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Thursday, December 16, 2021

History of photography

View from the Window at Le Gras 1826 or 1827, believed to be the earliest surviving camera photograph. Original (left) & colorized reoriented enhancement (right).

The history of photography began in remote antiquity with the discovery of two critical principles: camera obscura image projection and the observation that some substances are visibly altered by exposure to light. There are no artifacts or descriptions that indicate any attempt to capture images with light sensitive materials prior to the 18th century.

Around 1717, Johann Heinrich Schulze captured cut-out letters on a bottle of a light-sensitive slurry, but he apparently never thought of making the results durable. Around 1800, Thomas Wedgwood made the first reliably documented, although unsuccessful attempt at capturing camera images in permanent form. His experiments did produce detailed photograms, but Wedgwood and his associate Humphry Davy found no way to fix these images.

In the mid-1820s, Nicéphore Niépce first managed to fix an image that was captured with a camera, but at least eight hours or even several days of exposure in the camera were required and the earliest results were very crude. Niépce's associate Louis Daguerre went on to develop the daguerreotype process, the first publicly announced and commercially viable photographic process. The daguerreotype required only minutes of exposure in the camera, and produced clear, finely detailed results. The details were introduced to the world in 1839, a date generally accepted as the birth year of practical photography. The metal-based daguerreotype process soon had some competition from the paper-based calotype negative and salt print processes invented by William Henry Fox Talbot and demonstrated in 1839 soon after news about the daguerreotype reached Talbot. Subsequent innovations made photography easier and more versatile. New materials reduced the required camera exposure time from minutes to seconds, and eventually to a small fraction of a second; new photographic media were more economical, sensitive or convenient. Since the 1850s, the collodion process with its glass-based photographic plates combined the high quality known from the Daguerreotype with the multiple print options known from the calotype and was commonly used for decades. Roll films popularized casual use by amateurs. In the mid-20th century, developments made it possible for amateurs to take pictures in natural color as well as in black-and-white.

The commercial introduction of computer-based electronic digital cameras in the 1990s soon revolutionized photography. During the first decade of the 21st century, traditional film-based photochemical methods were increasingly marginalized as the practical advantages of the new technology became widely appreciated and the image quality of moderately priced digital cameras was continually improved. Especially since cameras became a standard feature on smartphones, taking pictures (and instantly publishing them online) has become a ubiquitous everyday practice around the world.

Etymology

The coining of the word "photography" is usually attributed to Sir John Herschel in 1839. It is based on the Greek φῶς (phots), (genitive: photo's) meaning "light", and γραφή (graphê), meaning "drawing, writing", together meaning "drawing with light".

Early history of the camera

Principle of a box camera obscura with mirror

A natural phenomenon, known as camera obscura or pinhole image, can project a (reversed) image through a small opening onto an opposite surface. This principle may have been known and used in prehistoric times. The earliest known written record of the camera obscura is to be found in Chinese writings by Mozi, dated to the 4th century BCE. Until the 16th century the camera obscura was mainly used to study optics and astronomy, especially to safely watch solar eclipses without damaging the eyes. In the later half of the 16th century some technical improvements were developed: a biconvex lens in the opening (first described by Gerolamo Cardano in 1550) and a diaphragm restricting the aperture (Daniel Barbaro in 1568) gave a brighter and sharper image. In 1558 Giambattista della Porta advised using the camera obscura as a drawing aid in his popular and influential books. Della Porta's advice was widely adopted by artists and since the 17th century portable versions of the camera obscura were commonly used — first as a tent, later as boxes. The box type camera obscura was the basis for the earliest photographic cameras when photography was developed in the early 19th century.

Before 1700: Light sensitive materials

The notion that light can affect various substances — for instance, the sun tanning of skin or fading of textile — must have been around since very early times. Ideas of fixing the images seen in mirrors or other ways of creating images automatically may also have been in people's minds long before anything like photography was developed. However, there seem to be no historical records of any ideas even remotely resembling photography before 1700, despite early knowledge of light-sensitive materials and the camera obscura.

In 1614 Angelo Sala noted that sunlight will turn powdered silver nitrate black, and that paper wrapped around silver nitrate for a year will turn black.

Wilhelm Homberg described how light darkened some chemicals in 1694.

1700 to 1802: earliest concepts and fleeting photogram results

Schulze's Scotophors: earliest fleeting letter photograms (circa 1717)

Around 1717, German polymath Johann Heinrich Schulze accidentally discovered that a slurry of chalk and nitric acid into which some silver particles had been dissolved was darkened by sunlight. After experiments with threads that had created lines on the bottled substance after he placed it in direct sunlight for a while, he applied stencils of words to the bottle. The stencils produced copies of the text in dark red, almost violet characters on the surface of the otherwise whitish contents. The impressions persisted until they were erased by shaking the bottle or until overall exposure to light obliterated them. Schulze named the substance "Scotophors" when he published his findings in 1719. He thought the discovery could be applied to detect whether metals or minerals contained any silver and hoped that further experimentation by others would lead to some other useful results. Schulze's process resembled later photogram techniques and is sometimes regarded as the very first form of photography.

De la Roche's fictional image capturing process (1760)

The early science fiction novel Giphantie (1760) by the Frenchman Tiphaigne de la Roche described something quite similar to (color) photography, a process that fixes fleeting images formed by rays of light: "They coat a piece of canvas with this material, and place it in front of the object to capture. The first effect of this cloth is similar to that of a mirror, but by means of its viscous nature the prepared canvas, as is not the case with the mirror, retains a facsimile of the image. The mirror represents images faithfully, but retains none; our canvas reflects them no less faithfully, but retains them all. This impression of the image is instantaneous. The canvas is then removed and deposited in a dark place. An hour later the impression is dry, and you have a picture the more precious in that no art can imitate its truthfulness." De la Roche thus imagined a process that made use of a special substance in combination with the qualities of a mirror, rather than the camera obscura. The hour of drying in a dark place suggests that he possibly thought about the light sensitivity of the material, but he attributed the effect to its viscous nature.

Scheele's forgotten chemical fixer (1777)

In 1777, the chemist Carl Wilhelm Scheele was studying the more intrinsically light-sensitive silver chloride and determined that light darkened it by disintegrating it into microscopic dark particles of metallic silver. Of greater potential usefulness, Scheele found that ammonia dissolved the silver chloride, but not the dark particles. This discovery could have been used to stabilize or "fix" a camera image captured with silver chloride, but was not picked up by the earliest photography experimenters.

Scheele also noted that red light did not have much effect on silver chloride, a phenomenon that would later be applied in photographic darkrooms as a method of seeing black-and-white prints without harming their development.

Although Thomas Wedgwood felt inspired by Scheele's writings in general, he must have missed or forgotten these experiments; he found no method to fix the photogram and shadow images he managed to capture around 1800 (see below).

Thomas Wedgwood and Humphry Davy: Fleeting detailed photograms (1790?–1802)

English photographer and inventor Thomas Wedgwood is believed to have been the first person to have thought of creating permanent pictures by capturing camera images on material coated with a light-sensitive chemical. He originally wanted to capture the images of a camera obscura, but found they were too faint to have an effect upon the silver nitrate solution that was recommended to him as a light-sensitive substance. Wedgwood did manage to copy painted glass plates and captured shadows on white leather, as well as on paper moistened with a silver nitrate solution. Attempts to preserve the results with their "distinct tints of brown or black, sensibly differing in intensity" failed. It is unclear when Wedgwood's experiments took place. He may have started before 1790; James Watt wrote a letter to Thomas Wedgwood's father Josiah Wedgwood to thank him "for your instructions as to the Silver Pictures, about which, when at home, I will make some experiments". This letter (now lost) is believed to have been written in 1790, 1791 or 1799. In 1802, an account by Humphry Davy detailing Wedgwood's experiments was published in an early journal of the Royal Institution with the title An Account of a Method of Copying Paintings upon Glass, and of Making Profiles, by the Agency of Light upon Nitrate of Silver. Davy added that the method could be used for objects that are partly opaque and partly transparent to create accurate representations of, for instance, "the woody fibres of leaves and the wings of insects". He also found that solar microscope images of small objects were easily captured on prepared paper. Davy, apparently unaware or forgetful of Scheele's discovery, concluded that substances should be found to eliminate (or deactivate) the unexposed particles in silver nitrate or silver chloride "to render the process as useful as it is elegant". Wedgwood may have prematurely abandoned his experiments because of his frail and failing health. He died at age 34 in 1805.

Davy seems not to have continued the experiments. Although the journal of the nascent Royal Institution probably reached its very small group of members, the article must have been read eventually by many more people. It was reviewed by David Brewster in the Edinburgh Magazine in December 1802, appeared in chemistry textbooks as early as 1803, was translated into French and was published in German in 1811. Readers of the article may have been discouraged to find a fixer, because the highly acclaimed scientist Davy had already tried and failed. Apparently the article was not noted by Niépce or Daguerre, and by Talbot only after he had developed his own processes.

Jacques Charles: Fleeting silhouette photograms (circa 1801?)

French balloonist, professor and inventor Jacques Charles is believed to have captured fleeting negative photograms of silhouettes on light-sensitive paper at the start of the 19th century, prior to Wedgwood. Charles died in 1823 without having documented the process, but purportedly demonstrated it in his lectures at the Louvre. It was not publicized until François Arago mentioned it at his introduction of the details of the daguerreotype to the world in 1839. He later wrote that the first idea of fixing the images of the camera obscura or the solar microscope with chemical substances belonged to Charles. Later historians probably only built on Arago's information, and, much later, the unsupported year 1780 was attached to it. As Arago indicated the first years of the 19th century and a date prior to the 1802 publication of Wedgwood's process, this would mean that Charles' demonstrations took place in 1800 or 1801, assuming that Arago was this accurate almost 40 years later.

1816 to 1833: Niépce's earliest fixed images

The earliest known surviving heliographic engraving, made in 1825. It was printed from a metal plate made by Joseph Nicéphore Niépce with his "heliographic process". The plate was exposed under an ordinary engraving and copied it by photographic means. This was a step towards the first permanent photograph from nature taken with a camera obscura.
 
View of the Boulevard du Temple, a daguerreotype made by Louis Daguerre in 1838, is generally accepted as the earliest photograph to include people. It is a view of a busy street, but because the exposure lasted for several minutes the moving traffic left no trace. Only the two men near the bottom left corner, one of them apparently having his boots polished by the other, remained in one place long enough to be visible.

In 1816, Nicéphore Niépce, using paper coated with silver chloride, succeeded in photographing the images formed in a small camera, but the photographs were negatives, darkest where the camera image was lightest and vice versa, and they were not permanent in the sense of being reasonably light-fast; like earlier experimenters, Niépce could find no way to prevent the coating from darkening all over when it was exposed to light for viewing. Disenchanted with silver salts, he turned his attention to light-sensitive organic substances.

Robert Cornelius, self-portrait, October or November 1839, an approximately quarter plate size daguerreotype. On the back is written, "The first light picture ever taken".
 
One of the oldest photographic portraits known, 1839 or 1840,[24] made by John William Draper of his sister, Dorothy Catherine Draper
 
A photograph captured by Mary Dillwyn in Wales in 1853.

The oldest surviving photograph of the image formed in a camera was created by Niépce in 1826 or 1827. It was made on a polished sheet of pewter and the light-sensitive substance was a thin coating of bitumen, a naturally occurring petroleum tar, which was dissolved in lavender oil, applied to the surface of the pewter and allowed to dry before use. After a very long exposure in the camera (traditionally said to be eight hours, but now believed to be several days), the bitumen was sufficiently hardened in proportion to its exposure to light that the unhardened part could be removed with a solvent, leaving a positive image with the light areas represented by hardened bitumen and the dark areas by bare pewter. To see the image plainly, the plate had to be lit and viewed in such a way that the bare metal appeared dark and the bitumen relatively light.

In partnership, Niépce in Chalon-sur-Saône and Louis Daguerre in Paris refined the bitumen process, substituting a more sensitive resin and a very different post-exposure treatment that yielded higher-quality and more easily viewed images. Exposure times in the camera, although substantially reduced, were still measured in hours.

1832 to 1840: early monochrome processes

Niépce died suddenly in 1833, leaving his notes to Daguerre. More interested in silver-based processes than Niépce had been, Daguerre experimented with photographing camera images directly onto a mirror-like silver-surfaced plate that had been fumed with iodine vapor, which reacted with the silver to form a coating of silver iodide. As with the bitumen process, the result appeared as a positive when it was suitably lit and viewed. Exposure times were still impractically long until Daguerre made the pivotal discovery that an invisibly slight or "latent" image produced on such a plate by a much shorter exposure could be "developed" to full visibility by mercury fumes. This brought the required exposure time down to a few minutes under optimum conditions. A strong hot solution of common salt served to stabilize or fix the image by removing the remaining silver iodide. On 7 January 1839, this first complete practical photographic process was announced at a meeting of the French Academy of Sciences, and the news quickly spread. At first, all details of the process were withheld and specimens were shown only at Daguerre's studio, under his close supervision, to Academy members and other distinguished guests. Arrangements were made for the French government to buy the rights in exchange for pensions for Niépce's son and Daguerre and present the invention to the world (with the exception of Great Britain, where an agent for Daguerre patented it) as a free gift. Complete instructions were made public on 19 August 1839. Known as the daguerreotype process, it was the most common commercial process until the late 1850s when it was superseded by the collodion process.

French-born Hércules Florence developed his own photographic technique in  in 1832 or 1833 with some help of pharmacist Joaquim Corrêa de Mello (1816–1877). Looking for another method to copy graphic designs he captured their images on paper treated with silver nitrate as contact prints or in a camera obscura device. He did not manage to properly fix his images and abandoned the project after hearing of the Daguerreotype process in 1839 and didn't properly publish any of his findings. He reportedly referred to the technique as "photographie" (in French) as early as 1833, also helped by a suggestion of De Mello. Some extant photographic contact prints are believed to have been made in circa 1833 and kept in the collection of IMS.

Henry Fox Talbot had already succeeded in creating stabilized photographic negatives on paper in 1835, but worked on perfecting his own process after reading early reports of Daguerre's invention. In early 1839, he acquired a key improvement, an effective fixer, from his friend John Herschel, a polymath scientist who had previously shown that hyposulfite of soda (commonly called "hypo" and now known formally as sodium thiosulfate) would dissolve silver salts. News of this solvent also benefited Daguerre, who soon adopted it as a more efficient alternative to his original hot salt water method.

A calotype showing the American photographer Frederick Langenheim, circa 1849. The caption on the photo calls the process "Talbotype".

Talbot's early silver chloride "sensitive paper" experiments required camera exposures of an hour or more. In 1841, Talbot invented the calotype process, which, like Daguerre's process, used the principle of chemical development of a faint or invisible "latent" image to reduce the exposure time to a few minutes. Paper with a coating of silver iodide was exposed in the camera and developed into a translucent negative image. Unlike a daguerreotype, which could only be copied by photographing it with a camera, a calotype negative could be used to make a large number of positive prints by simple contact printing. The calotype had yet another distinction compared to other early photographic processes, in that the finished product lacked fine clarity due to its translucent paper negative. This was seen as a positive attribute for portraits because it softened the appearance of the human face. Talbot patented this process, which greatly limited its adoption, and spent many years pressing lawsuits against alleged infringers. He attempted to enforce a very broad interpretation of his patent, earning himself the ill will of photographers who were using the related glass-based processes later introduced by other inventors, but he was eventually defeated. Nonetheless, Talbot's developed-out silver halide negative process is the basic technology used by chemical film cameras today. Hippolyte Bayard had also developed a method of photography but delayed announcing it, and so was not recognized as its inventor.

In 1839, John Herschel made the first glass negative, but his process was difficult to reproduce. Slovene Janez Puhar invented a process for making photographs on glass in 1841; it was recognized on June 17, 1852 in Paris by the Académie National Agricole, Manufacturière et Commerciale. In 1847, Nicephore Niépce's cousin, the chemist Niépce St. Victor, published his invention of a process for making glass plates with an albumen emulsion; the Langenheim brothers of Philadelphia and John Whipple and William Breed Jones of Boston also invented workable negative-on-glass processes in the mid-1840s.

1850 to 1900

In 1851, English sculptor Frederick Scott Archer invented the collodion process. Photographer and children's author Lewis Carroll used this process. (Carroll refers to the process as "Talbotype" in the story "A Photographer's Day Out".)

Herbert Bowyer Berkeley experimented with his own version of collodion emulsions after Samman introduced the idea of adding dithionite to the pyrogallol developer. Berkeley discovered that with his own addition of sulfite, to absorb the sulfur dioxide given off by the chemical dithionite in the developer, dithionite was not required in the developing process. In 1881, he published his discovery. Berkeley's formula contained pyrogallol, sulfite, and citric acid. Ammonia was added just before use to make the formula alkaline. The new formula was sold by the Platinotype Company in London as Sulphur-Pyrogallol Developer.

Nineteenth-century experimentation with photographic processes frequently became proprietary. The German-born, New Orleans photographer Theodore Lilienthal successfully sought legal redress in an 1881 infringement case involving his "Lambert Process" in the Eastern District of Louisiana.

Popularization

The daguerreotype proved popular in response to the demand for portraiture that emerged from the middle classes during the Industrial Revolution. This demand, which could not be met in volume and in cost by oil painting, added to the push for the development of photography.

Roger Fenton and Philip Henry Delamotte helped popularize the new way of recording events, the first by his Crimean War pictures, the second by his record of the disassembly and reconstruction of The Crystal Palace in London. Other mid-nineteenth-century photographers established the medium as a more precise means than engraving or lithography of making a record of landscapes and architecture: for example, Robert Macpherson's broad range of photographs of Rome, the interior of the Vatican, and the surrounding countryside became a sophisticated tourist's visual record of his own travels.

In 1839, François Arago reported the invention of photography to stunned listeners by displaying the first photo taken in Egypt; that of Ras El Tin Palace.

In America, by 1851 a broadsheet by daguerreotypist Augustus Washington was advertising prices ranging from 50 cents to $10. However, daguerreotypes were fragile and difficult to copy. Photographers encouraged chemists to refine the process of making many copies cheaply, which eventually led them back to Talbot's process.

Lapwing incubating its eggs - Photograph of a Lapwing (Vanellus vanellus), for which in 1895 R. B. Lodge received from the Royal Photographic Society the first medal ever presented for nature photography. Eric Hosking and Harold Lowes stated their belief that this was the first photograph of a wild bird.

Ultimately, the photographic process came about from a series of refinements and improvements in the first 20 years. In 1884 George Eastman, of Rochester, New York, developed dry gel on paper, or film, to replace the photographic plate so that a photographer no longer needed to carry boxes of plates and toxic chemicals around. In July 1888 Eastman's Kodak camera went on the market with the slogan "You press the button, we do the rest"."History- Kodak". www.kodak.com. Retrieved 4 December 2021. Now anyone could take a photograph and leave the complex parts of the process to others, and photography became available for the mass-market in 1901 with the introduction of the Kodak Brownie.

Stereoscopic photography

Charles Wheatstone developed his mirror stereoscope around 1832, but did not really publicize his invention until June 1838. He recognized the possibility of a combination with photography soon after Daguerre and Talbot announced their inventions and got Henry Fox Talbot to produce some calotype pairs for the stereoscope. He received the first results in October 1840, but was not fully satisfied as the angle between the shots was very big. Between 1841 and 1842 Henry Collen made calotypes of statues, buildings and portraits, including a portrait of Charles Babbage shot in August 1841. Wheatstone also obtained daguerreotype stereograms from Mr. Beard in 1841 and from Hippolyte Fizeau and Antoine Claudet in 1842. None of these have yet been located.

David Brewster developed a stereoscope with lenses and a binocular camera in 1844. He presented two stereoscopic self portraits made by John Adamson in March 1849. A stereoscopic portrait of Adamson in the University of St Andrews Library Photographic Archive, dated "circa 1845', may be one of these sets. A stereoscopic daguerreotype portrait of Michael Faraday in Kingston College's Wheatstone collection and on loan to Bradford National Media Museum, dated "circa 1848", may be older.

Color process

The first durable color photograph, taken by Thomas Sutton in 1861

A practical means of color photography was sought from the very beginning. Results were demonstrated by Edmond Becquerel as early as the year of 1848, but exposures lasting for hours or days were required and the captured colors were so light-sensitive they would only bear very brief inspection in dim light.

The first durable color photograph was a set of three black-and-white photographs taken through red, green, and blue color filters and shown superimposed by using three projectors with similar filters. It was taken by Thomas Sutton in 1861 for use in a lecture by the Scottish physicist James Clerk Maxwell, who had proposed the method in 1855. The photographic emulsions then in use were insensitive to most of the spectrum, so the result was very imperfect and the demonstration was soon forgotten. Maxwell's method is now most widely known through the early 20th century work of Sergei Prokudin-Gorskii. It was made practical by Hermann Wilhelm Vogel's 1873 discovery of a way to make emulsions sensitive to the rest of the spectrum, gradually introduced into commercial use beginning in the mid-1880s.

Alim Khan photographed by Sergey Prokudin-Gorsky using Maxwell's method, 1911

Two French inventors, Louis Ducos du Hauron and Charles Cros, working unknown to each other during the 1860s, famously unveiled their nearly identical ideas on the same day in 1869. Included were methods for viewing a set of three color-filtered black-and-white photographs in color without having to project them, and for using them to make full-color prints on paper.

The first widely used method of color photography was the Autochrome plate, a process inventors and brothers Auguste and Louis Lumière began working on in the 1890s and commercially introduced in 1907. It was based on one of Louis Duclos du Haroun's ideas: instead of taking three separate photographs through color filters, take one through a mosaic of tiny color filters overlaid on the emulsion and view the results through an identical mosaic. If the individual filter elements were small enough, the three primary colors of red, blue, and green would blend together in the eye and produce the same additive color synthesis as the filtered projection of three separate photographs.

A color portrait of Mark Twain by Alvin Langdon Coburn, 1908, made by the recently introduced Autochrome process

Autochrome plates had an integral mosaic filter layer with roughly five million previously dyed potato grains per square inch added to the surface. Then through the use of a rolling press, five tons of pressure were used to flatten the grains, enabling every one of them to capture and absorb color and their microscopic size allowing the illusion that the colors are merged. The final step was adding a coat of the light-capturing substance silver bromide, after which a color image could be imprinted and developed. In order to see it, reversal processing was used to develop each plate into a transparent positive that could be viewed directly or projected with an ordinary projector. One of the drawbacks of the technology was an exposure time of at least a second in bright daylight, with the time required quickly increasing in poor light. An indoor portrait required several minutes with the subject stationary. This was because the grains absorbed color fairly slowly, and a filter of a yellowish-orange color was required to keep the photograph from coming out excessively blue. Although necessary, the filter had the effect of reducing the amount of light that was absorbed. Another drawback was that the image could only be enlarged so much before the many dots that made up the image would become apparent.

Competing screen plate products soon appeared, and film-based versions were eventually made. All were expensive, and until the 1930s none was "fast" enough for hand-held snapshot-taking, so they mostly served a niche market of affluent advanced amateurs.

A new era in color photography began with the introduction of Kodachrome film, available for 16 mm home movies in 1935 and 35 mm slides in 1936. It captured the red, green, and blue color components in three layers of emulsion. A complex processing operation produced complementary cyan, magenta, and yellow dye images in those layers, resulting in a subtractive color image. Maxwell's method of taking three separate filtered black-and-white photographs continued to serve special purposes into the 1950s and beyond, and Polachrome, an "instant" slide film that used the Autochrome's additive principle, was available until 2003, but the few color print and slide films still being made in 2015 all use the multilayer emulsion approach pioneered by Kodachrome.

Development of digital photography

Walden Kirsch as scanned into the SEAC computer in 1957

In 1957, a team led by Russell A. Kirsch at the National Institute of Standards and Technology developed a binary digital version of an existing technology, the wirephoto drum scanner, so that alphanumeric characters, diagrams, photographs and other graphics could be transferred into digital computer memory. One of the first photographs scanned was a picture of Kirsch's infant son Walden. The resolution was 176x176 pixels with only one bit per pixel, i.e., stark black and white with no intermediate gray tones, but by combining multiple scans of the photograph done with different black-white threshold settings, grayscale information could also be acquired.

The charge-coupled device (CCD) is the image-capturing optoelectronic component in first-generation digital cameras. It was invented in 1969 by Willard Boyle and George E. Smith at AT&T Bell Labs as a memory device. The lab was working on the Picturephone and on the development of semiconductor bubble memory. Merging these two initiatives, Boyle and Smith conceived of the design of what they termed "Charge 'Bubble' Devices". The essence of the design was the ability to transfer charge along the surface of a semiconductor. It was Dr. Michael Tompsett from Bell Labs however, who discovered that the CCD could be used as an imaging sensor. The CCD has increasingly been replaced by the active pixel sensor (APS), commonly used in cell phone cameras. These mobile phone cameras are used by billions of people worldwide, dramatically increasing photographic activity and material and also fueling citizen journalism.

The web has been a popular medium for storing and sharing photos ever since the first photograph was published on the web by Tim Berners-Lee in 1992 (an image of the CERN house band Les Horribles Cernettes). Since then sites and apps such as Facebook, Flickr, Instagram, Picasa (discontinued in 2016), Imgur and Photobucket have been used by many millions of people to share their pictures.

Camera

From Wikipedia, the free encyclopedia
 
Leica Camera (1950s)
 
Hasselblad 500 C/M with Zeiss lens

A camera is an optical instrument that captures a visual image. At a basic level, cameras are sealed boxes (the camera body) with a small hole (the aperture) that allows light through to capture an image on a light-sensitive surface (usually photographic film or a digital sensor). Cameras have various mechanisms to control how the light falls onto the light-sensitive surface. Lenses focus the light entering the camera, and the size of the aperture can be widened or narrowed. A shutter mechanism determines the amount of time the photosensitive surface is exposed to light.

The still image camera is the main instrument in the art of photography. Captured images may be reproduced later as a part of the process of photography, digital imaging, or photographic printing. Similar artistic fields in the moving image camera domain are film, videography, and cinematography.

The word camera comes from camera obscura, the Latin name of the original device for projecting an image onto a flat surface (literally translated to "dark chamber"). The modern photographic camera evolved from the camera obscura. The first permanent photograph was made in 1825 by Joseph Nicéphore Niépce.

Mechanics

Basic elements of a modern digital single-lens reflex (SLR) still camera

A camera captures light photons, usually from the visible spectrum, but could also capture other portions of the electromagnetic spectrum.

All cameras use the same basic design: light enters an enclosed box through a converging or convex lens and an image is recorded on a light-sensitive medium. A shutter mechanism controls the length of time that light enters the camera.

Most cameras also have a viewfinder, which shows the scene to be recorded and allows the ability to control focus and exposure.

Exposure control

Aperture

Different apertures of a lens

The aperture, sometimes called the diaphragm or iris, is the opening through which light enters the camera. Typically located in the lens, this opening can widen or narrow to alter the amount of light that strikes the film. The aperture is controlled by the movements of overlapping plates or blades that rotate together or apart, which function to shrink or expand the hole (aperture) at the center. The diameter of the aperture can be set manually, typically by adjusting a dial on the camera body or lens. Automatic adjustments can also occur based on calculations influenced by an internal light meter.

The size of the opening is set at standard increments, typically called f-stops (but also f-numbers, stop numbers, or simply steps or stops), that usually range from f/1.4 to f/32 in standard increments: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and 32. As the numbers increase, each increment halves the amount of light entering the camera. Conversely, the lower the number, the larger the opening, and the more light enters the camera.

The wider opening at lower f-stops narrows the range of focus so the image background is blurry when the foreground is in focus, and vice versa. This depth of field increases as the aperture closes. As a result, objects at differing distances from the camera can both be in focus; when the aperture is at its narrowest, the foreground and background are both in sharp focus.

Shutter

The shutter, along with the aperture, is one of two ways to control the amount of light entering the camera. The shutter determines the duration that the light-sensitive surface is exposed to light. The shutter opens, light enters the camera and exposes the film or sensor to light, and then the shutter closes.

There are two types of mechanical shutters. The leaf-type uses a circular iris diaphragm maintained under spring tension inside or just behind the lens that rapidly opens and closes when the shutter is released.

A focal-plane shutter. In this shutter, the metal shutter blades travel vertically.

More commonly, a focal-plane shutter is used. This shutter operates close to the film plane and employs metal plates or cloth curtains with an opening that passes across the light-sensitive surface. The curtains or plates have an opening that is pulled across the film plane during exposure. The focal-plane shutter is typically used in single-lens reflex (SLR) cameras, since covering the film (rather than blocking the light passing through the lens) allows the photographer to view the image through the lens at all times, except during the exposure itself. Covering the film also facilitates removing the lens from a loaded camera (many SLRs have interchangeable lenses).

A digital camera may use a mechanical or electronic shutter, the latter of which is common in smartphone cameras. Electronic shutters either record data from the entire sensor at the same time (a global shutter) or record the data line by line across the sensor (a rolling shutter).

In movie cameras, a rotary shutter opens and closes in sync with the advancement of each frame of film.

The duration is called the shutter speed or exposure time. The longer the shutter speed, the slower it is. Typical exposure times can range from one second to 1/1,000 of a second, though longer and shorter durations are not uncommon. In the early stages of photography, exposures were often several minutes long. These long exposure times often resulted in blurry images, as a single object is recorded in multiple places across a single image for the duration of the exposure. To prevent this, shorter exposure times can be used. Very short exposure times can capture fast-moving action and eliminate motion blur.

Like aperture settings, exposure times increment in powers of two. The two settings determine the exposure value (EV), a measure of how much light is recorded during the exposure. There is a direct relationship between the exposure times and aperture settings so that if the exposure time is lengthened one step but the aperture opening is also narrowed one step, then the amount of light that contacts the film or sensor is the same.

Metering

A handheld digital light meter showing an exposure of 1/200th at an aperture of f/11, at ISO 100. The light sensor is on top, under the white diffusing hemisphere.

In most modern cameras, the amount of light entering the camera is measured using a built-in light meter or exposure meter. Taken through the lens (called TTL metering), these readings are taken using a panel of light-sensitive semiconductors. They are used to calculate optimal exposure settings. These settings are typically determined automatically as the reading is used by the camera's microprocessor. The reading from the light meter is incorporated with aperture settings, exposure times, and film or sensor sensitivity to calculate the optimal exposure.

Light meters typically average the light in a scene to 18% middle gray. More advanced cameras are more nuanced in their metering, weighing the center of the frame more heavily (center-weighted metering), considering the differences in light across the image (matrix metering), or allowing the photographer to take a light reading at a specific point within the image (spot metering).

Lens

The lens of a camera captures light from the subject and focuses it on the sensor. The design and manufacturing of the lens are critical to photo quality. A technological revolution in camera design during the 19th century modernized optical glass manufacturing and lens design. This contributed to the modern manufacturing processes of a wide range of optical instruments such as reading glasses and microscopes. Pioneering companies include Zeiss and Leitz.

Camera lenses are made in a wide range of focal lengths. Examples include extreme wide angle, standard, and medium telephoto. Lenses either have a fixed focal length (prime lens) or a variable focal length (zoom lens). Each lens is best suited to certain types of photography. Extreme wide-angles might be preferred for architecture due to their ability to capture a wide view of buildings. Standard lenses commonly have a wide aperture, and because of this, they are often used for street and documentary photography. The telephoto lens is useful in sports and wildlife, but is more susceptible to camera shake, which might cause motion blur.

Focus

An image of flowers, with one in focus. The background is out of focus.
The distance range in which objects appear clear and sharp, called depth of field, can be adjusted by many cameras. This allows for a photographer to control which objects appear in focus, and which do not.

Due to the optical properties of a photographic lens, only objects within a limited range of distance from the camera will be reproduced clearly. The process of adjusting this range is known as changing the camera's focus. There are various ways to accurately focus a camera. The simplest cameras have fixed focus and use a small aperture and wide-angle lens to ensure that everything within a certain range of distance from the lens, usually around 3 meters (10 ft.) to infinity, is in reasonable focus. Fixed focus cameras are usually inexpensive types, such as single-use cameras. The camera can also have a limited focusing range or scale-focus that is indicated on the camera body. The user will guess or calculate the distance to the subject and adjust the focus accordingly. On some cameras, this is indicated by symbols (head-and-shoulders; two people standing upright; one tree; mountains).

Rangefinder cameras allow the distance to objects to be measured employing a coupled parallax unit on top of the camera, allowing the focus to be set with accuracy. Single-lens reflex cameras allow the photographer to determine the focus and composition visually using the objective lens and a moving mirror to project the image onto a ground glass or plastic micro-prism screen. Twin-lens reflex cameras use an objective lens and a focusing lens unit (usually identical to the objective lens) in a parallel body for composition and focus. View cameras use a ground glass screen which is removed and replaced by either a photographic plate or a reusable holder containing sheet film before exposure. Modern cameras often offer autofocus systems to focus the camera automatically by a variety of methods.

Experimental cameras such as the planar Fourier capture array (PFCA) do not require focusing to take pictures. In conventional digital photography, lenses or mirrors map all of the light originating from a single point of an in-focus object to a single point at the sensor plane. Each pixel thus relates an independent piece of information about the far-away scene. In contrast, a PFCA does not have a lens or mirror, but each pixel has an idiosyncratic pair of diffraction gratings above it, allowing each pixel to likewise relate an independent piece of information (specifically, one component of the 2D Fourier transform) about the far-away scene. Together, complete scene information is captured and images can be reconstructed by computation.

Some cameras support post-focusing. Post focusing refers to taking photos that are later focused on a computer. The camera uses many tiny lenses on the sensor to capture light from every camera angle of a scene, which is known as plenoptic technology. A current plenoptic camera design has 40,000 lenses working together to grab the optimal picture.

Image capture on film

Traditional cameras capture light onto photographic plates or photographic film. Video and digital cameras use an electronic image sensor, usually a charge-coupled device (CCD) or a CMOS sensor to capture images which can be transferred or stored in a memory card or other storage inside the camera for later playback or processing.

A wide range of film and plate formats have been used by cameras. In the early history plate sizes were often specific for the make and model of cameras although there quickly developed some standardization for the more popular cameras. The introduction of roll film drove the standardization process still further so that by the 1950s only a few standard roll films were in use. These included 120 films providing 8, 12 or 16 exposures, 220 films providing 16 or 24 exposures, 127 films providing 8 or 12 exposures (principally in Brownie cameras) and 135 (35mm film) providing 12, 20 or 36 exposures – or up to 72 exposures in the half-frame format or bulk cassettes for the Leica Camera range.

For cine cameras, film 35 mm wide and perforated with sprocket holes was established as the standard format in the 1890s. It was used for nearly all film-based professional motion picture production. For amateur use, several smaller and therefore less expensive formats were introduced. 17.5 mm film, created by splitting 35 mm film, was one early amateur format, but 9.5 mm film, introduced in Europe in 1922, and 16 mm film, introduced in the US in 1923, soon became the standards for "home movies" in their respective hemispheres. In 1932, the even more economical 8 mm format was created by doubling the number of perforations in 16 mm film, then splitting it, usually after exposure and processing. The Super 8 format, still 8 mm wide but with smaller perforations to make room for substantially larger film frames, was introduced in 1965.

Film speed (ISO)

Traditionally used to tell the camera the film speed of the selected film on film cameras, film speed numbers are employed on modern digital cameras as an indication of the system's gain from light to numerical output and to control the automatic exposure system. Film speed is usually measured via the ISO 5800 system. The higher the film speed number, the greater the film sensitivity to light, whereas with a lower number, the film is less sensitive to light.

White balance

In digital cameras, there is electronic compensation for the color temperature associated with a given set of lighting conditions, ensuring that white light is registered as such on the imaging chip and therefore that the colors in the frame will appear natural. On mechanical, film-based cameras, this function is served by the operator's choice of film stock or with color correction filters. In addition to using white balance to register the natural coloration of the image, photographers may employ white balance to aesthetic end, for example, white balancing to a blue object to obtain a warm color temperature.

Camera accessories

Flash

A flash provides a short burst of bright light during exposure and is a commonly used artificial light source in photography. Most modern flash systems use a battery-powered high-voltage discharge through a gas-filled tube to generate bright light for a very short time (1/1,000 of a second or less).

Many flash units measure the light reflected from the flash to help determine the appropriate duration of the flash. When the flash is attached directly to the camera—typically in a slot at the top of the camera (the flash shoe or hot shoe) or through a cable—activating the shutter on the camera triggers the flash, and the camera's internal light meter can help determine the duration of the flash.

Additional flash equipment can include a light diffuser, mount and stand, reflector, soft box, trigger and cord.

Other accessories

Accessories for cameras are mainly for care, protection, special effects, and functions.

  • Lens hood: used on the end of a lens to block the sun or other light source to prevent glare and lens flare (see also matte box).
  • Lens cap: covers and protects the lens during storage.
  • Lens adapter: allows the use of lenses other than those for which the camera was designed.
  • Filters: allow artificial colors or change light density.
  • Lens extension tubes allow close focus in macro photography.
  • Care and protection: including camera case and cover, maintenance tools, and screen protector.
  • Camera monitor: provides an off-camera view of the composition with a brighter and more colorful screen, and typically exposes more advanced tools such as framing guides, focus peaking, zebra stripes, waveform monitors (oftentimes as an "RGB parade"), vectorscopes and false color to highlight areas of the image critical to the photographer.
  • Large format cameras use special equipment which includes magnifier loupe, viewfinder, angle finder, focusing rail /truck.
  • Battery and sometimes a charger.
  • Some professional SLR could be provided with interchangeable finders for eye-level or waist-level focusing, focusing screens, eye-cup, data backs, motor-drives for film transportation or external battery packs.
  • Tripod, primarily used for keeping the camera steady while recording video, doing a long exposure, and time-lapse photography.
  • Microscope adapter, an adapter used to connect a camera to a microscope to photograph what the microscope is examining.
  • Cable release, a remote shutter button that can be connected to the camera via a cable to remotely control the shutter, it can be used to lock the shutter open for the desired period. It is also commonly used to prevent camera shake from pressing the built-in camera shutter button.
  • Dew shield – Prevents moisture build-up on the lens.
  • UV filter: can protect the front element of a lens from scratches, cracks, smudges, dirt, dust, and moisture while keeping a minimum impact on image quality.

Primary types

Single-lens reflex (SLR) camera

Nikon D200 digital camera

In photography, the single-lens reflex camera (SLR) is provided with a mirror to redirect light from the lens to the viewfinder prior to releasing the shutter for composing and focusing an image. When the shutter is released, the mirror swings up and away, allowing the exposure of the photographic medium, and instantly returns after the exposure is finished. No SLR camera before 1954 had this feature, although the mirror on some early SLR cameras was entirely operated by the force exerted on the shutter release and only returned when the finger pressure was released. The Asahiflex II, released by Japanese company Asahi (Pentax) in 1954, was the world's first SLR camera with an instant return mirror.

In the single-lens reflex camera, the photographer sees the scene through the camera lens. This avoids the problem of parallax which occurs when the viewfinder or viewing lens is separated from the taking lens. Single-lens reflex cameras have been made in several formats including sheet film 5x7" and 4x5", roll film 220/120 taking 8,10, 12, or 16 photographs on a 120 roll, and twice that number of a 220 film. These correspond to 6x9, 6x7, 6x6, and 6x4.5 respectively (all dimensions in cm). Notable manufacturers of large format and roll film SLR cameras include Bronica, Graflex, Hasselblad, Mamiya, and Pentax. However the most common format of SLR cameras has been 35 mm and subsequently the migration to digital SLR cameras, using almost identical sized bodies and sometimes using the same lens systems.

Almost all SLR cameras use a front-surfaced mirror in the optical path to direct the light from the lens via a viewing screen and pentaprism to the eyepiece. At the time of exposure, the mirror is flipped up out of the light path before the shutter opens. Some early cameras experimented with other methods of providing through-the-lens viewing, including the use of a semi-transparent pellicle as in the Canon Pellix and others with a small periscope such as in the Corfield Periflex series.

Large-format camera

The large-format camera, taking sheet film, is a direct successor of the early plate cameras and remained in use for high-quality photography and technical, architectural, and industrial photography. There are three common types: the view camera, with its monorail and field camera variants, and the press camera. They have extensible bellows with the lens and shutter mounted on a lens plate at the front. Backs taking roll film and later digital backs are available in addition to the standard dark slide back. These cameras have a wide range of movements allowing very close control of focus and perspective. Composition and focusing are done on view cameras by viewing a ground-glass screen which is replaced by the film to make the exposure; they are suitable for static subjects only and are slow to use.

Plate camera

19th-century studio camera with bellows for focusing

The earliest cameras produced in significant numbers were plate cameras, using sensitized glass plates. Light entered a lens mounted on a lens board which was separated from the plate by extendible bellows. There were simple box cameras for glass plates but also single-lens reflex cameras with interchangeable lenses and even for color photography (Autochrome Lumière). Many of these cameras had controls to raise, lower, and tilt the lens forwards or backward to control perspective.

Focusing of these plate cameras was by the use of a ground glass screen at the point of focus. Because lens design only allowed rather small aperture lenses, the image on the ground glass screen was faint and most Photographers had a dark cloth to cover their heads to allow focusing and composition to be carried out more easily. When focus and composition were satisfactory, the ground glass screen was removed and a sensitized plate put in its place protected by a dark slide. To make the exposure, the dark slide was carefully slid out and the shutter opened, and then closed and the dark slide replaced.

Glass plates were later replaced by sheet film in a dark slide for sheet film; adapter sleeves were made to allow sheet film to be used in plate holders. In addition to the ground glass, a simple optical viewfinder was often fitted.

Medium-format camera

Medium-format cameras have a film size between the large-format cameras and smaller 35 mm cameras. Typically these systems use 120 or 220 roll film. The most common image sizes are 6×4.5 cm, 6×6 cm and 6×7 cm; the older 6×9 cm is rarely used. The designs of this kind of camera show greater variation than their larger brethren, ranging from monorail systems through the classic Hasselblad model with separate backs, to smaller rangefinder cameras. There are even compact amateur cameras available in this format.

Twin-lens reflex camera

Twin-lens reflex camera

Twin-lens reflex cameras used a pair of nearly identical lenses, one to form the image and one as a viewfinder. The lenses were arranged with the viewing lens immediately above the taking lens. The viewing lens projects an image onto a viewing screen which can be seen from above. Some manufacturers such as Mamiya also provided a reflex head to attach to the viewing screen to allow the camera to be held to the eye when in use. The advantage of a TLR was that it could be easily focused using the viewing screen and that under most circumstances the view seen in the viewing screen was identical to that recorded on film. At close distances, however, parallax errors were encountered and some cameras also included an indicator to show what part of the composition would be excluded.

Some TLR had interchangeable lenses but as these had to be paired lenses they were relatively heavy and did not provide the range of focal lengths that the SLR could support. Most TLRs used 120 or 220 films; some used the smaller 127 films.

Compact cameras

Instant camera

After exposure, every photograph is taken through pinch rollers inside of the instant camera. Thereby the developer paste contained in the paper 'sandwich' is distributed on the image. After a minute, the cover sheet just needs to be removed and one gets a single original positive image with a fixed format. With some systems, it was also possible to create an instant image negative, from which then could be made copies in the photo lab. The ultimate development was the SX-70 system of Polaroid, in which a row of ten shots – engine driven – could be made without having to remove any cover sheets from the picture. There were instant cameras for a variety of formats, as well as adapters for instant film use in medium- and large-format cameras.

Subminiature camera

Subminiature spy camera

Subminiature cameras were first produced in the nineteenth century and use film significantly smaller than 35mm. The expensive 8×11mm Minox, the only type of camera produced by the company from 1937 to 1976, became very widely known and was often used for espionage (the Minox company later also produced larger cameras). Later inexpensive subminiatures were made for general use, some using rewound 16 mm cine film. Image quality with these small film sizes was limited.

Folding camera

The introduction of films enabled the existing designs for plate cameras to be made much smaller and for the base-plate to be hinged so that it could be folded up, compressing the bellows. These designs were very compact and small models were dubbed vest pocket cameras. Folding roll film cameras were preceded by folding plate cameras, more compact than other designs.

Box camera

9Box cameras were introduced as budget-level cameras and had few, if any controls. The original box Brownie models had a small reflex viewfinder mounted on the top of the camera and had no aperture or focusing controls and just a simple shutter. Later models such as the Brownie 127 had larger direct view optical viewfinders together with a curved film path to reduce the impact of deficiencies in the lens.

Rangefinder camera

Rangefinder camera, Leica c. 1936

As camera lens technology developed and wide aperture lenses became more common, rangefinder cameras were introduced to make focusing more precise. Early rangefinders had two separate viewfinder windows, one of which is linked to the focusing mechanisms and moved right or left as the focusing ring is turned. The two separate images are brought together on a ground glass viewing screen. When vertical lines in the object being photographed meet exactly in the combined image, the object is in focus. A normal composition viewfinder is also provided. Later the viewfinder and rangefinder were combined. Many rangefinder cameras had interchangeable lenses, each lens requiring its range- and viewfinder linkages.

Rangefinder cameras were produced in half- and full-frame 35 mm and roll film (medium format).

Motion picture cameras

A movie camera or a video camera operates similarly to a still camera, except it records a series of static images in rapid succession, commonly at a rate of 24 frames per second. When the images are combined and displayed in order, the illusion of motion is achieved.

Cameras that capture many images in sequence are known as movie cameras or as cine cameras in Europe; those designed for single images are still cameras. However, these categories overlap as still cameras are often used to capture moving images in special effects work and many modern cameras can quickly switch between still and motion recording modes.

A ciné camera or movie camera takes a rapid sequence of photographs on an image sensor or strips of film. In contrast to a still camera, which captures a single snapshot at a time, the ciné camera takes a series of images, each called a frame, through the use of an intermittent mechanism.

The frames are later played back in a ciné projector at a specific speed, called the frame rate (number of frames per second). While viewing, a person's eyes and brain merge the separate pictures to create the illusion of motion. The first ciné camera was built around 1888 and by 1890 several types were being manufactured. The standard film size for ciné cameras was quickly established as 35mm film and this remained in use until the transition to digital cinematography. Other professional standard formats include 70 mm film and 16 mm film whilst amateur filmmakers used 9.5 mm film, 8 mm film, or Standard 8 and Super 8 before the move into digital format.

The size and complexity of ciné cameras vary greatly depending on the uses required of the camera. Some professional equipment is very large and too heavy to be handheld whilst some amateur cameras were designed to be very small and light for single-handed operation.

Professional video camera

Arri Alexa, a digital movie camera

A professional video camera (often called a television camera even though the use has spread beyond television) is a high-end device for creating electronic moving images (as opposed to a movie camera, that earlier recorded the images on film). Originally developed for use in television studios, they are now also used for music videos, direct-to-video movies, corporate and educational videos, marriage videos, etc.

These cameras earlier used vacuum tubes and later electronic image sensors.

Camcorders

A camcorder is an electronic device combining a video camera and a video recorder. Although marketing materials may use the colloquial term "camcorder", the name on the package and manual is often "video camera recorder". Most devices capable of recording video are camera phones and digital cameras primarily intended for still pictures; the term "camcorder" is used to describe a portable, self-contained device, with video capture and recording its primary function.

Digital camera

A digital camera (or digicam) is a camera that encodes digital images and videos digitally and stores them for later reproduction. They typically use semiconductor image sensors. Most cameras sold today are digital, and digital cameras are incorporated into many devices ranging from mobile phones (called camera phones) to vehicles.

Digital and film cameras share an optical system, typically using a lens of variable aperture to focus light onto an image pickup device. The aperture and shutter admit the correct amount of light to the imager, just as with film but the image pickup device is electronic rather than chemical. However, unlike film cameras, digital cameras can display images on a screen immediately after being captured or recorded, and store and delete images from memory. Most digital cameras can also record moving videos with sound. Some digital cameras can crop and stitch pictures and perform other elementary image editing.

Consumers adopted digital cameras in the 1990s. Professional video cameras transitioned to digital around the 2000s–2010s. Finally, movie cameras transitioned to digital in the 2010s.

The first camera using digital electronics to capture and store images was developed by Kodak engineer Steven Sasson in 1975. He used a charge-coupled device (CCD) provided by Fairchild Semiconductor, which provided only 0.01 megapixels to capture images. Sasson combined the CCD device with movie camera parts to create a digital camera that saved black and white images onto a cassette tape. The images were then read from the cassette and viewed on a TV monitor. Later, cassette tapes were replaced by flash memory.

In 1986, Japanese company Nikon introduced an analog-recording electronic single-lens reflex camera, the Nikon SVC.

The first full-frame digital SLR cameras were developed in Japan from around 2000 to 2002: the MZ-D by Pentax, the N Digital by Contax's Japanese R6D team, and the EOS-1Ds by Canon. Gradually in the 2000s, the full-frame DSLR became the dominant camera type for professional photography.

On most digital cameras a display, often a liquid crystal display (LCD), permits the user to view the scene to be recorded and settings such as ISO speed, exposure, and shutter speed.

Camera phone

Smartphone with built-in camera

In 2000, Sharp introduced the world's first digital camera phone, the J-SH04 J-Phone, in Japan. By the mid-2000s, higher-end cell phones had an integrated digital camera, and by the beginning of the 2010s, almost all smartphones had an integrated digital camera.

Gene structure

From Wikipedia, the free encyclopedia

Gene structure is the organisation of specialised sequence elements within a gene. Genes contain the information necessary for living cells to survive and reproduce. In most organisms, genes are made of DNA, where the particular DNA sequence determines the function of the gene. A gene is transcribed (copied) from DNA into RNA, which can either be non-coding (ncRNA) with a direct function, or an intermediate messenger (mRNA) that is then translated into protein. Each of these steps is controlled by specific sequence elements, or regions, within the gene. Every gene, therefore, requires multiple sequence elements to be functional. This includes the sequence that actually encodes the functional protein or ncRNA, as well as multiple regulatory sequence regions. These regions may be as short as a few base pairs, up to many thousands of base pairs long.

Much of gene structure is broadly similar between eukaryotes and prokaryotes. These common elements largely result from the shared ancestry of cellular life in organisms over 2 billion years ago. Key differences in gene structure between eukaryotes and prokaryotes reflect their divergent transcription and translation machinery. Understanding gene structure is the foundation of understanding gene annotation, expression, and function.

Common features

he structures of both eukaryotic and prokaryotic genes involve several nested sequence elements. Each element has a specific function in the multi-step process of gene expression. The sequences and lengths of these elements vary, but the same general functions are present in most genes. Although DNA is a double-stranded molecule, typically only one of the strands encodes information that the RNA polymerase reads to produce protein-coding mRNA or non-coding RNA. This 'sense' or 'coding' strand, runs in the 5' to 3' direction where the numbers refer to the carbon atoms of the backbone's ribose sugar. The open reading frame (ORF) of a gene is therefore usually represented as an arrow indicating the direction in which the sense strand is read.

Regulatory sequences are located at the extremities of genes. These sequence regions can either be next to the transcribed region (the promoter) or separated by many kilobases (enhancers and silencers). The promoter is located at the 5' end of the gene and is composed of a core promoter sequence and a proximal promoter sequence. The core promoter marks the start site for transcription by binding RNA polymerase and other proteins necessary for copying DNA to RNA. The proximal promoter region binds transcription factors that modify the affinity of the core promoter for RNA polymerase. Genes may be regulated by multiple enhancer and silencer sequences that further modify the activity of promoters by binding activator or repressor proteins. Enhancers and silencers may be distantly located from the gene, many thousands of base pairs away. The binding of different transcription factors, therefore, regulates the rate of transcription initiation at different times and in different cells.

Regulatory elements can overlap one another, with a section of DNA able to interact with many competing activators and repressors as well as RNA polymerase. For example, some repressor proteins can bind to the core promoter to prevent polymerase binding. For genes with multiple regulatory sequences, the rate of transcription is the product of all of the elements combined. Binding of activators and repressors to multiple regulatory sequences has a cooperative effect on transcription initiation.

Although all organisms use both transcriptional activators and repressors, eukaryotic genes are said to be 'default off', whereas prokaryotic genes are 'default on'. The core promoter of eukaryotic genes typically requires additional activation by promoter elements for expression to occur. The core promoter of prokaryotic genes, conversely, is sufficient for strong expression and is regulated by repressors.


The image above contains clickable links
The structure of a eukaryotic protein-coding gene. Regulatory sequence controls when and where expression occurs for the protein coding region (red). Promoter and enhancer regions (yellow) regulate the transcription of the gene into a pre-mRNA which is modified to remove introns (light grey) and add a 5' cap and poly-A tail (dark grey). The mRNA 5' and 3' untranslated regions (blue) regulate translation into the final protein product.
 

An additional layer of regulation occurs for protein coding genes after the mRNA has been processed to prepare it for translation to protein. Only the region between the start and stop codons encodes the final protein product. The flanking untranslated regions (UTRs) contain further regulatory sequences. The 3' UTR contains a terminator sequence, which marks the endpoint for transcription and releases the RNA polymerase. The 5’ UTR binds the ribosome, which translates the protein-coding region into a string of amino acids that fold to form the final protein product. In the case of genes for non-coding RNAs the RNA is not translated but instead folds to be directly functional.

Eukaryotes

The structure of eukaryotic genes includes features not found in prokaryotes. Most of these relate to post-transcriptional modification of pre-mRNAs to produce mature mRNA ready for translation into protein. Eukaryotic genes typically have more regulatory elements to control gene expression compared to prokaryotes. This is particularly true in multicellular eukaryotes, humans for example, where gene expression varies widely among different tissues.

A key feature of the structure of eukaryotic genes is that their transcripts are typically subdivided into exon and intron regions. Exon regions are retained in the final mature mRNA molecule, while intron regions are spliced out (excised) during post-transcriptional processing. Indeed, the intron regions of a gene can be considerably longer than the exon regions. Once spliced together, the exons form a single continuous protein-coding regions, and the splice boundaries are not detectable. Eukaryotic post-transcriptional processing also adds a 5' cap to the start of the mRNA and a poly-adenosine tail to the end of the mRNA. These additions stabilise the mRNA and direct its transport from the nucleus to the cytoplasm, although neither of these features are directly encoded in the structure of a gene.

Prokaryotes

The overall organisation of prokaryotic genes is markedly different from that of the eukaryotes. The most obvious difference is that prokaryotic ORFs are often grouped into a polycistronic operon under the control of a shared set of regulatory sequences. These ORFs are all transcribed onto the same mRNA and so are co-regulated and often serve related functions. Each ORF typically has its own ribosome binding site (RBS) so that ribosomes simultaneously translate ORFs on the same mRNA. Some operons also display translational coupling, where the translation rates of multiple ORFs within an operon are linked. This can occur when the ribosome remains attached at the end of an ORF and simply translocates along to the next without the need for a new RBS. Translational coupling is also observed when translation of an ORF affects the accessibility of the next RBS through changes in RNA secondary structure. Having multiple ORFs on a single mRNA is only possible in prokaryotes because their transcription and translation take place at the same time and in the same subcellular location.

The operator sequence next to the promoter is the main regulatory element in prokaryotes. Repressor proteins bound to the operator sequence physically obstructs the RNA polymerase enzyme, preventing transcription. Riboswitches are another important regulatory sequence commonly present in prokaryotic UTRs. These sequences switch between alternative secondary structures in the RNA depending on the concentration of key metabolites. The secondary structures then either block or reveal important sequence regions such as RBSs. Introns are extremely rare in prokaryotes and therefore do not play a significant role in prokaryotic gene regulation.

 

Operator (computer programming)

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Operator_(computer_programmin...