Several measures can be used to assess the quality of still photographs, the most discussed being the pixel count, presumed to correlate with spatial resolution. This is measured by the quantity of picture elements (pixels) in the image sensor, usually counted in millions and called "megapixels". Measuring the resolution of both film and digital photographs involves numerous issues. For film, this issue depends upon the size of film used ( 35mm,Medium format or Large format), the speed of the film used and the quality of lens fitted to the camera.
There is no major difference in principle between a lens used for a camera, a telescope, a microscope, or other apparatus, but the detailed design and construction are different.
A lens may be permanently fixed to a camera, or it may be interchangeable with lenses of different focal lengths, apertures, and other properties.
The Impact of New Technology
The development of the 35-mm or “candid” camera by Oskar Barnack of the Ernst Leitz company, first marketed in 1925, made documentarians infinitely more mobile and less conspicuous, while the manufacture of faster black-and-white film enabled them to work without a flash in situations with a minimum of light. Color film for transparencies (slides) was introduced in 1935 and color negative film in 1942. Portable lighting equipment was perfected, and in 1947 the Polaroid Land camera, which could produce a positive print in seconds, was placed on the market. All of these technological advances granted the photojournalist enormous and unprecedented versatility.
The Aesthetics of Photography
Seeking to determine the particular aesthetics of photography, the American Berenice Abbott and the Frenchmen Eugène Atget, André Kertész, and Henri Cartier-Bresson developed intensely personal styles. The exponents of surrealism in France and of futurism in Italy and the various German art movements that were focused in the Bauhaus all explored the medium of photography. The international exhibition “Film und Foto,” held in Stuttgart in 1929, helped to make formal a purely photographic aesthetic. The works exhibited combined elements of functionalism and abstraction. Photographic subject matter shifted from the past to the present—a present of new forms in machinery and architecture, new concern with the experience of the working classes, and a new interest in the timeless forms of nature.
Art and Documentary Photography
The fight to certify photography as a fine art has been among the medium's dominant philosophical preoccupations since its inception. Photography's legitimacy as an art form was challenged by artists and critics, who seized upon the mechanical and chemical aspects of the photographic process as proof that photography was, at best, a craft. Perhaps because so many painters came to rely so heavily on the photograph as a source of imagery, they insisted that photography could only be a handmaiden to the arts.
Further Developments
E. J. Marey, the painter Thomas Eakins, and Eadweard Muybridge all devised means for making stop-action photographs that demonstrated the gap between what the mind thinks it sees and what the eye actually perceives. Muybridge's major work, Animal Locomotion (1887), remains a basic source for artists and scientists alike. As accessory lenses were perfected, the camera's vision extended both telescopically and microscopically; the moon and the microorganism became accessible as photographic images.
The Impact of Early Photography
With the advent of the collodion process came mass production and dissemination of photographic prints. The inception of these visual documents of personal and public history engendered vast changes in people's perception of history, of time, and of themselves. The concept of privacy was greatly altered as cameras were used to record most areas of human life. The ubiquitous presence of photographic machinery eventually changed humankind's sense of what was suitable for observation. The photograph was considered incontestable proof of an event, experience, or state of being.
The Calotype
The calotype's paper negative made possible the reproduction of photographic images. The unavoidably coarse paper base for the negative, however, eliminated the delicate detail that made the daguerreotype so appealing. This lack of precision was understood and used to advantage by the Scottish painter David Octavius Hill and his assistant, Robert Adamson. From 1843 to 1848 they made an extensive series of calotype portraits of Scottish clergymen, intended to serve only as studies for a group portrait in oils, that stands today among the major bodies of work in the medium. Hill and Adamson composed their portraits in broad planes, juxtaposing bold masses of light and dark, creating works that are monumental in feeling despite their small size.
The Daguerreotype
Daguerreotypy spread rapidly, except in England, where Daguerre had secretly patented his process before selling it to the French government. The legal problems attending the pursuit of photography as a profession account in part for the widespread influence of amateurs (e.g., Nadar, the French pioneer photographer) on the early development of the medium. The popularity of the daguerreotype is attributable to two principal factors. The first of these was the Victorian passion for novelty and for the accumulation of material objects, which found its perfect paradigm in these silvery, exquisitely detailed miniatures. The second was the greatly increasing demand from a rising middle class for qualitatively good but—compared to a painter's fee—inexpensive family portraits. The cheaper tintype eventually made such likenesses available to all.
The Invention of Photography
The necessary first breakthrough in photography was in a different, not eye-centered area—that of making permanent photographic images. Employing data from the researches of Johann Heinrich Schulze—who, in 1727, discovered that silver nitrate darkened upon exposure to light—Thomas Wedgwood and Sir Humphry Davy, early in the 19th cent., created what we now call photograms. These were made by placing assorted objects on paper soaked in silver nitrate and exposing them to sunlight. Those areas of the paper covered by the objects remained white; the rest blackened after exposure to the light. Davy and Wedgwood found no way of arresting the chemical action at this stage, however, and their images lasted only a short time before darkening entirely.
Early Developments
The camera itself is based on optical principles known at least since the age of Aristotle; indeed, a filmless version was in use in the mid-1500s as a sketching device for artists. Called the camera obscura (Lat.,=dark chamber), it consisted of a small, lightproof box with a pinhole or lens on one side and a translucent screen on the opposite side. This screen registered, in a manner suitable for tracing, the inverted image transmitted through the lens. The human eye was the prototype for this device, which functioned as a primitive extension of seeing. Most experiments in photographic technology were directed toward perfecting the medium as a surrogate, more sophisticated eye.
Digital photography
The process of electronic acquisition, the equivalent of taking a photograph, is often referred to as image capture.
Light intensity is detected in digital camera by an photosensor. This is normally a charge-coupled device (CCD), although complementary metal oxide silicon (CMOS) devices are beginning to appear in some systems.
When photons strike the sensor, they give up energy. This causes electrons to be emitted, turning the energy of the photons into electrical energy. The number of electrons that are emitted can be measured to determine how many photons struck the capture element, and from this the scanner can generate a value for the intensity of light arriving from the point on the original being analyzed.
The best conventional between-the-lens shutters rarely yield exposures shorter than 1/500 s. Some focal plane shutters are rated at 1/2000 or 1/4000 s but may take 1/100 s to traverse the film format. Substantially shorter exposures are possible with magnetooptical shutters (using the Faraday effect) with electrooptical shutters (using the Kerr effect), or with pulsed electron image tubes. Alternatively, a capping shutter may be used in combination with various pulsed light sources which provide intense illumination for very short durations, including pulsed xenon arcs (electronic flash), electric arcs, exploding wires, pulsed lasers, and argon flash bombs. Flash durations ranging from 1 millisecond to less than 1 nanosecond are possible. Similarly, high-speed radiographs have been made by discharging a short-duration high-potential electrical pulse through the x-ray tube.
Two distinct classes of photography rely on ultraviolet radiation. In the first, the recording material is exposed directly with ultraviolet radiation emitted, reflected, or transmitted by the subject; in the other, exposure is made solely with visible radiation resulting from the fluorescence of certain materials when irradiated in the ultraviolet. In the direct case, the wavelength region is usually restricted by the camera lens and filtration to 350–400 nm, which is readily detected with conventional black-and-white films. Ultraviolet photography is accomplished at shorter wavelengths in spectrographs and cameras fitted with ultraviolet-transmitting or reflecting optics, usually with specialized films. In ultraviolet-fluorescence photography, ultraviolet radiation is blocked from the film by filtration over the camera lens and the fluorescing subject is recorded readily with conventional color or panchromatic films. Both forms of ultraviolet photography are used in close-up photography and photomicrography by mineralogists, museums, art galleries, and forensic photographers.
Emulsions made with special sensitizing dyes can respond to radiation at wavelengths up to 1200 nanometers, though the most common infrared films exhibit little sensitivity beyond 900 nm. One specialized color film incorporates a layer sensitive in the 700–900-nm region and is developed to false colors to show infrared-reflecting subjects as bright red.
The process of forming stable or permanent visible images directly or indirectly by the action of light or other forms of radiation on sensitive surfaces. Traditional photography uses the action of light to cause changes in a film of silver halide crystals in which development converts exposed silver halide to (nonsensitive) metallic silver. Following exposure in a camera or other device, the film or plate is developed, fixed in a solution that dissolves the undeveloped silver halide, washed to remove the soluble salts, and dried. Printing from the original, if required, is done by contact or optical projection onto a second emulsion-coated material, and a similar sequence of processing steps is followed. Digital photography captures images directly with an electronic photosensor.
