1.
Exposure (photography)
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In photography, exposure is the amount of light per unit area reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value, in photographic jargon, an exposure generally refers to a single shutter cycle. For the same speed, the accumulated photometric exposure should be similar in both cases. A photograph may be described as underexposed when it has a loss of detail, that is. As the adjacent image shows, these terms are technical ones, there are three types of settings, manual, automatic and exposure compensation. Radiant exposure of a surface, denoted He and measured in J/m2, is given by H e = E e t, where Ee is the irradiance of the surface, measured in W/m2, t is the exposure duration, measured in s. Luminous exposure of a surface, denoted Hv and measured in lx⋅s, is given by H v = E v t, many photographic materials are also sensitive to invisible light, which can be a nuisance, or a benefit. The use of units is appropriate to characterize such sensitivity to invisible light. In sensitometric data, such as curves, the log exposure is conventionally expressed as log10. Photographers more familiar with base-2 logarithmic scales can convert using log2 ≈3.32 log10, correct exposure may be defined as an exposure that achieves the effect the photographer intended. A more technical approach recognises that a film has a physically limited useful exposure range. If, for any part of the photograph, the exposure is outside this range. In a very simple model, for example, out-of-range values would be recorded as black or white rather than the precisely graduated shades of colour and this ensures that no significant information is lost during capture. However, it is much easier to discard recorded information during post processing than to try to re-create unrecorded information. In a scene with strong or harsh lighting, the ratio between highlight and shadow luminance values may well be larger than the ratio between the maximum and minimum useful exposure values. A photograph may be described as underexposed when it has a loss of detail, that is. In manual mode, the photographer adjusts the lens aperture and/or shutter speed to achieve the desired exposure, manual exposure calculations may be based on some method of light metering with a working knowledge of exposure values, the APEX system and/or the Zone System. A camera in automatic exposure mode automatically calculates and adjusts exposure settings to match the subjects mid-tone to the mid-tone of the photograph, for most cameras this means using an on-board TTL exposure meter
2.
Film speed
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Film speed is the measure of a photographic films sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. A closely related ISO system is used to measure the sensitivity of digital imaging systems, highly sensitive films are correspondingly termed fast films. In both digital and film photography, the reduction of exposure corresponding to use of higher sensitivities generally leads to reduced image quality, in short, the higher the sensitivity, the grainier the image will be. Ultimately sensitivity is limited by the efficiency of the film or sensor. The speed of the emulsion was then expressed in degrees Warnerke corresponding with the last number visible on the plate after development. Each number represented an increase of 1/3 in speed, typical speeds were between 10° and 25° Warnerke at the time. The concept, however, was built upon in 1900 by Henry Chapman Jones in the development of his plate tester. In their system, speed numbers were inversely proportional to the exposure required, for example, an emulsion rated at 250 H&D would require ten times the exposure of an emulsion rated at 2500 H&D. The methods to determine the sensitivity were later modified in 1925, the H&D system was officially accepted as a standard in the former Soviet Union from 1928 until September 1951, when it was superseded by GOST 2817-50. The Scheinergrade system was devised by the German astronomer Julius Scheiner in 1894 originally as a method of comparing the speeds of plates used for astronomical photography, Scheiners system rated the speed of a plate by the least exposure to produce a visible darkening upon development. ≈2 The system was extended to cover larger ranges and some of its practical shortcomings were addressed by the Austrian scientist Josef Maria Eder. Scheiners system was abandoned in Germany, when the standardized DIN system was introduced in 1934. In various forms, it continued to be in use in other countries for some time. The DIN system, officially DIN standard 4512 by Deutsches Institut für Normung, was published in January 1934, International Congress of Photography held in Dresden from August 3 to 8,1931. The DIN system was inspired by Scheiners system, but the sensitivities were represented as the base 10 logarithm of the sensitivity multiplied by 10, similar to decibels. Thus an increase of 20° represented an increase in sensitivity. ≈3 /10 As in the Scheiner system, speeds were expressed in degrees, originally the sensitivity was written as a fraction with tenths, where the resultant value 1.8 represented the relative base 10 logarithm of the speed. Tenths were later abandoned with DIN4512, 1957-11, and the example above would be written as 18° DIN, the degree symbol was finally dropped with DIN4512, 1961-10
3.
F-number
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The f-number of an optical system such as a camera lens is the ratio of the systems focal length to the diameter of the entrance pupil. It is a number that is a quantitative measure of lens speed. It is also known as the ratio, f-ratio, f-stop. The f-number is commonly indicated using a hooked f with the format f/N, the f-number N or f# is given by, N = f D where f is the focal length, and D is the diameter of the entrance pupil. It is customary to write f-numbers preceded by f/, which forms a mathematical expression of the pupil diameter in terms of f and N. Ignoring differences in light transmission efficiency, a lens with a greater f-number projects darker images, the brightness of the projected image relative to the brightness of the scene in the lenss field of view decreases with the square of the f-number. Doubling the f-number decreases the brightness by a factor of four. To maintain the same photographic exposure when doubling the f-number, the time would need to be four times as long. Most lenses have a diaphragm, which changes the size of the aperture stop. The entrance pupil diameter is not necessarily equal to the aperture stop diameter, a 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A200 mm focal length f/4 lens has a pupil diameter of 50 mm. The 200 mm lenss entrance pupil has four times the area of the 100 mm lenss entrance pupil, a T-stop is an f-number adjusted to account for light transmission efficiency. The word stop is sometimes confusing due to its multiple meanings, a stop can be a physical object, an opaque part of an optical system that blocks certain rays. In photography, stops are also a used to quantify ratios of light or exposure. The one-stop unit is known as the EV unit. On a camera, the setting is traditionally adjusted in discrete steps. Each stop is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop. This corresponds to a decrease of the pupil and aperture diameters by a factor of 1/2 or about 0.7071, each element in the sequence is one stop lower than the element to its left, and one stop higher than the element to its right
4.
Photography
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Typically, a lens is used to focus the light reflected or emitted from objects into a real image on the light-sensitive surface inside a camera during a timed exposure. With an electronic sensor, this produces an electrical charge at each pixel. A negative image on film is used to photographically create a positive image on a paper base, known as a print. The word photography was created from the Greek roots φωτός, genitive of φῶς, light and γραφή representation by means of lines or drawing, several people may have coined the same new term from these roots independently. Johann von Maedler, a Berlin astronomer, is credited in a 1932 German history of photography as having used it in an article published on 25 February 1839 in the German newspaper Vossische Zeitung. Both of these claims are now widely reported but apparently neither has ever been confirmed as beyond reasonable doubt. Credit has traditionally given to Sir John Herschel both for coining the word and for introducing it to the public. Photography is the result of combining several technical discoveries, later Greek mathematicians Aristotle and Euclid also independently described a pinhole camera in the 5th and 4th centuries BCE. Daniele Barbaro described a diaphragm in 1566, wilhelm Homberg described how light darkened some chemicals in 1694. The fiction book Giphantie, published in 1760, by French author Tiphaigne de la Roche, the discovery of the camera obscura that provides an image of a scene dates back to ancient China. Leonardo da Vinci mentions natural camera obscura that are formed by dark caves on the edge of a sunlit valley, a hole in the cave wall will act as a pinhole camera and project a laterally reversed, upside down image on a piece of paper. So the birth of photography was primarily concerned with inventing means to capture, renaissance painters used the camera obscura which, in fact, gives the optical rendering in color that dominates Western Art. The camera obscura literally means dark chamber in Latin and it is a box with a hole in it which allows light to go through and create an image onto the piece of paper. Around the year 1800, British inventor Thomas Wedgwood made the first known attempt to capture the image in a camera obscura by means of a light-sensitive substance and he used paper or white leather treated with silver nitrate. The shadow images eventually darkened all over, the first permanent photoetching was an image produced in 1822 by the French inventor Nicéphore Niépce, but it was destroyed in a later attempt to make prints from it. Niépce was successful again in 1825, in 1826 or 1827, he made the View from the Window at Le Gras, the earliest surviving photograph from nature. Because Niépces camera photographs required a long exposure, he sought to greatly improve his bitumen process or replace it with one that was more practical. With an eye to eventual commercial exploitation, the partners opted for total secrecy, Daguerres efforts culminated in what would later be named the daguerreotype process
5.
Exposure value
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In photography, exposure value is a number that represents a combination of a cameras shutter speed and f-number, such that all combinations that yield the same exposure have the same EV. Exposure value is used to indicate an interval on the photographic exposure scale, with a difference of 1 EV corresponding to a standard power-of-2 exposure step. The EV concept was developed by the German shutter manufacturer Friedrich Deckel in the 1950s and its intent was to simplify choosing among equivalent camera exposure settings by replacing combinations of shutter speed and f-number with a single number. This was especially helpful to beginners with limited understanding of the shutter speed and aperture. The concept became known as the Light Value System in Europe, the proper EV was determined by the scene luminance and film speed, it was intended that the system also include adjustment for filters, exposure compensation, and other variables. With all of elements included, the camera would be set by transferring the single number thus determined. Exposure value has been indicated in various ways, although all camera settings with the same EV nominally give the same exposure, they do not necessarily give the same picture. The f-number determines the depth of field, and the shutter speed determines the amount of motion blur, if the EV is known, it can be used to select combinations of exposure time and f-number, as shown in Table 1. Greater exposure values are appropriate for photography in more brightly lit situations, to avoid confusion, some authors have used camera exposure to refer to combinations of camera settings. The 1964 ASA standard for automatic exposure controls for cameras, ASA PH2. 15-1964, took the same approach, common practice among photographers is nonetheless to use “exposure” to refer to camera settings as well as to photometric exposure. If, for example, the f-number is changed, an equivalent exposure time can be determined from t 2 t 1 = N22 N12. Performing this calculation mentally is tedious for most photographers, but the equation is solved with a calculator dial on an exposure meter or a similar dial on a standalone calculator. If the camera controls have detents, constant exposure can be maintained by counting the steps as one control is adjusted, the ratio t/N2 could be used to represent equivalent combinations of exposure time and f-number in a single value. But for many such combinations used in photography, the ratio gives a fractional value with a large denominator. For example, beginning with 1 s and f/1, decreasing exposure gives the simple sequence 0,1,2,3, the last two values shown frequently apply when using ISO100 speed imaging media in outdoor photography. Current cameras do not allow direct setting of EV, and cameras with automatic exposure control generally obviate the need for it, EV can nonetheless be helpful when used to transfer recommended exposure settings from an exposure meter to an exposure calculator. Used as an indicator of camera settings, EV corresponds to actual combinations of shutter speed, when the actual EV matches that recommended by the light level and the ISO speed, these settings should result in the “correct” exposure. If the common value of K =12.5 is used, at EV =15 the luminance is 380 cd/ft2 or 4096 cd/m2
6.
Light meter
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A light meter is a device used to measure the amount of light. In photography, a meter is often used to determine the proper exposure for a photograph. Light meters are used in the fields of cinematography and scenic design. They are used in the field of lighting, where they can help to reduce the amount of waste light used in the home, light pollution outdoors. The earliest type of light meters were called extinction meters and contained a numbered or lettered row of neutral density filters of increasing density. The photographer would position the meter in front of his subject, the letter or number corresponding to the filter was used as an index into a chart of appropriate aperture and shutter speed combinations for a given film speed. Extinction meters suffered from the problem that they depended on the sensitivity of the human eye. Later meters removed the element and relied on technologies incorporating selenium, CdS. Selenium and silicon light meters use sensors that are photovoltaic, they generate a proportional to light exposure. Selenium sensors generate enough voltage for direct connection to a meter, they need no battery to operate, selenium sensors however cannot measure low light accurately and are altogether unable to measure very low light, such as candlelight, moonlight, starlight etc. Silicon sensors need an amplification circuit and require a source such as batteries to operate. CdS light meters use a sensor based on photoresistance, i. e. their electrical resistance changes proportionately to light exposure and these also require a battery to operate. Most modern light meters use silicon or CdS sensors and they indicate the exposure either with a needle galvanometer or on an LCD screen. Many modern consumer still and video include a built-in meter that measures a scene-wide light level and are able to make an approximate measure of appropriate exposure based on that. There are two types of light meters, reflected-light and incident-light. Reflected-light meters measure the light reflected by the scene to be photographed, all in-camera meters are reflected-light meters. Reflected-light meters are calibrated to show the appropriate exposure for “average” scenes and this pitfall is avoided by incident-light meters which measure the amount of light falling on the subject using an integrating sphere placed on top of the light sensor. Because the incident-light reading is independent of the reflectance, it is less likely to lead to incorrect exposures for subjects with unusual average reflectance
7.
Ansel Adams
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Ansel Easton Adams was an American photographer and environmentalist. His black and white photographs of the American West, especially Yosemite National Park, have been widely reproduced on calendars, posters, books. Adams and Fred Archer developed the Zone System as a way to determine proper exposure, the resulting clarity and depth characterized his photographs. He primarily used large-format cameras because their high resolution helped ensure sharpness in his images, Adams founded the photography group known as Group f/64, along with fellow photographers Willard Van Dyke and Edward Weston. Adams was born in the Western Addition of San Francisco, California and he was named after his uncle, Ansel Easton. His mothers family came from Baltimore, where his grandfather had a successful freight-hauling business but lost his wealth investing in failed mining. The Adams family came from New England, having migrated from Northern Ireland in the early 18th century and his paternal grandfather founded and built a prosperous lumber business which his father later ran, though his fathers natural talents lay more with sciences than with business. Later in life, Adams condemned that very same industry for cutting many of the great redwood forests. In 1907, his family moved 2 miles west to a new home near the Seacliff neighbourhood, the home had a splendid view of the Golden Gate and the Marin Headlands. San Francisco was devastated by the April 18,1906 San Francisco earthquake, the four year-old Ansel Adams was uninjured in the initial shaking but was tossed face-first into a garden wall during an aftershock three hours later, breaking and scarring his nose. Among his earliest memories was watching the smoke from the fire that destroyed much of the city a few miles to the east. A doctor recommended that his nose be reset once he reached maturity, Adams was a hyperactive child and prone to frequent sickness and hypochondria. He had few friends, but his home and surroundings on the heights facing the Golden Gate provided ample childhood activities. His father bought a telescope, and they enthusiastically shared the hobby of amateur astronomy. His father went on to serve as the paid secretary-treasurer of the Astronomical Society of the Pacific from 1925 to 1950, Ansels fathers business suffered great financial losses after the death of Ansels grandfather and the aftermath of the Panic of 1907. By 1912, the standard of living had dropped sharply. Ansel was dismissed from several schools for being restless and inattentive. Adams was then educated by tutors, his aunt Mary
8.
Reversal film
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In photography, reversal film is a type of photographic film that produces a positive image on a transparent base. The film is processed to produce transparencies or diapositives instead of negatives, Reversal film is produced in various sizes, from 35 mm roll film to 8×10 inch sheet film. A slide is a specially mounted individual transparency intended for projection onto a screen using a slide projector and this allows the photograph to be viewed by a large audience at once. The most common form is the 35 mm slide, with the image framed in a 2×2 inch cardboard or plastic mount. Reversal film is used as motion picture film, mostly in the 16 mm, Super 8 and 8 mm cine formats. This avoids the expense of using film, which requires additional film. The earliest practical and commercially successful color photography reversal process was the Lumière Autochrome and this was an additive method, using a panchromatic emulsion coated on a thin glass plate previously coated with a layer of dyed potato starch grains. Autochrome plates were discontinued in the 1930s after the introduction of Lumière Filmcolor in sheet film, also using the additive principle and reversal processing were the Agfa color screen plates and films and Dufaycolor film, all of which were discontinued by 1961. Leopold Godowsky, Jr. and Leopold Mannes, working with the Eastman Kodak Company, developed Kodachrome, Kodachrome was introduced in 1935 as 16mm motion picture film, and in 1936 as 35mm film for still cameras. The Kodachrome films contained no color dye couplers, these were added during processing, in late 1936, Agfacolor Neu was launched, Agfa having overcome earlier difficulties with color sensitivity problems. This film had the dye couplers incorporated into the emulsion, making processing simpler than for Kodachrome, amateurs who owned projection equipment used reversal films extensively because the cost of projection equipment and slide film was offset by not having to pay for prints. Eventually, print quality improved and prices decreased, and, by the 1970s, color negative film, digital media gradually replaced transparency film. All color reversal film sold today is developed with the E-6 process, the non-substantive Kodachrome films, the last of which was discontinued in 2009, were processed with the K-14 process. Polaroid produced an instant slide film called Polachrome and it was packaged in cassettes like normal 35mm film. A separate processing unit was used to develop it after exposure, black-and-white transparencies can be made directly with some modern black-and-white films, which normally yield negatives. The negative image is developed but not fixed, the remaining silver halide salts are re-exposed to light, developed and fixed, and the film is washed and dried. Black-and-white transparencies were once popular for presentation of materials using 3 1⁄4 by 4 glass-mounted slides. Such positive black-and-white projection is now rarely done, except in motion pictures, even where black-and-white positives are currently used, the process to create them typically uses an internegative with standard processing instead of a chemical reversal process
9.
Bracketing
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In photography, bracketing is the general technique of taking several shots of the same subject using different camera settings. Autobracketing is automatic bracketing by using a setting on the camera to take several bracketed shots, given the time it takes to accomplish multiple shots, it is typically, but not always, used for static subjects. Technically, this can be accomplished by changing either the speed or the aperture, or, with digital cameras. Exposure can also be changed by altering the level, for example using neutral-gray filters or changing the degree of illumination of the subject. Exposure bracketing is indicated when dealing with high-contrast subjects and/or media with limited dynamic range, exposure bracketing is also used to create fade-in or fade-out effects, for example in conjunction with multi-vision slide shows, or in combination with multiple exposure or flash. When shooting using negative film, the printing the pictures to paper must not compensate for the deliberately underexposed and overexposed pictures. Images produced using exposure bracketing are often combined in postprocessing to create a dynamic range image that exposes different portions of the image by different amounts. Flash bracketing is a technique of working with electronic flash, especially used as fill flash in combination with existing light. The amount of light provided by the flash is varied in a series in order to find the most pleasing combination of ambient light. If used for purpose, flash bracketing can be differentiated from normal exposure bracketing via flash. Alternatively, if the amount of light cannot be altered easily, it is also possible to alter the aperture instead, however. In the first case, it will change the amount of motion blur in the picture. In the second case, it may visibly affect image noise, combining DOF bracketing with multiple exposure, the so-called STF effect can be achieved as implemented in the Minolta Maxxum 7s automated STF function. This closely resembles the Bokeh-pleasing optical effect of the filter in the Minolta/Sony STF135 mm f/2.8 s special-purpose lens. Usually this involves the use of software with unsharp masking, an algorithm that removes out-of-focus portions of each exposure. The in-focus portions are then stacked, combined into a single image, focus stacking is challenging, in that the subject must stay still and that as the focal point changes, the magnification of the images change. This must then be corrected in an application by transforming the image. ISO bracketing is a form of simulated exposure bracketing in which aperture and this is analogous to pushing or pulling in film processing, and as in film processing, will affect the amount of grain or image noise
10.
Minolta
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Minolta Co. Ltd. was a Japanese worldwide manufacturer of cameras, camera accessories, photocopiers, fax machines, and laser printers. Minolta was founded in Osaka, Japan, in 1928 as Nichi-Doku Shashinki Shōten and it is perhaps best known for making the first integrated autofocus 35mm SLR camera system. In 1931, the company adopted its current name, an acronym for Mechanism, Instruments, Optics, in 1933, the brand name first appeared on a camera, a copy of the Plaubel Makina simply called Minolta. In 2003, Minolta merged with Konica Corporation to form Konica Minolta,1928, Kazuo Tashima establishes Nichi-Doku Shashinki Shōten. 1929, Marketed the companys first camera, the Nifcarette,1937, The Minolta Flex is Japans second twin-lens reflex camera. 1947, Introduction of the long lived 35mm rangefinder camera Minolta-351958,1962, John Glenn takes a specially modified Ansco-logoed Minolta Hi-Matic camera into space aboard Friendship 7. The company changes its name to Minolta Camera Co. Ltd,1966, The Minolta SR-T101 SLR camera is one of the first with TTL full aperture light metering. The first is Topcon RE Super from 1963,1972, Minolta signs an agreement to cooperate with Leica in SLR development,1973, The Minolta CL is the first fruit of this agreement. 1976, The Leica R3 is introduced, Minolta produces the R3, R4, and R5 models in the Leica R series. Subsequent cameras are built in Germany by Leica themselves,1977, The Minolta XD-11 is introduced, the worlds first multi mode SLR offering M, A, S modes, with a Program override in S mode effected by a computer chip, the worlds first Program mode. 1981, Implementation of Minoltas invention and patent of TTL OTF exposure metering, the Minolta X-700 manual-focus SLR is introduced, this model is sold until 1999 and is enormously successful. The Minolta XD-11 is the first Minolta product branded with an updated logo,1985, The Minolta Maxxum 7000 Alpha Mount Camera becomes the worlds first autofocus 35mm SLR with in-camera autofocus motor. 1987, Honeywell files lawsuit against Minolta for patent infringement over autofocus technologies,1991, Minoltas autofocus design was found to infringe on the patents of Honeywell, a U. S. corporation. 1994, The company changes its name to Minolta Co. Ltd. because it no longer is primarily a camera company,1995, Introduction of the Minolta RD-175, a 1. 75-megapixel digital SLR camera. 1996, The Minolta Vectis camera is a completely new SLR system designed around the Advanced Photo System film format,1998, The Minolta Maxxum 9 autofocus SLR is introduced. This system is targeted toward the photographer and has many features not duplicated by the competition. 2003, DiMAGE A1 introduced worlds first sensor-based anti-shake, and was the final Minolta product branded prior to the Konica Minolta merger,2004, Minolta and Konica officially merge to become Konica Minolta Holdings, Inc. 2005, The company announces joint venture with Sony on CCD,2006, Konica Minolta announces it is discontinuing all film and digital camera production, ending a 78-year history as a camera manufacturer
11.
Shutter speed
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The amount of light that reaches the film or image sensor is proportional to the exposure time. 1/500th of a second will let half as much light in as 1/250th, the cameras shutter speed, the lenss aperture, and the scenes luminance together determine the amount of light that reaches the film or sensor. Exposure value is a quantity that accounts for the shutter speed and this will achieve a good exposure when all the details of the scene are legible on the photograph. Too much light let into the results in an overly pale image while too little light will result in an overly dark image. Multiple combinations of speed and f-number can give the same exposure value. According to exposure value formula, doubling the exposure time doubles the amount of light, for example, f/8 lets 4 times more light into the camera as f/16 does. In addition to its effect on exposure, the speed changes the way movement appears in photographs. Very short shutter speeds can be used to freeze fast-moving subjects, very long shutter speeds are used to intentionally blur a moving subject for effect. Short exposure times are called fast, and long exposure times slow. Adjustments to the aperture need to be compensated by changes of the speed to keep the same exposure. The agreed standards for shutter speeds are, With this scale, camera shutters often include one or two other settings for making very long exposures, B keeps the shutter open as long as the shutter release is held. T keeps the open until the shutter release is pressed again. The ability of the photographer to take images without noticeable blurring by camera movement is an important parameter in the choice of the slowest possible speed for a handheld camera. Through practice and special techniques such as bracing the camera, arms, or body to minimize movement, using a monopod or a tripod. If a shutter speed is too slow for hand holding, a support, usually a tripod. Image stabilization on digital cameras or lenses can often permit the use of shutter speeds 3–4 stops slower, Shutter priority refers to a shooting mode used in cameras. It allows the photographer to choose a shutter speed setting and allow the camera to decide the correct aperture and this is sometimes referred to as Shutter Speed Priority Auto Exposure, or TV mode, S mode on Nikons and most other brands. Shutter speed is one of methods used to control the amount of light recorded by the cameras digital sensor or film
12.
Aperture
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In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of a system determine the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are, which is of importance for the appearance at the image plane. If an aperture is narrow, then highly collimated rays are admitted, a wide aperture admits uncollimated rays, resulting in a sharp focus only for rays coming from a certain distance. This means that a wide aperture results in an image that is sharp for things at the correct distance, the aperture also determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane. In the human eye, the pupil is the aperture, an optical system typically has many openings or structures that limit the ray bundles. In general, these structures are called stops, and the stop is the stop that primarily determines the ray cone angle. In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the stop or the opening itself. For example, in a telescope, the stop is typically the edges of the objective lens or mirror. One then speaks of a telescope as having, for example, 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 large stop to be the aperture stop for the system. In astrophotography, the aperture may be given as a measure or as the dimensionless ratio between that measure and the focal length. In other photography, it is given as a ratio. Sometimes stops and diaphragms are called apertures, even when they are not the stop of the system. The word aperture is 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 usually corresponds to a window around the image of a star within which the light intensity is assumed. The aperture stop is an important element in most optical designs and its most obvious feature is that it limits the amount of light that can reach the image/film plane. This can be unavoidable, as in a telescope where one wants to collect as much light as possible, or deliberate
