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 Photographs serve as scientific evidence, conveyers of news, historical documents, works of art, and records of family life.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.

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.