Learn about Photography with help from Grolier

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Camera's could be considered to be consumer electronics these days. I found this description of photography and thought that it would make an interesting post. I stole it from Grolier cause they make you pay for the information. I thought that was kinda rotten.


photography

Table of Contents

The term photography (from the Greek, phos, "light," and graphein, "to draw") means the production of a negative or positive black-and-white or colored record initiated by the action of radiant energyusually in the form of light upon a sensitive surface.

The fundamental physical principle of photography is that light falling briefly on the grains of certain insoluble silver salts (silver chloride, bromide, or iodide) produces small, invisible changes in the grains. When placed in certain solutions known as developers, the affected grains are converted into a black metallic silver. In color photography, further interactions take place with color-forming agents.

When a photograph is taken with a camera, light reflected from the object passes through the lens, diaphragm, and shutter, to form an inverted image. For the brief period during which the shutter is open, this image falls on the surface of a film or plate sensitized by silver salts and causes an invisible latent image to be recorded on it.

With an instant (Polaroid-type) camera, development and printing are carried out both while the film is still in the camera and just after the picture has been ejected. In most cameras, however, pictures are taken until all the film has been exposed; then the still-undeveloped film must be removed in darkness or in greatly subdued light and placed in a developing solution. This chemical solution darkens the affected grains of silver salt and converts the latent image into a negative image, in which dark and light areas in the object are recorded as light and dark areas, respectively, on the negative. The negative is then placed in a fixing solution, which dissolves the unaffected grains of silver salt and prevents any further action by light on the image.

To produce a positive contact print, light is passed through the negative so that it falls on a piece of printing paper held in close contact with the negative. Once again a latent image is produced; it is then developed into a positive image and fixed. Just as with the formation of the negative, this process reverses the dark and light areas and reproduces the original tones of the subject in a positive print.

Negatives today are usually too small to make a useful contact print. An optical enlarger is therefore used to project a magnified image of the negative onto the printing paper, which is then developed and fixed to yield a large positive print called an enlargement.

Camera
A camera consists essentially of a box carrying a lens, diaphragm, and shutter that are arranged to project an image of the scene to be recorded onto a sensitive film or plate.

The lens is usually made up of several components. It forms a real, inverted image of the object. In the popular 35-mm cameras the focal length is typically 50 mm (2 in), but it can be shorter or longer according to the size of the camera.

In the focusing mechanism provision is made for moving parts of the lens backward or forward to focus the image on the film. Three main methods are used to determine the position of the lens: focusing scale, range finder, and reflex finder.

Two types of shutters are commonly used. The between-the-lens shutter is mounted between the components of the lens. The focal-plane shutter consists of a roller blind containing a slit that moves rapidly across the plane in front of the film. In popular cameras the shutter provides a range of exposures from 1 second or longer, to 1/1,000 of a second or much shorter.

The diaphragm may also be placed between the components of the lens. It provides a roughly circular hole whose variable size regulates the amount of light that reaches the film.

If the light is weak, or if a short exposure is required, the diaphragm is opened wide to admit sufficient light. Under good lighting conditions with moderate exposures the diaphragm is set to a smaller aperture, thus reducing the amount of light reaching the film. The smaller aperture can also reduce the effects of some optical aberrations and of any error in focusing, thus producing a sharper picture (see depth of field).

Of the various kinds of viewfinders, the simplest consists of a small hole, which serves to position the eye, and a wire frame a few inches in front of the hole, which delimits the field of view that corresponds to the image on the film. Most cameras today use optical or through-the-lens viewfinders.

Types of Cameras. The wide range of camera types is largely determined by the degree of compactness and portability required. The range extends from the ultraminiature camera, which weighs a few ounces and has a picture size of the order of 1 cm (0.4 in) square, to the large studio or view cameras weighing many kilograms or pounds and taking a picture of 20 25 cm (8 10 in) or more in size.

Specialized types of cameras are also used, for example, for aerial or underwater photography. In some cameras the film is advanced by a motor drive for taking a short, rapid succession of still photos. Others operate in conjunction with multiple flash to produce a number of superimposed pictures. Digital cameras (see digital photography) convert light into recorded digital signals. Digitized images can be viewed immediately on a TV or computer screen and can be manipulated using computer image processing.

Black-and-White Photography

Film Composition. Film for black-and-white photography consists of a transparent base on which is spread a thin layer of an emulsion consisting of a suspension of minute mixed crystals of silver halides, such as silver bromide and silver iodide, in gelatin. The gelatin not only holds the grains but also greatly increases their sensitivity to light. The formation of the latent image was explained above.

Almost all films, both black-and-white and color, are panchromatic that is, they are sensitive to almost all visible wavelengths. Sensitizing dyes are added to the emulsion to extend the sensitivity to include the long-wave (red) end of the spectrum.

Grain and Speed. Films vary in the size of the grains they contain. This variation, in turn, has an effect on the film's speed, or sensitivity to light. Coarse-grain films are faster than those of fine grain, probably because a larger grain intercepts more light. Faster films can therefore be used with a shorter exposure, in a weaker light, or at a smaller aperture. The resultant picture, however, is apt to show grain, especially when greatly enlarged. Slower, fine-grain films are used for work of highest quality. The speed of a film is measured using a number of systems; for example, the International Standards Organization (ISO) scale is one common measure. Slow films have speeds of around ISO 50; ISO 200 400 is satisfactory for general purposes; ISO 600 designates a fast film. The scale is linear, so that films of speeds ISO 100 and 200 require one-half and one-quarter, respectively, of the exposure required for a film of ISO 50.

Developing. The exposed film is transferred in darkness into a developing solution, which causes any affected grain of the latent image to be wholly converted into silver. After a brief rinse in a stop bath of acid or water to neutralize developer and stop development, the film is fixed in a solution of sodium thiosulfate (often called hypo), which dissolves and removes the unchanged silver salts, thus rendering the film insensitive to light. (Roll film, such as 35 mm, can be wound onto a spiral developing reel, which is then placed in a light-proof tank into which the successive processing solutions are poured.) Finally the film is washed and dried. Each picture is now a transparent negative in which light parts of the object are represented by dark areas and dark parts by light areas.

Printing. A large negative can be used to make a contact print, but small negatives are usually placed in an enlarger, where light from a lamp throws an enlarged and accurately focused image onto a piece of printing paper that has been coated with an emulsion similar to that on the film. The paper is then developed and fixed to form a positive print.

A print in which the dark areas are extremely black and the light areas extremely white is said to show high contrast, while one in which only various shades of gray appear is said to be of low contrast. The degree of contrast depends on the scene itself, but it also depends on the enlarger, the film, the exposure, and the development time (longer development gives higher contrast). Different grades of paper or different filters used with variable contrast paper range from hard (contrasty) to soft (low contrast) and are also used to control the degree of contrast in the final print.

Lighting
A photograph is no more than a visual record of the variation in the brightnesses of different parts of a scene. Because the ratio of intensity of illumination from bright sunlight to the dim lighting of a dark interior is of the order of a million to one, two fundamental requirements for good photography are a way to provide appropriate lighting and a way to measure how much light is available.

Natural daylight from two hours after sunrise to two hours before sunset varies according to weather conditions by a factor of only about 10, corresponding to a varying aperture of slightly more than three stops of the diaphragm; under these conditions, the exposure may be set by following some simple rules based on the conditions: bright sunlight, cloudy bright, open shade, or cloudy dull.

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Exposure Meter. Under more complicated lighting conditions, an exposure meter should be used (see actinometer). One type of exposure meter consists of a photoelectric cell connected to an ammeter. Light falling on the cell causes a current to flow; the deflection of a needle or some other indication gives a reading of the intensity of the light.

The meter can be separate from the camera or can form an integral part of it. A popular arrangement is to have the photocell mounted inside the camera so that it receives a part of the light forming the photographic image. Light can be directed onto the cell in one of several ways without seriously impairing the image itself. The method is known as TTL (through-the-lens) metering.

In one TTL arrangement, called match-the-needle metering, the photographer adjusts the exposure time, the f-stop (diaphragm setting), or both to bring a needle in the viewfinder into alignment with a fixed mark. A more modern arrangement is for the photographer to preset one of the two settings (shutter speed or aperture). Other metering modes allow the camera to choose both settings.

A TTL measurement method that depends on light reflected from the subject is excellent because it totally eliminates the effect of variations in the intensity of the incident light; in practice it is effective with average subjects. It does have its drawbacks, however. For example, suppose automatic aperture control is used to take three positive color transparencies: the first of a white, the second of a gray, and the third of a black piece of paper (black paper reflects about 10% of the incident light). The automatic aperture control would produce three images of identical intensity. With black-and-white film, the three pictures will show an identical shade of gray even though the three original subjects varied considerably in brightness.

An alternative method, incident light measurement, eliminates the above drawback. The incident light meter is placed in the scene to be photographed and is usually pointed at the light source. The meter reading indicates the exposure (combination of shutter speed and f-stop) appropriate for a film of a given speed. Dark and light objects in the scene will receive identical exposures and will be appropriately recorded as dark or light in the picture. A TTL meter can achieve the same effect if it is directed at a gray card placed in the scene to be photographed; the reading will depend only on the incident light intensity, making the method an incident light method.

Color Filters. Filters placed over the lens of the camera are used to modify the light passing through the lens and onto the film. Because filters do not transmit all the light that reaches them, the exposure must usually be increased when a filter is used. A yellow filter absorbs blue rays and greatly improves many black-and-white pictures by darkening the sky. Its use requires an approximate doubling of the exposure. A red filter gives similar but stronger effects and may produce dramatic pictures of mountain scenery. Color filters may also be used to increase contrast. Suppose a dress has a pattern of orange and blue. A blue filter will allow blue rays to pass but will block orange rays, so that in the picture the orange areas will be darkened. A polarizing filter appropriately oriented will block the polarized light from the sky and is useful in darkening blue sky in color photographs; it can also be used to eliminate undesirable reflections in the picture.

Lighting with Electronic Flash. When light is weak, additional light can be created using various electronic flash devices. (The flashbulb and the flashcube, used on older cameras, are now obsolete. They were both electrically activated.) The flash is produced by a capacitor charged to a high voltage discharging through a flashtube in as little as 1/10,000 sec, a time short enough to freeze rapid action. The firing of the flash is usually synchronized with the shutter action. A sensitive electronic eye, often mounted on the camera itself, may be used to operate a mechanism that extinguishes the flash when sufficient light for exposure of the film has been reflected back from the subject. Multiple flash may be used to produce a number of superimposed pictures; such a technique is useful for analyzing movement in sports, for example (see flash photography).

Color Photography
Like black-and-white photography, color photography depends fundamentally upon the darkening of silver salts. It is possible because any color can be made from mixtures of the three primary colors: red, green, and blue. (See color perception.)

Each of these three primary colors has a complementary color, which when mixed with it gives white. When red is mixed with blue green (cyan), green with magenta, and blue with yellow, each mixture yields white. Thus red and cyan are complementary colors, as are green and magenta, and blue and yellow.

Note that two successive transformations into the complementary color restore the original color for example, green, into magenta and magenta back into green. This effect forms the basis of the negative-positive process.

The Negative-Positive Process. The negative film consists of three superimposed layers: one blue-sensitive, one green-sensitive, and one red-sensitive. Exposure in the camera is followed by development, which produces deposits in black silver grains representing the blue, the green, and the red parts of the optical image. During development, substances known as dye couplers incorporated in the three layers are converted into dyes that stain each of the three silver images in a color complementary to the color of the light that formed it. The silver itself is bleached and removed so that only the transparent color remains. In the negative, therefore, the blue parts of the picture are represented by yellow, the green parts by magenta, and the red parts by cyan.

Next, the negative is placed in an enlarger, where the image is thrown onto printing paper coated with three superimposed layers of emulsion that are essentially the same as those in the negative color film; they are developed in essentially the same way. Just as the negative forms colors complementary to those in the optical image, so the paper forms colors complementary to those in the negative. The result is that the original colors of the subject are reproduced. (The stages in the processing of the negative and print are actually more elaborate; the time taken is about one hour, and stringent control of temperature of the solutions is required.)

In producing the color print, care is needed to ensure accurate color rendition. In one method, three successive exposures are made in the enlarger one with blue, one with red, and one with green light and the relative durations are adjusted from indications given by test strips. In an alternative method, a white light source is used with a single exposure. As successive test strips are made, color-correcting filters placed in the light path are adjusted until a satisfactory test strip is obtained.

The Reversal Method for Color Transparencies. As explained, red light plus cyan light, green light plus magenta light, and blue light plus yellow light each equals white. Thus, cyan is the color produced from white light when red light is removed from it. Therefore cyan light can be thought of as minus red, magenta as minus green, and yellow as minus blue.

Positive, or reversal, film consists of three superimposed emulsion layers sensitive to the red, green, and blue parts of the spectrum, respectively. As in the previous development method, after exposure in the camera and a first development, three superimposed black-and-white images in silver are formed that represent the red, green, and blue parts of the optical image. In the reversal method, however, these negative images are not stained. The film is next exposed to light from a lamp, which induces the formation of a latent image in the previously unaffected grains of silver salt that is, in those grains unaffected by the red, green, and blue lights in the optical image. A different dye coupler is incorporated in each of the three layers. When the film is given the second development, a second set of three silver images is formed. The dye couplers form dyes that stain the newly forming second images, but not the first ones, coloring them as follows: The red-sensitive layer produces the color cyan (minus red); the green-sensitive layer produces magenta (minus green); and the blue-sensitive layer produces yellow (minus blue). Both sets of silver images are bleached and washed away, so that only the transparent dyes remain.

For example, in an area where the optical image was red, a black deposit was initially formed in the red-sensitive area; it is bleached and washed away, so that the layer is clear. In the green- and blue-sensitive layers the second silver images were stained minus green and minus blue, respectively, and only these colors remained after bleaching. When the transparency is viewed by projection of white light through it, the minus-green and minus-blue dyes remove the green and the blue components of the white light, leaving only the red. Thus the original red color in the optical image is correctly reproduced. Similar processes can be applied to the green and the blue parts of the optical image.

Polaroid instant color film is developed through a process called diffusion transfer. The film itself is composed of many discrete, microscopically thin layers. As the exposed film emerges from the camera, a chemical developing agent is forced between the top layers, which hold the color dyes, and the bottom layers, on which the image is imprinted. The image develops below, while the chemicals producing the colors diffuse through the upper layers.

Harry Asher

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Reviewed by Stephen Petegorsky

Bibliography: Adams, Ansel, The Camera (1980; repr. 1995), The Negative (1981; repr. 1995), and The Print (1983; repr. 1995); Butkowski, Joel, and Kemper, Van, Using Digital Cameras (1998); Craven, George, How Photography Works (1986); Eastman Kodak Staff, The Complete Book of Photography (1986) and More Joy of Photography (1988); Eisenstaedt, Alfred, Eisenstaedt's Guide to Photography (1981); Focal Encyclopedia of Photography, 3d ed. (1993); Freeman, Michael, ed., The Image (1987); Hirsch, Robert, Exploring Color Photography, 3d ed. (1996); Horenstein, Henry, Black and White Photography, 2d ed., rev. (1983); Koschatzky, Walter, The Art of Photography (1985; repr. 2002); London, Barbara, ed., Photography, 7th ed. (2001); London, Barbara, and Stone, Jim, A Short Course in Photography: An Introduction to Black and White Photographic Technique, 4th ed. (2000); Lyons, Nathan, Photographers on Photography (1966); Patterson, Freeman, Photography and the Art of Seeing, rev. ed (1989); Sawyer, Ben, The Digital Camera Companion (1996).