic objective) is an optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion ...
can be adjusted to control the amount of
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
reaching the
film A film, also called a movie, motion picture or moving picture, is a work of visual art The visual arts are art forms such as painting Painting is the practice of applying paint Paint is any pigmented liquid, liquefiable, ...
or
image sensor An image sensor or imager is a sensor A sensor is a device that produces an output signal for the purpose of sensing of a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects e ...
. In combination with variation of
shutter speed In photography Photography is the art Art is a diverse range of (products of) human activities Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs ...

shutter speed
, 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 Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...
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 (color), an ambiguous color term, usually referring to shades ranging from blue-violet to violet *Iris (mantis), ''Iris'' (mantis), a genus of insects *Iris (mythology), a Greek god ...
of the
eye Eyes are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is t ...

eye
 – it controls the effective
diameter In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position ...

diameter
of the lens opening. Reducing the aperture size increases the
depth of field For many cameras, depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. The depth of field can be calculated based on focal length The focal length of an optical ...

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 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 Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

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. 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 setting 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 the maximum aperture (the widest opening), 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 35 or XXXV may refer to: * 35 (number) 35 (thirty-five) is the natural number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and total order, ordering (as in "this is ...
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 Image file formats are standardized means of organizing and storing digital image A digital image is an composed of s, also known as ''pixels'', each with ', ' of numeric representation for its or that is an output from its fed as input ...
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 approximately equivalent in size to the film negative in its C ("Classic") format, of 25.1×16.7 mm, an aspect ratio of 3:2. It is therefore also equivalent in size to the motion picture film form ...
need to project a much smaller
image circle The image circle is the cross section Cross section may refer to: * Cross section (geometry), the intersection of a 3-dimensional body with a plane * Cross section (electronics), a common sample preparation technique in electronics * Cross secti ...

image circle
than a lens used for
large format Large format refers to any imaging format of 9×12 cm (3-1/2x4-3/4 inch) or larger. Large format is larger than "medium format . Medium-format film lacks the sprocket holes of 35 mm film. Medium format has traditionally referre ...
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 VoigtländerCosina Voigtländer refers to photographic products manufactured by Cosina under the Voigtländer name since 1999. Cosina leases rights to the Voigtländer name from RINGFOTO GmbH & Co. ALFO Marketing KG in Germany. Cosina Voigtländer products have ...
17.5mm , 25mm and 42.5mm manual focus lenses for the Micro Four-Thirds System. 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. He is frequently cited as one of the greatest filmmakers Filmmaking (or, in any context, film production) is ...
's film ''
Barry Lyndon ''Barry Lyndon'' is a 1975 period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Period, a descr ...
'' 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 9×12 cm (3-1/2x4-3/4 inch) or larger. Large format is larger than "medium format . Medium-format film lacks the sprocket holes of 35 mm film. Medium format has traditionally referre ...

large format
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 The San Francisco Bay Area, popularly referred to as the Bay Area, is a populous region surrounding the San Francisco Bay, San Francisco, San Pablo Bay, San P ...
. 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 great ...

macro photography
, 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 engi ...

pinhole
optic for
Lensbaby Lensbaby is a line of Photographic lens, camera lenses for single lens reflex camera, SLR cameras that combine a simple lens with a Bellows (photography), bellows or ball and socket mechanism for use in special-effect photography. A lensbaby ca ...
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 Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

focal length
''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 Visual perception is the ability to interpret the surrounding environment (biophysical), environment through photopic vision (daytime vision), color vision, scotopic visio ...

angle of view
, and the same aperture area, they gather the same amount of light from the scene. In that case, the relative focal-plane
illuminance In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical o ...

illuminance
, 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 Torkel Korling (April 24, 1903 – October 22, 1998) was a Swedish-born American industrial, commercial, portrait and botanical photographer. Early life Torkel Korling was born into a 400-year line of Lutheranism, Lutheran Church choir directors a ...
had invented and patented for the
Graflex Graflex was a manufacturer Manufacturing is the creation or production Production may be: Economics and business * Production (economics) * Production, the act of manufacturing goods * Production, in the outline of industrial organization, ...
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 SLR may refer to: Technology * Satellite laser ranging In satellite laser ranging (SLR) a global network of observation stations measures the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflecto ...
manufacturers separately developed ''automatic aperture control'' (the Miranda T 'Pressure Automatic Diaphragm', and other solutions on the Exakta Varex IIa and
Praktica FX2
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 35mm35 mm may refer to: * 135 film, film rolls * 35 mm movie film, motion picture film stock * 35 mm format for still photography (36×24 mm) {{Letter-NumberCombDisambig ... single-lens ref ...

Pentax Spotmatic
) 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 In photography Photography is the art Art is a diverse range of (products of) human activities Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairle ...
. For some lenses, including a few long telephotos, lenses mounted on
bellows A bellow or pair of bellows is a device constructed to furnish a strong blast of air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together comp ...
, 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 focus (optics), in focus. This does not relate to depth of field which describes apparent sharpness. Reducing the size of the aperture will increase the d ...
, 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 The Sony Cyber-shot DSC-RX10 is a digital superzoom bridge camera made by Sony. It was announced on October 16, 2013. It has 20 Megapixels and supports video recording with 1080p at up 60 fps. Its single-axis articulating screen can tilt upwards ...

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 Handwriting is the writing Writing is a medium of human communication that involves the representation of a languag ...

Drum scanner
, an
image sensor An image sensor or imager is a sensor A sensor is a device that produces an output signal for the purpose of sensing of a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects e ...
, 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 may refer to: *Sampling (signal processing), converting a continuous signal into a discrete signal *Sample (graphics), Sampling (graphics), converting continuous colors into discrete color components *Sampling (music), the reuse of a sound ...
a signal waveform. For example,
film grain Film grain or granularity is the random optical texture of processed photographic film Photographic film is a strip or sheet of transparent film baseA film base is a transparent substrate which acts as a support medium for the photosensi ...
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 Optics is the branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, ...

Numerical aperture
*
Antenna aperture In electromagnetics Electromagnetism is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matte ...
*
Angular resolution Angular resolution describes the ability of any image-forming device In optics, an image-forming optical system is a system capable of being used for Image, imaging. The diameter of the aperture of the main objective is a common criterion for c ...

Angular resolution
*
Diaphragm (optics) In optics Optics is the branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, o ...
*
Waterhouse stop File:Diaphragms Cassells 1911.png, 350px, ''Cassell's Cyclopedia of Photography'' 1911 identifies John Waterhouse of Halifax as the inventor in 1858 of Waterhouse stops, "a separate stop being required for each opening." The Waterhouse stop or Wate ...
*
Bokeh In photography, bokeh ( or ; ) is the aesthetic quality of the blur produced in defocus aberration, 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 opt ...

Bokeh
*
Shallow focus Shallow focus is a photographic Photography is the art, application, and practice of creating durable image An SAR radar imaging, radar image acquired by the SIR-C/X-SAR radar on board the Space Shuttle Endeavour shows the Teide volcano ...
*
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 Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and t ...
*
Exit pupil In optics Optics is the branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, ...

Exit pupil
*
Lyot stop A Lyot stop (also called a glare stop) is an optical stop, invented by French astronomer Bernard Lyot Bernard Ferdinand Lyot (27 February 1897 in Paris – 2 April 1952 in Cairo) was a France, French astronomer. Biography An avid reader of t ...


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
Large Format page


External links



{{Authority control Science of photography Geometrical optics Physical optics Observational astronomy