Etymologically, photography derives from two Greek words which essentially mean "writing with light," which is an accurate description of the process. In its most basic expression, photography is the exposure of a light-sensitive material to light, typically through a lens or other means of focusing light into a sharp point. After chemical development of the light-sensitive material, the image of the objects off which the light striking the film has reflected will appear.
Photography has many uses, most of which are beyond the scope of this article. Photography is used for portraits of people, holiday snapshots, art, journalism, and the preparation of materials for printed reproduction. The following article will briefly discuss the history of photography, and then discuss the various elements needed for modern photography. The article will then turn to graphic arts photography, or the shooting of line art and halftones for printed reproduction. The article will conclude with an overview of the newly emerging digital photographic techniques, from Photo CDs to digital cameras.
HISTORY OF PHOTOGRAPHY
Odd as it may seem, the camera preceded what we think of as photography by over 700 years. In 1039, the Arab scholar Alhazen described a device in use at the time (later called the camera obscura) which essentially consisted of a dark, box-like room with a small hole cut in one wall. Light passed into the room through the hole and cast the image of scenes taking place outside the box on the opposite wall. People could sit in the box and watch a "picture show" of activities transpiring outside. In the mid-sixteenth century, several Italian scientists fitted the camera obscura hole with a lens and diaphragm apparatus so as to provide a sharper image. The camera obscura was used for a type of "photography," but there was no way of capturing the images except by manually drawing what was cast on the wall, which is how many painters actually worked. (The Dutch painter Jan Vermeer [1632:75], for example, is believed to have used a camera obscura for his highly-detailed domestic scenes.)
Meanwhile, in 1727, the German physicist Johannes Heinrich Schulze discovered the sensitivity of certain silver salts to light. However, Schulze's interest in the chemistry of the process blinded him to the commercial exploitation of it. Several advances were made in the use of the silver salts for the capture of images, and several years later the English chemists Thomas Wedgwood and Humphy Davy were able to coat a paper with silver salts and photographically create silhouettes of leaves and other objects. However, unexposed salts could not as yet be dissolved away, the result being that the captured images were not permanent. In the 1820s, French chemist Joseph Nicéphore Niepce duplicated the work of Wedgwood and Davy, but found that bitumen (a material similar to asphalt) became insoluble when exposed to light. He coated bitumen on metal plates, exposed them to an image, and dissolved the unexposed bitumen (corresponding to non-image areas). Applying acid to the plate, the acid did not dissolve the exposed bitumen, and Niepce therefore had a relief plate which could be used on a letterpress printing press. Image quality, however, was not very high. Niepce also applied the camera obscura to this technique and the first attempts to combine a camera and a photosensitive material were made. Although Niepce called the process heliography (literally "sun writing"), the world's first true photograph, of the view outside Niepce's window, was made in 1826.
Niepce (and, after Niepce's death in 1833, his son Isidore) formed a partnership with the French painter Louis Jacques Mandé Daguerre to develop the photographic techniques. Daguerre abandoned bitumen as the photosensitive material and returned to silver compounds. In 1839, Daguerre announced the process of daguerreotypy, in which a polished silver plate was exposed to iodine vapor. The resulting layer of silver iodide was light-sensitive, and after exposure mercury vapor was used to develop the plate. Common salt could then be used to remove the unexposed silver iodide. The process caught on, and soon better materials (first sodium thiosulfate and then a mixture of silver iodide and silver bromide) were found to yield better results and reduce the required exposure time enough to make portrait-taking practical.
At about the same time that Daguerre was preparing his paper on the daguerreotype process, an English archaeologist and philologist named William Henry Fox Talbot had invented a similar process, which he called calotypy (also known as the talbotype process). Talbot exposed silver iodide-treated paper with a camera and developed it in gallic acid. This process produced a paper negative, which could then be transferred to a second sensitized paper to produce a positive print. Tabot's images, however, ended up much grainier and blurrier than Daguerre's (due to the coarseness of the paper Talbot used), but the process of making a single negative from which multiple positives could be made became the basis of modern photography.
Until it was realized that silver salts were only light-sensitive when in the presence of certain organic media (such as that found in paper), the use of other base materials for photographic plates (glass in particular) was impossible. However, in the mid-1840s, Niepce de Saint-Victor discovered this, and by coating glass plates with a layer of albumen (an organic colloid found in egg whites), potassium iodide, and sodium chloride, the added silver nitrate was rendered light-sensitive. Saint-Victor was able to produce negatives on glass plates, which produced much sharper images than Talbot's original paper negatives. In 1851, Frederick Scott Archer invented the wet-plate process, which replaced the albumen with a substance called collodion (essentially a solution of guncotton in ether and alcohol). These glass plates needed to be exposed and developed before they were allowed to dry (they lost their sensitivity to light when dry), but the process was very popular among some photographers, who built portable darkrooms so as to be able to photograph events "on location." In particular, famed Civil War photographer Matthew Brady used the wet-plate process. The wet-plate process was very popular for studio work, however, especially early graphic arts photography, but for portable photographic work a solution wasn't found until 1871, when English physician Richard Maddox developed an emulsion made from gelatin rather than collodion, and further refinements made the resulting dry plates practical.
Meanwhile, the camera itself was undergoing changes. The first daguerreotype cameras essentially comprised two wooden boxes, one being slid in or out of the other one for focusing. In 1861, the first single-lens reflex camera (a camera in which light was reflected through the lens—which could be focused without moving the rest of the camera—both to the film and to the viewfinder) was invented, and in 1880 the twin-lens reflex camera (which had two lenses, one which led to the viewfinder and one which led to the film) was invented. In 1888, George Eastman introduced the first roll-film camera. The first model of the Kodak camera (named, legend has it, after the sound the camera makes when the shutter button is pressed, although some believe this to be apocryphal) used photographic paper coated with a gelatin emulsion, which could be peeled off and transferred to glass for developing. Subsequent models replaced the paper backing with celluloid-based film (invented several years earlier by John Corbutt). The small size of the Kodak cameras made amateur photography extraordinarily popular. In 1885, a pocket camera was introduced which, designed by Frank Brownell, was known familiarly as the "Brownie." The earliest Kodak cameras came pre-loaded with film which, after shooting all the pictures, was sent—camera and all—back to the company for developing. These were replaced by reloadable cameras, but in the 1990s the so-called "disposable camera" made a comeback. Plus ça change, plus c'est la même chose'.
The problem with celluloid film—which, by the way, also made possible the motion picture industry, as its transparency meant that light could be projected through it after development—was that it was highly flammable. (Celluloid, made from guncotton mixed with alcohol and camphor, was invented by John Hyatt, an Albany, New York, printer for a completely different purpose than film: he wanted to win a prize offered by a billiard-ball manufacturer who had been having difficulties obtaining ivory, the original material for the balls. Although Hyatt's billiard balls caught on, the celluloid was rather unstable, and during a game of billiards they had a tendency to explode, or at the very least emit a sound much like a gunshot. When used in Western saloons filled with trigger-happy cowboys, one can imagine a variety of unfortunate events arising.) At any rate, non-flammable celluloid acetate "safety film" was introduced in the 1920s, and had completely replaced the flammable celluloid film completely by the 1950s.
Further refinements over the years have made high-quality cameras smaller and more portable, color photographs possible and of greater tonal quality, film faster and exposure times much quicker.
BASICS OF PHOTOGRAPHY
Although new digital cameras and other technologies may someday make film obsolete, it still remains one of the essential parts of the photographic process. And who knows? Someday, maybe even the camera itself will become obsolete!
'Camera'. Any camera, regardless if it is a small hand-held model or a room-sized graphic arts process camera, is essentially the same device: a light-proof box with a lens at one end which can focus incoming light onto securely-mounted film for a set period of time. A lens is a curved piece of glass which refracts incoming light and focuses it at a specific point, producing a sharp image of whatever it is that the light is reflecting off of. A camera's focal length is a measure of the distance between the center—or node—of the lens and the point at which distant objects are in focus, or the focal point. Portable cameras have focal lengths measured in millimeters, but in graphic arts arts cameras they may be measured in inches. In most cameras (except movie cameras), the focal length is equal to the diagonal size of the negative.
Light passes through the lens into the camera through an aperture, a small hole which is opened for set periods of time (the length of time the aperture remains open, allowing light to strike the film, is also called the aperture). Light entering the camera is controlled by the shutter, the "door" which when closed prevents light from entering and when open allows light to enter. The amount of time the shutter remains open is called the shutter speed. In addition to shutter speed and aperture, the aperture diameter (adjusted by means of a diaphragm) is also an important measurement in photography. The diameter of the aperture and the focal length are related mathematically to each other by means of the f/stop system. A camera's f/number is essentially the ratio of the focal length to the aperture diameter. For example, if a camera has a focal length of 8 inches and the aperture diameter is set at 1 inch, then the f/stop value would be f/8 (f/number = focal length ÷ aperture diameter or 8 ÷ 1 = 8). If the aperture diameter were reduced to H inch, then the f/stop value would be f/16 (1 ÷ 1/2 = 16). Although in theory any f/number can be created, there is a set of default f/stops prescribed by lens manufacturers, which are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, and f/64. This set of f/numbers is based on a factor of 2; moving from one f/number to the next doubles the amount of light entering the camera. (For a constant focal length, the higher the f/number, the smaller the aperture. Thus changing from f/32 to f/22 doubles the aperture, while changing from f/16 to f/22 halves it.) As a result of this relationship, an equivalent exposure time can be determined, as a means of minimizing the length of exposure while at the same time ensuring that the same amount of light strikes the film. Thus, at f/16 with an exposure time of 60 seconds, an equivalent exposure could be determined to be 30 seconds at f/11. Since f/stop values are mathematical ratios of focal length to aperture diameter, they are not specific to any single camera. On both a 35mm portable camera and a large graphic arts process camera, an f/stop of f/16 will allow the same quantity of light to enter the camera.
Another factor involved in photography is the field, both the size and depth of field. In most cameras, the field (or the size of the area the lens can capture) for any given lens is related to the focal length. A lens with a focal length of 50 mm would be able to cover about a 53º field. The depth of field is that range of distances at which all objects will be in acceptable focus when the lens is focused on a particular point. So, for example, a lens focused on an object 16 feet away would have all objects within the range of 13:25 feet in acceptable focus. Depth of field varies with lens size (i.e., focal length). (See Lens.)
Another important consideration in photography is shutter speed, or the length of time the shutter remains open. This can range from the smallest fractions of a second to several minutes, depending on the illumination level, the type of film used, the type of lens used, and other such factors.
'Film'. The basis of film (or any photosensitive material) is the chemical change in an emulsion that occurs when light of a specific wavelength strikes it. Most photographic film is formulated to be sensitive to light in the visible spectrum. Other materials (such as printing plate materials, for example) are sensitive to light in the ultraviolet portion of the electromagnetic spectrum. Basically, photographic film consists of an emulsion containing silver halides (a class of chemicals including bromides, iodides, and chlorides) suspended in a gelatin, which is then applied to a plastic or paper base. After exposure to an image (or, rather, light reflecting off an image), immersion of the film in a developing solution converts the silver halide to metallic silver in direct proportion to the amount of exposure received; the greater the exposure, the less metallic silver is produced. The light areas of the original are represented (on a photographic negative) by heavy deposits of silver, while the dark areas of the original are very light or transparent, containing very light silver deposits. (On a film or paper positive, the reverse is the case.) Different types of film require different types of developing chemicals and developing temperatures to be effective. During developing, when the desired image density is reached, the action of the developing chemical must be stopped, often using an acid- or water-based stop bath. Immersion in a fixing solution (often sodium or ammonia thiosulfate) dissolves the residual silver halide and effectively "fixes" the image on the film. Automated film processing machines are widely used to save time and produce consistent results.
A photographic negative is used to prepare either film positives or prints. A recurring problem with photographic films and papers (especially now that glass is no longer used as the backing material) is the dimensional stability of the plastic or paper. Such materials tend to increase or decrease in size with changes in temperature. In cases where high degrees of registration are vital, a variety of base materials are available which keep their shape (and thus prevent image distortion) readily.
Some films exhibited the problem of halation, or a blurring effect of photographic images caused by light passing through the emulsion and base material, striking the back of the base material, and passing back out through the emulsion, essentially exposing the film twice. Films now use an antihalation dye, which is applied to the back of the base material and effectively absorbs the light striking it, preventing the light from bouncing back up through the emulsion.
Different types of films demonstrate different types of color sensitivity, or in other words, each type of film is sensitive to light in certain portions of the spectrum. Blue-sensitive film, as the name indicates, is only sensitive to light in the blue and ultraviolet portion of the spectrum, and is also often known as color blind film. Orthochromatic film is sensitive to all the wavelengths of the spectrum except for red. Orthochromatic (often called simply "ortho") film is the most commonly used, since its lack of sensitivity to red light means that it can be developed in a darkroom under a red safelight. In contrast, panchromatic film is sensitive to all wavelengths of visible light and as a result must be developed in total darkness. The color sensitivity of a film is usually indicated by a wedge spectrogram, which is a graph or histogram that shows the relative sensitivity of a film across a range of wavelengths.
A film's contrast refers to the degree to which the tones of the original (either a graphic arts mechanical or a "real-life" scene) have been compressed or expanded. Film manufacturers often provide a characteristic curve, or a graph illustrating the film density as a function of exposure time, for various films.
Another important issue in photographic film is film speed, which essentially refers to the length of time the film needs to be exposed in order to register an image, or in other words how much light is required. "Fast" film requires less light, while "slow" film requires more light. Film speed is measured using the ASA system; the higher the ASA rating, the faster the film. Although conventional film is given a general ASA number, detailed exposure indexes give different ASA values for different light sources.
Effective photography—be it conventional or graphic arts—is a balancing act between all the above-mentioned variables: f/number, shutter speed, exposure time, film speed, and a variety of lighting conditions.
GRAPHIC ARTS PHOTOGRAPHY
There are basically two varieties of photography: creative and graphic arts. Creative photography—the variety with which most everyone is at least slightly familiar, includes professional portrait photography, amateur holiday photography, and photojournalism, among many other things. Graphic arts photography, on the other hand, comprises those prepress processes used to prepare copy and art for printed reproduction. Although digital prepress processes are working to replace graphic arts photography, some of the basic principles involved in optimizing copy and art apply to both photographic and digital processes.
Graphic arts cameras (also called process camera's) are very large devices. They commonly have a camera area under regular room light, while the camera back and controls are built into the wall of a darkroom (and are thus called darkroom cameras), which allows the operator to load and remove film without having to leave the darkroom. Other models, called galley cameras, operate under ambient light and the film needs to be loaded into special light-tight containers and brought out of the darkroom and loaded into the camera. Some process cameras have components—such as suspension units—attached to either the floor or ceiling. Most graphic arts cameras possess a horizontal image plane (in which case they are also known as horizontal cameras); some cameras—called vertical cameras—have the copyboard, lens, and camera back aligned vertically. Vertical cameras have an advantage over horiziontal models in that they save space.
Graphic arts cameras use symmetric coated lenses, ground within very strict tolerances so as to reduce and/or eliminate distortion as much as possible. Most are of the achromat type (also called apochromatic) which fully corrects the lens for any color deviations throughout the visible spectrum. Process camera lenses have f/numbers ranging from f/8 to f/11, which—due to the large focal lengths (ranging from 8 to 48 inches, depending on the size of the camera) correspond to small aperture diameters. Due to the large size of some mechanicals, process cameras can capture images up to 40-inch square.
Process cameras use high-resolution, high-contrast films, which are characterized by slow film speeds. As a result, high-intensity pulsed xenon lamps (for color reproduction) or quartz iodine lamps (for monochrome reproduction) are used. Lamps are often built onto the unit itself, one lamp on the right and one on the left, and are oriented such that the incident light rays strike the copyboard at a 45º angle. Many advanced cameras utilize computer-controlled light integrators which use photoelectric cells to monitor the intensity of the light and automatically adjust the exposure time to compensate for any deviations. The lamps are often controlled by the camera shutter; when the shutter opens (controlled by a user-set timer) the lights usually click on. When the shutter closes, the lights turn off.
The original art is mounted on a copyboard, a glass-covered frame facing the lens. On horizontal cameras, the copyboard is oriented vertically, but can usually be rotated to a horizontal position to facilitate the mounting of copy. The copyboard can also be moved toward or away from the lens, depending upon whether enlargement or reduction, respectively, is desired. When moving the copyboard—often by means of rotating wheels on the control panel of the camera—the lens also usually needs to be adjusted and refocused. Accurate enlargement and reduction percentages can be effected using line-up tapes, guide numbers, or via computer-controlled mechanisms. During enlargement or reduction, the distance of the copyboard from the light source also changes, which can be accounted for either by varying the f/number or by using the above-mentioined light integrators.
Enlargements and reductions are denoted as percentages of the original copy. A reproduction that is 100% of the original is the same size as (or is at a 1:1 ratio to) the original. Percentages lower than 100% are reductions (such as 50%, which is half the size of, or at a ratio of 1:2 to, the original), while those greater than 100% are enlargements (such as 200%, which is twice the size of, or at a ratio sf 2:1 to, the original).
At the other end of the camera is the filmboard, a flat plane to which the film is mounted, commonly using a vacuum system to hold it in place. The filmboard, like the copyboard, can be in a horizontal position for easy mounting of film, and is swung into a vertical position behind the lens for exposure.
During exposure, a step tablet or grayscale is placed on the copyboard next to the copy to be photographed (but not in a position that obscures detail of the original copy!). A step tablet consists of a series of discrete steps of gray progressing from white to black. When the film is developed, the individual steps darken with increasing development, which serves as important cues in the proper development of the image. When a predetermined gray step has filled in, it is time to stop the developing process. Afterward, the grayscale can be evaluated to gauge the usability of the film.
Graphic arts photography is used to make photostats of line art and other elements, which can then be pasted onto a mechanical, or it can be used to generate the negatives (or positives) for stripping and platemaking, or it can be used to make color separations. (See Prepress: Graphic Arts Photography and Flat Assembly and Color Separation.) Regardless, however, there are two basic varieties of graphic arts photography which must be recognized: line photography and halftone photography.
'Line Photography'. Like line art, line photography involves the photographing of simple lines, shapes, text matter, solids, etc. (as opposed to continuous-tone images). Line photography is essentially high-contrast photography: either black or white, ink or no ink. Line photography is relatively simple to accomplish, and is often performed using orthochromatic film. Line photography is used in the preparation of negatives of text and line art for stripping. Although most such photography consists of the making of negatives from original copy, contact prints are often made as well. Contact printing essentially involves the exposure of a previously-exposed positive or negative to a piece of film, producing a negative or positive, respectively. Special duplicating film is used to make negatives from other negatives and positives from other positives.
'Halftone Photography'. The conventional printing processes are essentially "binary" media; they are incapable of printing continuous tones. As a result, any continuous-tone image (such as a photograph) that is to be printed needs to be converted to a series of very small, discrete dots. Such an image that consists of dots is called a halftone. Each dot is some shade of gray (or color) and it is the packing of these dots in specific densities that provides the illusion of a continuous-tone image. In order to optimize a continuous-tone image for printing, then, it needs to be converted to a dot pattern. This is accomplished using either a halftone screen or, more and more commonly, electronic dot generation. (The remainder of this section on halftones will primarily concern itself with the photographic techniques for halftone production; see also Halftone for a discussion of electronic dot generation.)
Originally, halftones were produced by shooting a continuous-tone image through a glass screen, which was essentially two glass "sheets" cemented together, each of which contained a grid of inked lines which, when placed together at right angles to each other, formed a fine screen that separated the contone image into small discrete square dots. The glass screen is now obsolete, but the principle survives in the use of the contact screen. Contact screens are a descendent of glass screens (and are usually prepared from them), but are on film (i.e., acetate) bases and utilize vignetted dots, with gradient density from the center of each dot (the greatest density) outward to the perimeter. There are a variety of different types of screens, such as gray screens on which the dots are in silver, and magenta screens in which the dots are formed using a magenta dye. Other screen patterns include elliptical and square dots, as well as other shapes, which produce different effects which enhance different portions of an image, such as shadows, midtones, or highlights. Contact screen photography includes placing the screen against the unexposed film, and exposing it to the original continuous-tone image. The density variation of the dots on the screen images variations in the density of the original as smaller or larger dots, depending upon whether a region is a highlight or a shadow, respectively, or, in other words, based on how much light is being reflected or transmitted by the original. Various additional processes can enhance certain aspects of the image: flashing—or exposing the film as a whole to a yellow light—reduces contrast in shadow regions, while no-screen exposure (also called bump exposure)—or removing the screen for a short period of the exposure—increases contrast in the highlights. The specially-dyed screens also help increase or reduce contrast in desired portions of an image.
The screen process most commonly yields a halftone negative which is stripped into a flat prior to platemaking, but in some cases, a screen print can be made from a negative which can then be added to a mechanical and shot with line art prior to stripping. Called copy-dot reproduction, it is most often used in newspapers, catalogs, in-house newsletters and reports, etc. These prints are most often made by a diffusion transfer method.
'Color Separation'. When printing process color—or "full-color" photographs or other images—all the reproducible colors that exist are produced by overprinting halftone dots of the four process colors (cyan, magenta, yellow, and black) at various densities to create secondary colors. As a result, each of those four colors needs its own plate, and therefore its own negative. These negatives are produced from color separations in which a full-color, continuous-tone image is photographed four times through a series of color filters, so as to produce negatives or transparencies of just one color at a time. Although color separations were once produced exclusively photographically, they are more often than not performed digitally these days. (See Color Separation.)