help me.....please

[مـــــــهــــات]

سلاموووووووو
انا محتاجه مساعدتكم
لا تتخلو عني..........أبي موضوع باللغة الإنجليزية يتكلم عن اي شي بس المهم يكون شامل عن الموضووع نفسه من اكثر الجوانب
مثل: الانترنت ، تكنلوجيا، عن دوله معينه .......any thing
محتاجه الموضوع للإختبار .....والاختبار يوم الاربعاء الجاي

واللي يقدر باسرع وقت يرسلي الموضووع

اكووون له من الشاكرين

تحياتي.......

مــــهـــــات

[Petals]

عندي موسوعة تقدم مواضيع أولية ومن ثم تتوسع في الموضوع

بما انك خيرتينا فحيرتينا فاخترت ليكي موضوعين الأول عن التصوير.. لو في بالك موضوع معين اكتبيه وأدور عليه في إنكارتا...اختصري بحسب رأس الفقرة...

Photography
I INTRODUCTION

Photography, method of picture making developed in the early 19th century, based on principles of light, optics, and chemistry. The word photography comes from Greek words and means “drawing with light.” Photographs serve as scientific evidence, conveyers of news, historical documents, works of art, and records of family life. Millions of people around the world own cameras and enjoy taking pictures; every year more than 10 billion exposures are made with still cameras.

This article discusses how photographs are produced using film, cameras, and lenses. It also outlines techniques of modern photography, such as filtration and electronic flash, and surveys how photographic technologies have evolved since the medium's invention. For information on the history of photography and its artistic practice, see History of Photography. For information on motion picture technology and history, see Motion Pictures; History of Motion Pictures.

II GENERAL PRINCIPLES

Light is the most essential ingredient in photography. Nearly all forms of photography are based on the fact that certain chemicals are photosensitive—that is, they change in some way when exposed to light. Photosensitive materials abound in nature; plants that close their blooms at night are one example. The films used in photography depend on a limited number of chemical compounds that darken when exposed to light. The compounds most widely used today are silver halide crystals, which are salts consisting of silver and chemicals called halogens (usually bromine, chlorine, or iodine).

For the purpose of producing a photograph, these silver salts are distributed in gelatin to make a mixture called an emulsion, which is applied to film or another supporting material in a thin layer. When the emulsion is exposed to light, the silver halide crystals undergo chemical changes and, after further processing, an image becomes visible. The stronger the light that strikes the crystals, the denser or more opaque that part of the film becomes. Most types of film produce a negative image, from which a positive final copy can be printed on sensitized paper. The dense (or dark) areas of the negative translate into light areas on the final photograph. Almost all modern photography relies on this negative-to-positive process.

In most cases the camera and its lens determine the appearance of the photographic image. Cameras work on the basic principle of the camera obscura, a device that artists once used to project a temporary image of something they wanted to draw. In both the camera obscura and the modern camera, light passes through a lens fitted into an otherwise lightproof box. Light passing through the lens casts an image of the camera’s subject—the object, person, or scene in front of the camera—onto the inside of the box, which in a modern camera contains film. The camera and lens control how much light strikes the film in what is called an exposure.

The purpose of the lens is refraction, the bending of light. The camera’s glass or plastic lens bends the light rays reflected from the subject so that these rays cross and reappear upside-down on the other side of the lens. The area where they re-form an image of the subject inside the camera is called the plane of focus. The photographer, or an automatic mechanism in some cameras, must adjust the distance between the lens and the film so that the plane of focus falls exactly where the film lies, making the resulting image appear in focus.

Various types of lenses admit different amounts of light and permit different angles of view. Lenses that take in a wide angle of view make the subject seem farther away; lenses that take in a narrow angle make the subject seem magnified. The photographer can switch a modern zoom lens from wide to narrow angles of view by turning a collar or pressing a button.

The amount of light that a lens allows to fall on the film is controlled by a lens diaphragm, a mechanism built of overlapping metal blades. The diaphragm controls the size of the aperture, or circular opening of the lens. A device called a shutter controls how long light strikes the film; the shutter speed can range from a small fraction of a second (1/1000 or less) to minutes or even hours.

The combination of choices that a photographer makes—film type, camera size, focus, angle of view, lens aperture, shutter speed—influences the appearance of the photograph as much as the choice of subject and the time of day. To take one example, thousands of people have stood in the same spot to take photographs of the Grand Canyon over the years, but their photographs look different because the photographers made different choices with these controls.

III PHOTOGRAPHIC FILMS

Modern film consists of a transparent material, usually acetate, which has been coated with one or more light-sensitive emulsions. It is available in a variety of shapes and sizes determined by the format of the camera. Typical formats are 35-millimeter and 6-centimeter roll films, 4-by-5 and 8-by-10 inch sheet films, and most recently, Advanced Photo System (APS), a type of roll film that incorporates various conveniences for amateur photographers. Within each film format there are a range of film types (black and white, color print, or color transparency) and sensitivity levels, called film speeds, that are appropriate for different lighting conditions.

A A Brief History of Film

Scientists recognized the photosensitivity of certain silver compounds, particularly silver nitrate and silver chloride, during the 18th century. In the early 19th century English scientists Thomas Wedgwood and Sir Humphry Davy used silver nitrate in an attempt to transfer a painted image onto leather or paper. While they succeeded in producing a negative image, it was not permanent; the entire surface blackened after continued exposure to light.

A French inventor, Joseph Nicéphore Niépce, is credited with having made the first successful photograph in 1826. He achieved this by placing a pewter plate coated with bitumen, another light-sensitive material, in the back of a camera obscura. Niépce later switched from pewter to copper plates and from bitumen to silver chloride. French painter Louis Jacques Mandé Daguerre continued Niépce’s pioneering work and in 1839, after Niépce's death, announced an improved version of the process, which he called the daguerreotype.

The daguerreotype process produced a detailed, positive image on a shiny copper plate small enough to be held in the hand. Daguerreotypes remained popular through the 1850s, but were eventually replaced by a negative/positive process. English inventor William Henry Fox Talbot devised this process and perfected it in the 1840s. Talbot’s process produced a paper negative, from which he could produce any number of paper positives. He exposed silver-sensitized paper briefly to light and then treated it with other chemicals to produce a visible image. Beginning in 1850 glass replaced paper as a support for the negative, and the silver salts were suspended in collodion, a thick liquid. The smooth glass negatives could produce sharper images than paper ones, because the details were no longer lost in the texture of the paper. This refinement became known as the wet collodion process.

Because the wet collodion (or wet plate) process required photographers to coat the glass support just before taking a picture, experimenters sought a dry version of the same process. Dry plates, pieces of glass coated in advance with an emulsion of gelatin and silver bromide, were invented in 1878. A few years later American inventor George Eastman devised a flexible version of this system, a long paper strip that could replace the glass plate. In 1889 he improved on this by using a type of plastic called celluloid instead of paper, producing the first photographic film. Eastman's invention paved the way for all modern films, which are made of acetate or polyester, plastics that are less flammable than celluloid.

Except for some isolated experiments, color films were not invented until the 20th century. The first commercially successful material for making color photographs, called Autochrome, became available in 1907 and was based on a process devised by French inventors Auguste and Louis Lumière. But the era of color photography did not really begin until the advent of Kodachrome color film in 1935 and Agfacolor in 1936. Both of these films produced positive color transparencies, or slides. The Kodak company introduced Kodacolor film for color negatives in 1942, which gave amateurs the same negative/positive process they had long enjoyed in black and white.

B How Film Works

To understand how film works, it is first necessary to understand a few things about light. Light is the visible portion of a broad range of energy called electromagnetic radiation, which also includes invisible energy in the form of radio waves, gamma rays, X rays, and infrared and ultraviolet radiation. The narrow band of electromagnetic waves that the human eye can detect is called the visible spectrum, which we see as colors. Our eyes perceive the longest wavelengths as red, the shortest as violet, with orange, yellow, green, and blue in between. (A rainbow or a prism shows all the colors of the visible spectrum.)

B1 Dyes and Emulsions

Photographic films vary in the way they react to different wavelengths of visible light. Early black-and-white films were sensitive to only the shorter wavelengths of the visible spectrum, primarily to light perceived as blue. So, for example, in a picture of blue, red, and orange flowers, the blue flowers would appear too light, whereas the red and orange flowers would look unrealistically dark. To correct this, specialized compounds called dye sensitizers were incorporated into the emulsion.

Today, with a few specialized exceptions, films are sensitive to all colors of the visible spectrum. Even black-and-white films record colors as different shades of gray. Most color films are coated with three emulsions, typically cyan (a greenish blue), yellow, and magenta (a purplish red). Each emulsion responds to only one color of light and is coupled with a dye layer, which produces the actual color that resembles what the eye sees. Other layers act as filters to screen the light these emulsions receive, and to prevent light from scattering within the film.

Color transparency films produce a positive color image for viewing with the help of a slide projector or an illuminated surface called a light table. These films are also known as reversal films because the initial developed image is chemically reversed during processing, turning what would otherwise be a negative image into a positive one. The dyes in some brands of transparency film are added during development; in others, they are built into the film itself.

Color negative films, also known as print films, produce positive prints. As with some transparency films, dyes built into the emulsion chemically react with the silver salts that form the image. The colors on the processed negative are the complements of the colors in the original scene. For instance, if you took a picture of the Ecuadorian flag, which is red, yellow, and blue, the colored dyes on the negative would be a blue-green color called cyan (the complement of red), blue (the complement of yellow), and yellow (the complement of blue). When light shines through this negative onto color-sensitive print paper, the colors return to positive form, and the print shows a flag properly striped with red, yellow, and blue.

One type of black-and-white film, called chromogenic film, makes use of color-film technology to produce a negative that has just a single dye layer. When exposed to conventional black-and-white photo paper, the negative provides an image nearly identical to that of conventional black-and-white film. But when exposed to paper for printing color photographs it produces an image composed of different shades of a single color.

B2 Positive/Negative Development

When film is processed in a chemical agent called a developer, large particles of metallic silver form in areas of the film that were exposed to light. Exposure to lots of light causes many particles to form, while exposure to dim light or exposure for a very short time causes just a few particles to form. The resulting image produced on the film is called a negative because the tonal values of the subject photographed are reversed; areas in the subject that were dark appear light on the negative, and areas that were bright appear dark. The tonal values of the negative are reversed again in the printing process—or in the case of transparencies (slides), during the development of the film—creating a positive image.

Chromogenic color films, in which the dye is built in, exhibit dye images rather than silver images, although silver is also essential to the process. During processing, the chemical action of the developer creates initial images in metallic silver, just as in black-and-white processing. But in color processing the developer also stimulates dye couplers (chemicals that react to a specific color of light and cause corresponding dyes to be released) to form cyan, magenta, and yellow dye images. The silver is then removed, leaving a negative image in the three colors. Different combinations of those colors create the more complex colors visible on the final print. In color transparency films, unexposed silver halide crystals that are not converted to metallic silver and washed away during the initial development remain to be converted during a second development. As these remaining silver halides are converted to metal, they again combine with dye couplers to form the final color image, before the second layer of metallic silver is also washed away.

Photographic print papers are constructed much like films, but generally require fewer layers. The so-called paper support (today, commonly made of plastic or paper) is coated with a light-sensitive emulsion, just as films are. Black-and-white papers have a single layer of emulsion; color papers have at least three layers. When these papers are exposed to light shone through a negative, the end result is a positive.

C Film Characteristics

Certain characteristics help people determine which film will work best in a particular situation. Films may vary in their sensitivity to different kinds of light and in their ability to record fine details or quickly moving subjects.

C1 Sensitivity and Color Balance

Most films now in use are panchromatic, meaning that they respond to all colors of light and can record each color’s relative strength with a fair degree of accuracy. Color films also must be designed to respond to the specific quality or energy of light illuminating the scene, which may be outdoor sunlight, incandescent lamps, or electronic flash.

Each of these kinds of light has a distinct characteristic referred to as color temperature. While the theory of color temperature is complicated, the practical concept is simple: color films are balanced to perform best in specific lighting conditions. So-called daylight films, the most widely used, are designed for both outdoor photography and pictures taken indoors with electronic flash. Tungsten films are designed to be used indoors without flash, specifically with certain types of bulbs manufactured for such situations called photofloods.

Distinguishing between daylight and tungsten film types is important mainly with transparency or slide films, which produce direct positive images that cannot be altered. The color in print films, which produce negatives, can be adjusted during printing to compensate for different lighting conditions. Nonetheless, because print films are balanced for daylight, pictures from them often have an orange cast when taken indoors without flash. All color films will produce pictures with unpleasant green or purple casts when taken indoors under fluorescent light, as in an office. For more information about eliminating color casts, see the Filtration section of this article.

C2 Exposure Latitude

In any lighting situation there is an optimal exposure that will produce a perfect image on film. Film exposed to light for a longer than optimal time is said to be overexposed and produces prints that look bleached out and blurred. Too short an exposure and the image is underexposed, which shows most visibly as insufficient contrast between dark and light.

Every film has a characteristic exposure latitude, a range of settings within which it can accurately render the color and tonal values (contrasts of light and dark) of the subject photographed. With films that have a narrow exposure latitude, the margin for error is small; an exposure adjusted for a shady area is likely to result in overexposure of adjacent sunny areas. The wider a film's latitude, the greater its ability to provide satisfactory prints or slides in a range of lighting conditions.

Films that produce negatives generally offer much greater latitude than transparency films. In addition, many high-speed films have a greater exposure latitude than slower films. Staying within a given film’s exposure latitude can ensure an acceptable range of tones in the picture. But to achieve the best-possible image quality, including full detail throughout the picture, the exposure time and aperture size need to be precisely set to fit the lighting conditions.

C3 Speed and Grain

Film is also classified by speed, a rating that provides a measure of the film’s sensitivity to light. For each film, this rating determines the amount of exposure required to photograph a subject under a given lighting condition. The manufacturer of the film assigns it a standardized numerical rating known as the ISO number (ISO stands for the International Standards Organization). High ISO numbers correspond to highly light-sensitive, fast films, and low numbers to less sensitive, slow films.

Today, slow-speed films typically have a rating between ISO 25 and ISO 100, but films that are even slower exist. Films in the ISO 125 to ISO 200 range are considered medium speed, while films above ISO 200 are considered fast. A photographer can push the limits of a film by overriding the recommended exposure for that film speed and shortening the exposure time. With some cameras the photographer will need to manually adjust the ISO number; with other cameras, setting an exposure compensation dial will trick the camera into making this adjustment for you. The photographer must then make sure that the development time is lengthened to compensate for the underexposure.

Whether fast or slow, all films exhibit a pattern called grain. Film grain is the visible trace of the metallic silver that forms the image. The individual grains of silver are generally larger and more obvious in faster film than in slower film. For this reason, photographs taken with slow-speed film appear less grainy, especially when enlarged. Because of the small size of its silver halide grains, slow-speed film generally has a higher resolution—that is, it renders fine details with greater sharpness. Slow-speed film also produces a smoother range of tones and more intense colors than fast film. Despite these advantages, slow films are not as desirable as fast films in certain situations, such as when photographing a rapidly moving subject.

C4 DX Coding

DX coding is a recent innovation in film and camera technology that eliminates the need to set the film speed by hand in the camera's built-in exposure meter. On cartridges of 35-millimeter film, manufacturers print a checkerboard pattern that corresponds to an electronic code. This code tells the camera’s computer the ISO rating of the film as well as the number of frames on the roll. Most cameras with electronic controls are equipped with DX sensors that can read this information and automatically adjust exposures accordingly. The DX code is also placed on the film itself to inform the developing laboratory of this information.

D Color Films in Use Today

A range of color film types is available to photographers. These types include color print films; reversal films, used to make color slides and larger transparencies; Polaroid films, which develop into prints without additional processing; and a number of specialty films such as X-ray and infrared.

D1 Print Films

Color print films, which produce prints through the classic negative-to-positive process, include such brand names as Kodacolor, Fujicolor, and Agfacolor. Ideal for amateur use, they are designed to provide excellent color rendition out of doors and with electronic flash. Each manufacturer supplies its brand in several speeds: ISO 100, 200, and 400 are the most common. Films are available in several sizes, or formats, including the popular 35-millimeter format (in which a single frame of the film is 35 millimeters wide). Manufacturers also offer premium films in most formats, which provide better color and smaller grain size.

D2 Slide Films

Kodachrome, Ektachrome, Fujichrome, and Agfachrome are examples of films that produce 35-millimeter slides and larger transparencies. Both daylight and tungsten versions of these films are generally available. Manufacturers also design films for such specific tasks as slide duplication. Film speeds of slide films commonly range from a very slow ISO 25 to a very fast ISO 3200.

D3 Polaroid

In 1947 American physicist Edwin Herbert Land invented the Polaroid process, a type of photography that produces prints almost immediately after exposure. Although the process takes one or more minutes, it was quickly dubbed instant photography. Today Polaroid films are available in both black-and-white and color, for both special Polaroid cameras and for standard-format cameras (see Polaroid Corporation).

The processing chemicals and conventional silver halide emulsions in instant film are combined in a self-contained paper envelope or within the print itself. A chemical diffusing agent transfers the negative image to the paper, producing a print. Older Polaroid films use a system in which the negative peels away from the final print. Polaroid SX-70 film, on the other hand, has no separate negative, and users can watch the image develop before their eyes.

D4 Infrared, X-ray, and Special Films

Some special-purpose films are sensitive to wavelengths beyond the visible spectrum of light. Infrared film responds to the invisible, infrared portion of the spectrum in addition to visible light. Film manufacturers also design specialized emulsions for medical and scientific films that respond to X rays and other forms of electromagnetic radiation.

IV CAMERAS

The most important tool of photography is the camera itself. Basically, a camera is a lighttight box with a lens on one side and light-sensitive film on the other. Improvements in camera technology over the years have given photographers more control over the quality of their photographs.

A A Brief History of Cameras

Today’s cameras all derive from the 16th-century camera obscura. The earliest form of this device was a darkened room with a tiny hole in one wall. Light entered the room through this hole and projected an upside-down image of the subject onto the opposite wall. Over the course of three centuries the camera obscura evolved into a handheld box with a lens replacing the pinhole and an angled mirror at the back. The mirror reflected an image onto a ground-glass viewing screen on the top of the box. Long before film was invented artists used this device to help them draw more accurately. They placed thin paper onto the viewing screen and could easily trace the reflected image.

The inventors of photography in the early 19th century adapted the camera obscura by adding a device for holding sensitized plates in the back of the box. This kind of camera, with some improvements, was used throughout the 19th century. One notable enhancement for the box, pleated leather sides called bellows, allowed the photographer to easily adjust the distance between the lens and the plane of focus. Professional photographers still use a similar camera today, a large-format camera known as the view camera.

In the 1880s the invention of more sensitive emulsions and better lenses led to the development of lens shutters, devices that could limit the time of exposure to a fraction of a second. At first the shutter was simply a blind dropped in front of the lens by the force of gravity, or by a spring. Later designs featured a set of blades just behind the optical lens. In 1888 George Eastman introduced the first Kodak camera, which used a cylindrical shutter that the photographer turned by pulling a string on the front of the camera. The Kodak was one of the earliest handheld cameras. It made photography available to amateurs for the first time and created a snapshot craze at the turn of the 20th century.

In 1925 the Leitz Company in Germany introduced the Leica, one of the first cameras to use 35-millimeter film, a small-sized film initially designed for motion pictures. Because of its compactness and economy, the Leica and other 35-millimeter cameras became popular with both amateur and professional photographers. All but the earliest Leicas used a focal-plane shutter, located just in front of the film. Because it blocks light from the film even when the lens is removed, the focal-plane shutter allows photographers to switch lenses safely in the middle of a film roll.

B Modern Camera Types

Cameras come in a variety of forms. Whereas cameras once required many decisions on the part of photographers, most of today’s cameras offer a range of automated features that greatly simplify picture taking and reduce the likelihood of error.

B1 Box Cameras

The Eastman Kodak Company introduced one of the first box cameras in 1888, and the simplicity of this easy-to-use design has assured its popularity ever since. Box cameras consist of a rigid box or body; a fixed, simple lens; a viewfinder window, through which the photographer looks to frame the scene; and a shutter with one or possibly two speeds. On most box cameras, the lens is set to an aperture and focus that produce reasonably sharp pictures of a subject at least 2 m (about 6 ft) away, when the camera is used outdoors in the sun. But because these settings are not adjustable, the photographer can do little to control the results.

The modern-day equivalents of the old Kodak box cameras are the disposable cameras now sold at drugstores and tourist shops. These cardboard-covered, plastic cameras come loaded with 35-millimeter color print film. After taking a roll of pictures, the user turns over the entire camera to a processing lab for development. Manufacturers now reuse or recycle many of the parts inside these cameras. Single-use cameras are also available in several advanced models—offering built-in flash, a waterproof body, or the ability to show panoramic views in extra-wide prints.

B2 View Cameras

View cameras are larger and heavier than most amateur cameras but allow for maximum precision in focus, aperture, and framing. They use large-format films, which are able to capture far greater detail than 35-millimeter films. The body configuration of the view camera, unlike that of most general-purpose cameras, is extremely adjustable. It has two independently moveable elements that ride on a track: The front element holds the lens and shutter, the rear holds a ground-glass panel, and the space in between is enclosed in an expandable leather bellows. The photographer frames and focuses the scene that appears in the glass panel at the back, then inserts a film holder in front of the glass, and takes the picture. The gap in time between framing and exposure makes the view camera useless for action shots, but it is ideal for carefully arranged studio shots, landscapes, or architectural photography. The photographer can shift, tilt, raise, or swing the front and rear elements separately, allowing for great variation in perspective and focus.

B3 Rangefinder Cameras

Rangefinder cameras were the first cameras to have an optical viewfinder—that is, a separate, window-like lens through which the photographer sees and frames the subject. The viewfinder is paired with an adjacent window called a rangefinder. To focus the camera, the photographer adjusts a ring or collar until the two views appear as one, at which point the camera has set the focus to precisely match the distance of the subject. Since the viewfinder window does not show the scene through the lens, but only one that closely approximates it, rangefinder cameras can be inaccurate for framing close-up shots.

Rangefinder cameras were once very popular with amateur photographers, but today’s point-and-shoot cameras have largely replaced them. Nevertheless, the modern rangefinder camera works well under certain circumstances, and some professionals still use it. Rangefinders are available in two formats, for use with either 35-millimeter film or the larger format 6-centimeter film. Unlike point-and-shoot cameras, modern rangefinders feature lenses that can be removed from the camera body so that photographers can choose a lens specifically suited to the subject.

B4 Point-and-Shoot Cameras

The most popular camera type today is the point-and-shoot camera. It has a number of automatic features that make it practically foolproof to operate while producing pictures of high quality. Point-and-shoot cameras feature battery-operated electronic systems that may include automatic controls for exposure, focusing, flash, film winding, and film rewinding. They are available with a fixed single-focal-length lens or a zoom lens; the lenses cannot be removed from the body. The cameras work with all types of 35-millimeter film; some also use a newer film type called Advanced Photo System (APS). (For more information, see the Recent Developments: APS section of this article.)

B5 Single-Lens-Reflex Cameras

With the single-lens-reflex (SLR) camera, the photographer uses a single lens for both viewing the scene and taking the picture. Light comes through the lens onto a mirror, which then reflects it through a five-sided prism into the viewfinder. The mirror is hinged; at the moment the photographer snaps the picture, a spring automatically pulls the mirror out of the path between lens and film. Because of this system, the image recorded on the film is almost exactly what the photographer sees in the viewfinder, a great advantage in many picture-taking situations.

Most SLRs are precision electronic instruments equipped with fast focal-plane shutters, precise automatic exposure systems, and built-in flash controls. Increasingly, camera manufacturers are producing SLRs with automatic focusing, an innovation originally reserved for less sophisticated cameras.

C Modern Camera Features

Modern cameras feature several components to help photographers control their results under widely varying conditions. In today’s cameras many of these features are automated.

C1 Viewfinders

A viewfinder enables photographers to frame their subject the way they would like it to appear in the finished photograph. Some viewfinders consist of a simple window on top of the camera that only approximates the view through the lens. A more complex and more accurate viewfinding system is the single-lens-reflex system, described above.

C2 Shutters

The shutter, a spring-activated mechanical device, keeps light from entering the camera except during the interval of exposure. Most modern cameras have focal-plane or leaf shutters. The focal-plane shutter consists of a black shade with a variable-size slit across its width. When released, the shade moves quickly across the film, exposing it progressively as the slit moves. In the leaf shutter, at the moment of exposure, a cluster of meshed blades springs apart to uncover the full lens aperture and then springs shut.

C3 Built-in Meters and Automatic Exposure

For early photographers, setting the correct aperture and shutter speed for an exposure was essentially an educated guess. But with the development of handheld photoelectric exposure meters in the 1930s, photographers were able to take precise readings of the light level and adjust the exposure accordingly. By the 1960s camera companies had begun to build exposure meters right into the camera body; such systems typically required the user to center a needle over a pointer inside the viewfinder. In the 1980s this process became automated: With built-in electronics, the camera could adjust itself to produce an appropriate exposure. Today all but the most inexpensive cameras feature such a system of automatic exposure.

C4 Autofocusing

Autofocus cameras use electronics and a small computer processor to automatically sample the distance between camera and subject and from this determine the exact plane of focus. The computer then signals a small mechanism that turns the lens barrel to this point.

There are two widely used methods for determining the focus automatically, called active and passive. An active autofocus system, used in most point-and-shoot cameras, emits either an infrared light beam or high-energy (ultrasonic) sound waves. When the light or sound waves bounce off the subject and return to the camera, they give an accurate reading of the distance to that subject. Passive systems, used in more sophisticated cameras, automatically adjust the focus of the lens until sensors detect that maximum contrast has been reached inside a rectangular target at the center of the focusing screen. The point of maximum contrast corresponds to the point of greatest sharpness.

Neither method is foolproof. If the primary subject is off to one side of the frame, for example, most autofocusing systems will ignore it. Active systems can be fooled by window glass, which interrupts their beams. Passive systems require a certain amount of detail—usually there must be discernable lines present in the target zone for this system to determine maximum contrast in the subject. A passive system would have trouble setting the correct focus, for instance, for a photograph in which the plain white sails of a boat took up the center of the frame.

C5 Film Loading and Transport

Most people today buy film in the form of lighttight cartridges or cassettes that they can insert into the camera in daylight; only professional photographers using sheet films still need to load their cameras in the dark. With 35-millimeter film, the user attaches a leader extending from the cartridge to a spool at one side of the camera, then drops the cartridge into a slot on the other side. Automatic cameras wind the film into position when the back is closed and rewind the exposed film into the cartridge when all exposures have been taken. With older cameras, the user must use a crank to rewind the film.

Most cameras now automatically advance the film to the next frame after an exposure has been made. Some cameras come with a motor drive, a more rapid way of advancing the film. Motor drives allow the photographer to snap a sequence of exposures in rapid succession while holding a finger on the shutter-release button; as many as three to five pictures per second can be taken this way.

V LENSES

The lens is the eye of the camera. Its function is to bring light from the subject into focus on the film. A camera can have a single lens or a complex set of lenses. Together with the shutter, the lens controls the amount of light that enters the camera.

A A Brief History of Lenses

The modern camera’s predecessor, the camera obscura, consisted of a simple pinhole in the side of a room or box. In the 17th century people discovered they could produce a brighter, sharper image by fitting a camera obscura with a convex (outward-curving) lens. The first such lens came from a pair of eyeglasses. Over the next 300 years, interest in telescopes and microscopes led to the development of better and brighter lenses.

With the invention of photography in the 19th century, the need for camera-specific lenses increased, leading to rapid developments in the field of lens making. These developments took place along two fronts: The first was the invention of new types of glass that refracted light more effectively; and the second was the discovery of ways to combine several pieces of glass, or elements, to control optical distortion.

Quality modern lenses are made of many individual elements of ground and polished glass (6 to 14 elements is common). These elements, each of a different shape and purpose, are cemented into groups; each group is then assembled in what is called a lens barrel. On a manually controlled camera, the lens barrel incorporates an aperture ring and a focusing ring. By turning the aperture ring, the photographer adjusts the opening of the lens diaphragm, which determines how much light reaches the film. The focusing ring is used to focus the image on the film plane by changing the distance between the element groups.

B Focal Lengths

Camera lenses are categorized according to their focal lengths and maximum apertures. The longer the focal length, the larger the image inside the camera will be. The greater the size of the aperture, the more light the lens will admit. Focal length is the distance from the optical center of the lens to the image formed inside the camera. Because this distance varies depending on how the camera is focused, focal length ratings are defined by measuring the distance when the focusing ring is set for photographing a distant subject (indicated on the focusing ring with the symbol ∞, called infinity). A lens with a short focal length is commonly called a wide-angle lens; with a long focal length, a telephoto lens. Lenses that approximate the angle of view of the human eye are called normal lenses.

Focal length determines the magnification and angle of view of the image. With the camera in a fixed position, objects photographed with a wide-angle lens will seem farther away than with a normal lens; seen through a telephoto lens, the same objects will seem closer (and closer together). The wide-angle can take in a broader angle of view than the eye can see, while the telephoto narrows this view.

The zoom lens offers a range of focal lengths, and is one of the most popular types of lenses today. The user can change the focal length by simply pushing a button or turning a ring on the lens barrel. So-called true zooms maintain focus while changing the focal length; this allows photographers with single-lens-reflex cameras to focus precisely at high magnification before framing the picture at a different focal length. Another type of zoom lens, the varifocal lens, must refocus as the focal length changes—a disadvantage only if the camera does not offer automatic focusing.

C Macro Lenses

Some photographic subjects require task-specific optics. The most common specialized task is close-up photography, for subjects ranging from flowers to coins. To cope with these small subjects, macro lenses were developed for single-lens-reflex cameras. Macro lenses for 35-millimeter cameras extend the focusing range to a matter of inches. On their own they can reproduce an object on film at one-half its actual size; with the addition of an extension ring, the camera can picture an object at life size.

Many modern zoom lenses come with a macro setting that allows a limited range of close-up focusing. However, these are no substitute for a true macro lens because, at best, they only reproduce an object at one-fifth its actual size. Extension rings or simple close-up lenses also can attach to a normal lens to allow close-ups. Magnification of a subject to greater than its actual size calls for more specialized equipment, such as a microscope, and is called photomicrography.

D Aperture

The lens diaphragm controls the size of the aperture, or lens opening, and thus the amount of light that passes through the lens. It operates in conjunction with the shutter. The aperture size is measured by numerical settings called f-stops. On a traditional, manually controlled camera the f-stops are inscribed on an adjustable ring that fits around the lens. Typical f-stops are f/2, f/2.8, f/4, f/5.6, f/8, f/11, and f/16. The setting f/2 represents a large aperture, f/16 a small aperture. With simple automatic-exposure cameras, a computer sets the aperture size; thus the aperture ring has disappeared from many of today's lenses.

Lenses come with a rating for their maximum aperture, indicating how much light can reach the film when the lens diaphragm is wide open. With single-lens-reflex cameras, the maximum aperture also influences how bright the image appears in the viewfinder. Within lens types, a lens with a large maximum aperture will have a larger diameter and weigh more than a lens with a smaller aperture. A telephoto lens requires a larger lens diameter and greater length to let in the same amount of light as a normal or wide-angle lens. Like telephoto lenses, zoom lenses are also physically large. To reduce their bulkiness and complexity, many manufacturers now design zoom lenses with a variable maximum aperture: The size of the aperture changes as the focal length of the lens goes from wide-angle to telephoto settings.

E Focusing

Technically, film captures only one plane of a picture in perfect focus. However, in practice we call a picture “in focus” when it appears reasonably sharp at a given magnification and viewing distance. Until recently photographers had to bring an image into focus manually, by turning a ring or a focusing collar on the camera lens. But most of today's cameras with built-in lenses will adjust the lens automatically, through use of a mechanism connected to an autofocusing sensor. Cameras with interchangeable lenses still have focusing collars to allow for manual adjustment. Most lenses will focus from a few feet in front of the camera to a point in the far distance, called infinity.

F Depth of Field

To help determine what will appear in focus in a picture, photographers make use of a concept called depth of field. This term refers to a zone of focus—that is, the area between the closest and farthest objects that will appear sharply focused in the photograph. A picture with a deeper zone of focus might be a landscape in which both the trees in the foreground and the mountains in the background appear in sharp focus. A picture with a shallow depth of field might be a close-up portrait, in which objects in the background are purposely blurred.

The factors that determine depth of field are lens aperture, focusing distance, and focal length. All other factors being equal, depth of field will be greatest when photographing a distant subject, using a short focal length (wide-angle) lens, and a small aperture. Conversely, depth of field will be most shallow when photographing a subject at close range, using a long focal length (telephoto) lens, with a wide aperture.

A photographer using a single-lens-reflex camera or view camera can judge the approximate depth of field by looking directly through the lens with the aperture set to the desired f-stop. In cameras with removable, manually adjusted lenses, a depth-of-field scale shows the approximate sharp-focus zone for the different aperture settings.

Automatic cameras are designed to focus precisely on a single subject at the center of the frame or, in more sophisticated designs, to focus on a band of details across the central picture area. In most cases, the photographer locks in the focus by pressing the shutter button part way. For capturing the image of a moving subject, certain cameras with motor drives will adjust the focus continuously while the photographer tracks the subject.

Focusing precisely on a central subject, however, does not necessarily provide the greatest depth of field. With manual focusing, photographers can obtain the maximum depth of field by turning the focusing collar until the infinity sign aligns with the outside depth-of-field mark for the f/stop they have chosen. A variant of this manual-focusing technique is called zone focusing: The photographer chooses an aperture and a focusing distance that together cover the range of distances at which the subject is likely to appear. Zone focusing is especially useful for candid photography.

G Lens Hoods and Coatings

One of the worst enemies of photographers is flare, unwanted light that enters the lens and causes strange reflections and a loss of contrast on the film. Flare is especially obvious when photographing with the sun in front of or just to the side of the lens. To decrease the incidence of flare, photographers can shade the front of the lens with a collar called a lens hood that prevents sunlight from striking the glass surfaces. Hoods for zoom lenses are less effective because they must angle away from the lens enough to accommodate the lens's widest angle of view.

Lens makers also combat the more subtle effects of flare by coating the exterior and interior surfaces of the lens’s glass elements with thin layers of reflection-absorbing material. These coatings enhance the contrast of the film image and account for the characteristic green and purple hues visible when one looks into the front of a modern lens.

VI EXPOSURE

All light-sensitive photographic materials—film or photographic print paper—produce their finest results when given the optimum exposure. Precise exposure, coupled with consistent development, is the technical key to excellent photographs.

A photographer can change the amount of exposure the film receives by adjusting either the shutter speed or the aperture setting. A one-stop change in shutter speed is equivalent to an aperture change of one f-stop, and vice versa. Thus, for a given lighting situation several different combinations of f-stop and shutter speed result in the same amount of light hitting the film.

For example, an exposure of f/5.6 at 1/15 second allows the same amount of light to strike the film as an exposure of f/2.8 at 1/60 second—the aperture is two stops larger, but the speed is two stops faster. The exposures are thus comparable, but they produce different pictorial results. If the photographer is holding the camera by hand the second option is preferable, because at speeds below 1/60 second, movement of the camera or of the subject is likely to blur the image. If the photographer is using a tripod to hold the camera still and photographing a still subject, the first option may be preferable because the smaller aperture provides greater depth of field.

When film is developed according to the manufacturer's specifications, every stop of increase in the exposure (one step up in either f-stop or shutter speed) effectively doubles the density of the negative. For example, an exposure at f/5.6 for 1/15 second produces twice the density of an exposure at the same f-stop for 1/30 second, and therefore a print made from it will be twice as light, unless the print exposure time is doubled. However, there are limits to this relationship, called reciprocity, between exposure and density. At the extremes of very little and very great amounts of exposure, this rule is less consistent and the resulting images will be noticeably underexposed.

A Light Metering

To help photographers determine the ideal exposure, and to help them avoid the problems associated with extremely high or extremely low exposure levels, manufacturers introduced photoelectric exposure meters in the 1930s. At first these meters were independent, handheld devices; later they were incorporated into the camera itself, with a sensor measuring the light as it came through the lens. The final development was automatic exposure, in which the camera uses data from its built-in exposure meter to automatically adjust the shutter speed and lens aperture.

All metering systems share one principle: They respond to the world as if it were a uniform shade of gray. This shade (called 18 percent gray for its ratio of reflection) represents the average amount of light reflected by an average outdoor subject. In most situations, basing the exposure on this average reading produces ideal results: the negative receives just the right amount of light.

The meters built into modern cameras are called reflected light meters: They measure the amount of light reflected into the lens by the subject. (Another type, the incident-light meter, measures the light that is falling on the scene or subject.) Most of these devices are also called averaging meters because they read a broad angle of the scene; those that read a narrow angle are called spot meters. Averaging meters provide somewhat less accuracy than spot meters but are easy to use. Spot meters give very precise readings, but the photographer must know how to correctly interpret these readings.

Newer, more sophisticated single-lens-reflex cameras try to increase the accuracy of their automatic-exposure systems with what is called a multipattern metering system. This type of system measures the light coming through the lens from several different areas within the picture frame. It then compares the results to a computerized formula to determine the best overall exposure. Based on the data gathered, these meters try to guess the kind of picture-taking situation at hand and compensate for some problems, such as an overly bright sky.

Despite all the advances in exposure technology, meter readings are not foolproof. For example, neither very dark nor very light skin tones reflect 18 percent of the light, so portrait photographers have to adjust their exposures to compensate. In backlit conditions, when a person is surrounded by a bright background, most meters will recommend too little exposure. Likewise, if the main subject is a snowman in a field of snow, automatic exposure systems will assume that the snow is an average shade of gray and underexpose it.

B Development and Exposure

Perfectly exposed film will produce imperfect pictures if it is not developed properly. By the same token, development can be adjusted to compensate for certain variations in exposure. For example, a roll of ISO 100 slide film exposed by mistake at a rating of ISO 200 can be pushed—that is, have its development time extended during processing to produce reasonable results. Lengthening development time lightens the resulting images, which otherwise would appear too dark.

In black-and-white photography, it is common to adjust the exposure and development of each picture individually to compensate for varying contrast conditions. If the lighting is harsh, resulting in high contrast between light and dark areas, a sophisticated photographer might overexpose the negative and then shorten its development time to subdue the harsh light. This technique is often used in large-format, view camera photography and is the foundation of the method used by American wilderness photographer Ansel Adams.

C Long and Short Exposure Times

Most films are intended for use at shutter speeds between 1/2 and 1/1000 second. At significantly slower or faster speeds the reciprocity, or one-to-one relationship between exposure and image density, fails. Pictures taken at either very fast or very slow speeds tend to result in underexposed images. With color films, the colors may also shift.

Exposure meters do not compensate for reciprocity effects; instead, the photographer must compensate by manually adjusting the exposure according to charts supplied by the film manufacturer. With black-and-white film, development times must also be increased.

For some fast-moving subjects—such as the wings of a hummingbird in flight or a golf club as a golfer swings it—even a shutter-speed setting of 1/1000 second is not sufficient to capture the image in focus. Flash photography can produce an effect equivalent to shorter exposure times. Special electronic flash units are able to limit the duration of their light output to as little as 1/100,000 second.

D Flash Photography

In the absence of adequate daylight, photographers use artificial light to illuminate scenes, both indoors and outdoors. The most commonly used sources of artificial illumination are electronic flash, tungsten lamps called photofloods, and quartz lamps. Another once-popular light source, the flashbulb, a disposable bulb filled with oxygen and a mass of fine magnesium alloy wire, has gone the way of the dinosaur.

Flash units vary in size from small, battery-powered, camera-mounted units to large studio units that plug into an electric wall socket. Generally speaking, the larger the unit, the greater the intensity of light produced. Camera-mounted flashes are adequate for snapshots of family and friends, but to illuminate a large scene evenly and with a single burst of light, a powerful studio unit is needed.

An electronic flash unit consists of a glass quartz tube filled with an inert gas—usually xenon. When a brief jolt of electricity is applied to the electrodes sealed at the ends of the tube, the gas produces an intense burst of light of very short duration. The process can be repeated thousands of times, sometimes in rapid succession, without wearing out the tube. Most flash exposures last from 1/1000 to 1/5000 second, although a duration of 1/100,000 second is now readily available. In 1931 the inventor of the electronic flash, American engineer Harold Eugene Edgerton, developed an electronic strobe light (see Stroboscope) with which he produced flashes of 1/500,000 second, enabling him to capture the image of a bullet in flight.

Flash units are designed either as part of the camera mechanism or as accessories. Some designs, called dedicated flash units, are made for use with a particular camera model and have circuitry that sets the shutter speed and illuminates a light in the viewfinder when the tube is ready to fire again. Setting the shutter speed is important because the shutter and the flash need to be synchronized—that is, the shutter must be open for the duration of the flash. In cameras with a focal-plane shutter (this includes most commonly used cameras), the maximum speed at which synchronization is possible is usually 1/125 second.

Modern dedicated flash units, as well as built-in camera units, contain automatic flash systems. They have a sensor that determines the appropriate amount of light from the flash tube, depending on the aperture set on the lens. This sensor is commonly located inside the camera, where it can gauge the amount of light at the film plane. Before automatic flash was invented, it was not possible to adjust the flash output; photographers could control the exposure only by adjusting the aperture.

Flash aimed directly at the subject usually produces harsh, flat lighting. When photographing people or animals in very dim conditions, using flash also causes a condition known as red eye, making the centers of the subject’s eyes appear red. With some flash units it is possible to achieve more pleasant results indoors by aiming the flash at the ceiling. As light bounces from ceiling to subject, it produces a softer, more even light and eliminates red eye.

Flash can also be used in daylight to fill in foreground areas where shadows may be too strong. For this type of picture, the exposure generally should be set to half of what would be required for the existing light. This technique, called fill-flash, lightens shadows without overriding the main source of light. The color temperature of electronic flash is practically the same as daylight so the two light sources do not produce noticeable color differences.

E Filters

Filters added to the front of a camera lens change the quantity or quality of the light that reaches the film. Made of gelatin or glass, filters may alter the color balance of light, change contrast or brightness, minimize haze, or create special effects. In black-and-white photography, color filters transmit light of one color while blocking light of a contrasting color. In a landscape photograph taken with a red filter, for example, much of the blue light of the sky is blocked, causing the sky to appear darker and thereby emphasizing clouds. A yellow filter produces a less extreme effect because more blue light is transmitted to the film. The medium-yellow filter is often used for outdoor black-and-white photography because it renders the tone of a blue sky in much the same way as the human eye does.

Another type of filter, called a conversion filter, changes the color balance of light when it is radically different from that of the film. Tungsten films, for example, are balanced for use indoors with light from photofloods or incandescent lightbulbs. Exposed in daylight, they produce pictures with a bluish cast. A series 85 conversion filter can correct this. Daylight film, which is balanced for sunlight at noon, has a yellow-amber cast when exposed indoors under incandescent light or photofloods. A series 80 conversion filter corrects this problem. Similar to conversion filters are light-balancing filters, which can adjust tungsten film designed for one type of artificial light to work with a second type of artificial light.

Color-compensating (CC) filters help balance fluorescent light for daylight film or indoor (tungsten) film. Photographers also use CC filters to make small changes in color rendition on the film or when printing in the darkroom. Some professional transparency films require CC filtration as a matter of course.

Skylight, or ultraviolet (UV), filters are familiar amateur accessories. They filter ultraviolet light, which is invisible to humans but which can register on film as blue. Screwed into the end of a lens, a UV filter eliminates most of the excess blue that appears in distant landscape photographs and secondarily provides a transparent protective cap for the lens.

A polarizing filter reduces reflections from the surfaces of shiny subjects such as windows. In color photography, polarizing filters also produce more intense colors.

All filters reduce the amount of light reaching the lens to some degree—with a polarizing filter the reduction can amount to two stops or more of exposure. All such reductions, called filter factors, must be calculated into manual exposures. With automatic exposure, which measures the light after it has come through the lens, filter factors are less relevant, but they still require slower shutter speeds or larger apertures.

VII DARKROOM PROCESSING

A darkroom is a room for processing photography materials. It must completely seal out light from outside the room. In the early days of the medium, many photographers traveled with portable darkrooms, which were housed in horse-drawn wagons or carried by servants. Today many people have a home darkroom built in their basement, laundry room, or closet.

A darkroom is divided into a dry side and a wet side. The dry side is used for loading, enlarging, and preparation; the wet side contains a sink with temperature-controlled running water, and is used for the chemical processing of films and prints. Because many processing chemicals are toxic, certain precautions are necessary: the darkroom should have an exhaust fan to expel fumes and dust, and the photographer should always wear latex gloves when handling wet materials and a dust mask when mixing powdered chemicals with water.

During the process of exposing and developing black-and-white printing paper, a special orange-colored light bulb called a safelight can provide some illumination. But during the processing of black-and-white films, color films, and color printing papers, the darkroom must be totally dark, because these materials are panchromatic—that is, they are sensitive to all types of light.

In the home darkroom, film is customarily developed in a lighttight tank, which holds metal reels onto which the exposed film has been wound. Photographers make prints with an enlarger, an upright device that functions much like a camera except that it contains its own light source. The enlarger light shines through the negative, the enlarger lens focuses this light, and a large image of the negative projects onto the printing paper, which sits on a flat easel at the base of the enlarger.

A Developing the Film

Photographers develop film by treating it with an alkaline chemical solution called a developer. This solution reactivates the process begun by the action of light when the film was exposed. It encourages large grains of silver to form around the minute particles of metal that already make up the latent (not yet visible) image.

As large particles of silver begin to form, a visible image develops on the film. The density of silver deposited in each area depends on the amount of light the area received during exposure. In order to arrest the action of the developer, photographers transfer the film to a solution called the stop bath, which chemically neutralizes the developer. After rinsing the film, they apply another chemical solution to the negative image to fix it—that is, to remove residual silver halide crystals unexposed to light. The solution used for this process is commonly referred to as hypo, or fixer.

After a short rinse, a fixer remover, or hypo-clearing agent, is applied to clear any remaining fixer from the film. The film must then be thoroughly washed in water, as residual fixer tends to destroy negatives over time. Finally, bathing the processed film in a washing aid promotes uniform drying and prevents formation of water spots or streaks.

B Printing the Photos

Photographers produce prints by either of two methods: contact or projection. The contact method works for making prints of exactly the same size as the negative. Using this method, they place the emulsion side of the negative in contact with the printing material and expose the two together to a source of light. Photographers with 35-millimeter cameras commonly use this method to print what is called a contact sheet, which shows all the exposures from a single roll of film in small size.

For projection printing, photographers first place the negative in the enlarger and place a piece of sensitized printing material on the flat easel at its base. Switching on the enlarger light source projects an enlarged image of the negative onto the paper. An aperture on the enlarging lens controls the exposure, along with a timer connected to the enlarger light. The exposure commonly lasts from ten seconds to a minute. By blocking part of the light source with hands or small tools, the photographer can reduce or increase the amount of light falling on selected portions of the image, thus lightening or darkening those areas in the final print. This technique is known as dodging when used to lighten an area and as burning when making it darker.

For either printing process, prints are made on sheets of paper or plastic that have been coated with a light-sensitive emulsion. This coating is similar to that used for film but is much less sensitive to light. After exposing the print, the photographer can then develop and fix the positive image by a process very similar to that used for developing film. To process black-and-white prints, the paper is usually placed in a series of open trays; for color prints, a drum or automatic roller processor is preferred.

VIII RECENT DEVELOPMENTS

The technology of photography continues to develop rapidly. Electronic technologies have not only changed the way that most cameras work, but are changing photography in such fundamental ways that the distinction has begun to blur between photography and other image-making systems, such as computers and the graphic arts.

A APS

In the early 1990s the Eastman Kodak Company introduced a new line of cameras and film designed for amateur photographers. Called the Advanced Photo System (APS), this technology challenges conventional 35-millimeter photography on several fronts. APS film is easier to load, since the APS film cartridge has no leader to thread into a take-up spool. And APS cameras magnetically encode information onto the exposed film that automated photofinishing machines can read. According to Kodak, this results in a higher percentage of well-exposed prints than with 35-millimeter processing. And although APS film is of a smaller format than 35-millimeter film, it is capable of results that nearly match the precision and sharpness of the older format.

Soon after Kodak’s introduction of APS, other film and camera makers also adopted the system; dozens of APS cameras are now available, including several single-lens-reflex models. However, the target market for APS remains the point-and-shoot camera user. In comparison to 35-millimeter point-and-shoot models, APS cameras are slightly smaller and lighter.

One of the biggest differences between APS and conventional photography is that photographers can have their pictures processed conventionally or have them scanned onto a compact disc (CD) for use with a computer. APS is not a digital photography system; unlike digital systems, which are explained in the next section, APS employs well-established color film technology, including silver halides and dye couplers.

B Digital Photography

Digital photography is a method of making images without the use of conventional photographic film. Instead, a machine called a scanner records visual information and converts it into a code of ones and zeroes that a computer can read. Photographs in digital form can be manipulated by means of various computer programs. Digital photography was widely used in advertising and graphic design in the late 1990s, and was quickly replacing conventional photographic technology in areas such as photojournalism.

Digital cameras are now available for both professional photographers and amateur enthusiasts. The more expensive professional cameras function as sophisticated 35-millimeter cameras but record the picture information as pixels, or digital dots of color (see Computer Graphics). There can be several million pixels in a high-resolution, full-color digital photograph. Some digital cameras are able to transfer their large picture files directly into a computer for storage. Others accept a disc or similar portable storage unit to achieve the same purpose. The original high-resolution image can later be reproduced in ink (in a magazine, for example) or as a conventional silver halide print.

Digital cameras aimed at the amateur photography market function much as point-and-shoot cameras do, with automatic focus, automatic exposure, and built-in electronic flash. Pictures from these cameras contain fewer pixels than those from a more expensive camera and are therefore not as sharp. After taking pictures, the user can connect the camera directly to a television set or video cassette recorder, so the whole family can look at snapshots together. Alternatively, image files can be transferred to a home computer, stored on disks, or sent to friends via electronic mail.

[Petals]

الموضوع التاني عن قمة إيفريست


Mount Everest


I INTRODUCTION


Everest, Mount, mountain peak in the Himalayas of southern Asia, considered the highest mountain in the world. Mount Everest is situated at the edge of the Tibetan Plateau (Qing Zang Gaoyuan), on the border of Nepal and the Tibet Autonomous Region of China.

Mount Everest was known as Peak XV until 1856, when it was named for Sir George Everest, the surveyor general of India from 1830 to 1843. The naming coincided with an official announcement of the mountain's height, taken as the average of six separate measurements made by the Great Trigonometrical Survey in 1850. Most Nepali people refer to the mountain as Sagarmatha, meaning “Forehead in the Sky.” Speakers of Tibetan languages, including the Sherpa people of northern Nepal, refer to the mountain as Chomolungma, Tibetan for “Goddess Mother of the World.”

The height of Mount Everest has been determined to be 8,850 m (29,035 ft). The mountain’s actual height, and the claim that Everest is the highest mountain in the world, have long been disputed. But scientific surveys completed in the early 1990s continued to support evidence that Everest is the highest mountain in the world. In fact, the mountain is rising a few millimeters each year due to geological forces. Global Positioning System (GPS) has been installed on Mount Everest for the purpose of detecting slight rates of geological uplift.

II GEOLOGICAL FORMATION


Mount Everest, like the rest of the Himalayas, rose from the floor of the ancient Tethys Sea. The range was created when the Eurasian continental plate collided with the Indian subcontinental plate about 30 to 50 million years ago. Eventually the marine limestone was forced upward to become the characteristic yellow band on the top of Mount Everest. Beneath the shallow marine rock lies the highly metamorphosed black gneiss (foliated, or layered, rock) of the Precambrian era, a remnant of the original continental plates that collided and forced up the Himalayas.

Mount Everest is covered with huge glaciers that descend from the main peak and its nearby satellite peaks. The mountain itself is a pyramid-shaped horn, sculpted by the erosive power of the glacial ice into three massive faces and three major ridges, which soar to the summit from the north, south, and west and separate the glaciers. From the south side of the mountain, in a clockwise direction, the main glaciers are the Khumbu glacier, which flows northeast before turning southwest; the West Rongbuk glacier in the northwest; the Rongbuk glacier in the north; the East Rongbuk glacier in the northeast; and the Kangshung glacier in the east.

III CLIMATE


The climate of Mount Everest is naturally extreme. In January, the coldest month, the summit temperature averages -36° C (-33° F) and can drop as low as -60° C (-76° F). In July, the warmest month, the average summit temperature is -19° C (-2° F). At no time of the year does the temperature on the summit rise above freezing. In winter and spring the prevailing westerly wind blows against the peak and around the summit. Moisture-laden air rises from the south slopes of the Himalayas and condenses into a white, pennant-shaped cloud pointing east; this “flag cloud” sometimes enables climbers to predict storms. When the wind reaches 80 km/h (50 mph), the flag cloud is at a right angle to the peak. When the wind is weaker, the cloud tilts up; when it is stronger, the flag tilts down.

From June through September the mountain is in the grip of the Indian monsoon, during which wind and precipitation blow in from the Indian Ocean. Masses of clouds and violent snowstorms are common during this time. From November to February, in the dead of winter, the global southwest-flowing jet stream moves in from the north, beating the summit with winds of hurricane force that may reach more than 285 km/h (177 mph). Even during the pre- and post-monsoon climbing seasons, strong winds may arise suddenly. When such storms develop, sand and small stones carried aloft, as well as beating snow and ice, pose problems for climbers.

Precipitation falls mostly during the monsoon season, while winter storms between December and March account for the rest. Unexpected storms, however, can drop up to 3 m (10 ft) of snow on unsuspecting climbers and mountain hikers.

Base Camp, which serves as a resting area and base of operations for climbers organizing their attempts for the summit, is located on the Khumbu glacier at an elevation of 5,400 m (17,600 ft); it receives an average of 450 mm (18 in) of precipitation a year.

IV CLIMBING MOUNT EVEREST


Traditionally, the people who live near Mount Everest have revered the mountains of the Himalayas and imagined them as the homes of the gods. Because the peaks were considered sacred, no local people scaled them before the early 1900s. However, when foreign expeditions brought tourist dollars and Western ideas to the area, people of the Sherpa ethnic group began to serve as high-altitude porters for them. Because Nepal had been closed to foreigners since the early 1800s, all pre-World War II (1939-1945) Everest expeditions were forced to recruit Sherpa porters from Dārjiling (Darjeeling), India, then circle through Tibet and approach Everest from the north.

In 1913 British explorer John Noel sneaked into Tibet, which was also closed at the time, and made a preliminary survey of the mountain’s northern approaches, where the topography is less varied than on the southern side. In 1921 the British began a major exploration of the north side of the mountain, led by George Leigh Mallory. Mallory’s expedition, and another that took place soon afterward, were unable to overcome strong winds, avalanches, and other hazards to reach the summit. In 1924 a third British expedition resulted in the disappearance of Mallory and a climbing companion only 240 m (800 ft) from the summit. More attempts were made throughout the 1930s and into the 1940s. Then, with the conquest of Tibet by China in the early 1950s, the region was closed to foreigners again and the northern approaches to the mountain were sealed off.

In 1950, the year after Nepal opened to foreigners, W. H. Tilman and C. Houston made the first ascent from the south and became the first people to see into the Khumbu cirque (a steep basin at the head of a mountain valley). A number of attempts to reach the mountain’s summit followed in the early 1950s. In 1952 the Swiss almost succeeded in climbing the mountain from the South Col, which is a major pass between the Everest and Lhotse peaks and is now the most popular climbing route to the summit. On May 29, 1953, under the tenth British expedition flag and the leadership of John Hunt, Edmund Hillary of New Zealand and Sherpa Tenzing Norgay of Nepal successfully completed the first ascent of Mount Everest via the South Col. Several expeditions have since followed. In 1975 Junko Tabei of Japan became the first woman to summit Mount Everest. Later, in 1978, Austrians Reinhold Messner and Peter Habeler established a new and rigorous standard by climbing to the summit without the use of supplemental oxygen, which, because of the thin air at Everest’s high altitude, is important for the energy, health, and thinking skills of the climbers. In 1991 Sherpas, who had carried the supplies for so many foreigners up Mount Everest, completed their own successful expedition to the summit. By the mid-1990s, 4,000 people had attempted to climb Everest—660 of them successfully reached the summit and more than 140 of them died trying.

The difficulties of climbing Mount Everest are legendary. Massive snow and ice avalanches are a constant threat to all expeditions. The avalanches thunder off the peaks repeatedly, sometimes burying valleys, glaciers, and climbing routes. Camps are chosen to avoid known avalanche paths, and climbers who make ascents through avalanche terrain try to cross at times when the weather is most appropriate. Hurricane-force winds are a well-known hazard on Everest, and many people have been endangered or killed when their tents collapsed or were ripped to shreds by the gales. Hypothermia, the dramatic loss of body heat, is also a major and debilitating problem in this region of high winds and low temperatures.

Another hazard facing Everest climbers is the famous Khumbu icefall, which is located not far above Base Camp and is caused by the rapid movement of the Khumbu glacier over the steep rock underneath. The movement breaks the ice into sérac (large, pointed masses of ice) cliffs and columns separated by huge crevasses, and causes repeated icefalls across the route between Base Camp and Camp I. Many people have died in this area. Exposed crevasses may be easy to avoid, but those buried under snow can form treacherous snow bridges through which unwary climbers can fall.

The standard climb of Mount Everest from the south side ascends the Khumbu glacier to Base Camp at 5,400 m (17,600 ft). Typical expeditions use four camps above Base Camp; these camps give the climbers an opportunity to rest and acclimate (adapt) to the high altitude. The route from Base Camp through the great Khumbu icefall up to Camp I at 5,900 m (19,500 ft) is difficult and dangerous; it usually takes one to three weeks to establish because supplies must be carried up the mountain in several separate trips. Once Camp II, at 6,500 m (21,300 ft), has been supplied in the same manner using both Base Camp and Camp I as bases, climbers typically break down Base Camp and make the trek from there to Camp II in one continuous effort. Once acclimatized, the climbers can make the move to Camp II in five to six hours. Camp III is then established near the cirque of the Khumbu glacier at 7,300 m (24,000 ft). The route up the cirque headwall from Camp III to the South Col and Camp IV at 7,900 m (26,000 ft) is highly strenuous and takes about four to eight hours. The South Col is a cold, windy, and desolate place of rocks, snow slabs, littered empty oxygen bottles, and other trash.

From the South Col to the summit is a climb of only 900 vertical m (3,000 vertical ft), although its fierce exposure to adverse weather and steep drop-offs poses many challenges. The section between 8,530 m (28,000 ft) and the South Summit at 8,750 m (28,700 ft) is particularly treacherous because of the steepness and unstable snow. From the South Summit there remains another 90 vertical m (300 vertical ft) along a terrifying knife-edged ridge. The exposure is extreme, with the possibility of huge vertical drops into Tibet on the right and down the southwest face on the left. A little more than 30 vertical m (100 vertical ft) from the summit is a 12-m (40-ft) chimney across a rock cliff known as the Hillary Step; this is one of the greatest technical challenges of the climb.

As the popularity of climbing Everest has increased in recent years, so have safety problems. To pay the high climbing permit fee charged by the Nepalese government, many experienced climbers have recruited wealthy, amateur climbers as teammates. The combination of inexperience, crowded summit conditions (more than 30 have been known to summit the peak on the same day), and extreme weather conditions has led to a number of tragedies in which clients and competent guides alike have died attempting the climb.

V ENVIRONMENTAL ISSUES


The large number of trekkers and climbers who visit Nepal and the Everest region contribute to the local economy but also cause serious environmental impact. Such impact includes the burning of wood for fuel, pollution in the form of human waste and trash, and abandoned climbing gear. Although some climbing gear is recycled by local residents either for their own use or for resale, it is estimated that more than 50 tons of plastic, glass, and metal were dumped between 1953 and the mid-1990s in what has been called “the world’s highest junkyard.” Up on the ice, where few local people go, the norm is to throw trash into the many crevasses, where it is ground up and consumed by the action of the ice. A few bits and pieces show up on the lower part of the glacier many years later as they are churned back to the surface, although organic matter is generally consumed or scavenged by local wildlife. At the high-elevation camps, used oxygen bottles are strewn everywhere.

Efforts have been made to reduce the negative environmental impact on Mount Everest. The Nepalese government has been using a portion of climbing fees to clean up the area. In 1976, with aid from Sir Edmund Hillary’s Himalayan Trust and the Nepalese government, the Sagarmatha National Park was established to preserve the remaining soil and forest around Mount Everest. By the mid-1990s the park comprised 1,240 sq km (480 sq mi). Trekking and climbing groups must bring their own fuel to the park (usually butane and kerosene), and the cutting of wood is now prohibited. Because the freedoms of Sherpas have been restricted by the park rules, they have not been sympathetic to the existence of the park. Additionally, the Sagarmatha Pollution Control, funded by the World Wildlife Fund and the Himalayan Trust, was established in 1991 to help preserve Everest’s environment. Climbing activity continues to increase, however, and the environmental future of the Mount Everest area remains uncertain.

[مـــــــهــــات]

many thanks for your kind assistent and support

[ملكة البحار]

[align=left]hello mahat
this website is really helpful
http://www.isoc.org/internet/history/index.shtml
contains alot subjects about the history of the internet

also you can use Google search to find any articles you need.

hope could help you[/align]