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A negative is said to be correctly exposed when it gives a satisfactory rendition of detail in both the deepest shadows and brightest highlights of the scene or subject. Fortunately, in many cases, there is more than a single exposure that will produce this result - there is a wide range of possible exposures within which satisfactory tone separation is possible. The "minimum" satisfactory exposure is one in which good tone separation is just attained in the deepest shadow areas. The "maximum" satisfactory exposure is one in which detail is just retained in the brightest highlight. Any additional exposure will cause this highlight detail to become flattened out or "blocked up." The range between these two exposures is known as latitude. This latitude may be narrow or wide, depending on the subject matter, lighting contrast, type of film and degree of development of the negative. In general, the black-and-white films you will be using have a greater margin for error than color films. Ignoring the influence of development for a moment, a softly lighted scene composed of objects that are, themselves, fairly uniform intone will allow a wide range of possible exposures that will produce a satisfactory rendition on the negative. This is caused by the narrow range of tonal values from highlight to shadows in the subject. On the other hand, a brilliantly lighted scene composed of a variety of tones, from jet black to snow white, may take up the entire usable range of the negative scale. Therefore, the exposure required for the proper rendition of the entire range of tonal values in this scene may be quite critical. In addition there are many scenes, such as interiors with sunlight coming through a window, that have a range of brightness so wide that no single exposure can produce both highlight and shadow detail in a black-and-white negative. When you increase exposure and reduce the amount of development, almost any ordinary extreme of brightness range can be accommodated on black-and-white film. The sensitivity of black-and-white and color film for still-camera use is also called the film speed, the ISO speed or simply the ISO. Earlier in this chapter, we pointed out that "ISO" is an acronym for International Standards Organization, a federation of all national standard bodies of the world, which has approved a uniform set of film-speed standards. These standards call for a universal expression of both arithmetic and logarithmic values with the ISO designation. Until early 1983 the emulsion speeds of still-camera film were expressed in ASA values (which are arithmetic) or in DIN values (which are logarithmic). ASA values were determined according to standards published by the American National Standards Institute, formerly American Standards Association from which the designation ASA came. The DIN values reflected the German standards established by the Deutsche Industrie Norm. Film speed is determined by the manufacturer according to the ISO standards. It will generally look like this: ISO 100/21 The number immediately following "ISO" is the ASA equivalent. It indicates that the speeds progress arithmetically, and any film marked ISO 100 has the same sensitivity as any other film marked ISO 100 - it is twice as fast as film marked ISO 50 and is half as fast as film marked ISO 200. The number with the degree symbol (0) is the DIN equivalent. The arithmetic speed number is intended for exposure meters or cameras marked for ISO or ASA speeds or exposure indexes. The logarithmic speed is intended for exposure meters or cameras marked for ISO or DIN settings. FILM TYPES Photographic films (and papers) are composed of two basic parts: the emulsion and the base, or support. The emulsion is the light-sensitive portion of a film or paper that records the image. The emulsion contains the silver halides and any special sensitizing dyes suspended in a binder of gelatin. The gelatin holds the silver halides evenly dispersed and prevents action by a developer until the silver halides have been made developable either by exposure to light or chemical action. The gelatin also acts as a sensitizer for the silver salts. In photographic films and papers, the main purpose of the base is to support or hold the emulsion in place. Depending on how the recorded image is to be used, the base or support may be transparent or opaque. A transparent base is used for transparencies viewed by transmitted light and for negatives printed with transmitted light. An opaque base is used for prints that are viewed by reflected light. The latest state of the art in light-sensitive materials used in photography is the use of the electronic medium. Still video disks do not contain an emulsion or a base. When video mediums are used, light is converted to electrical impulses, and these impulses are stored magnetically on a tape or disk. Since it is the camera itself that converts the light to electrical impulses, the recording medium and all stages of the photographic process can be carried out in normal room light. Black-and-White Film The characteristics and use of black-and-white film depends largely on the actual construction of the emulsion. These characteristics include the following: the degree of sensitivity to light, response to various colors of light (color or spectral sensitivity), contrast, exposure latitude, emulsion latitude, and emulsion definition. There are many types of black-and-white films available. Each differs from others in one or more characteristics. You should become acquainted with the characteristics of films. This knowledge is helpful in selecting the film most suitable for each photographic assignment. A color negative film records a scene in image densities opposite to the brightness of objects in the scene, the same as a black-and-white negative film. Color films can be recognized because they contain the suffix "color," such as Vericolor, Kodacolor and Fujicolor. These color films are used when a print is the final product. Most color negatives (except for color film used for aerial photography) has an orange mask incorporated init. This orange mask increases the color separation which reproduces colors more accurately in the final print. During development, colors that are complementary to the color in the original scene are formed in the emulsion. For example, a red object in the scene is recorded as cyan in the negative. A combination of yellow, magenta, and cyan record all the other colors that we see in the scene. Color dyes in the emulsion layers control the colors of light passing through the color negative. To produce color prints or color transparencies, you can print color negative film images on color positive materials such as color paper and color print film. Color negatives also can be printed on a special panchromatic black-and-white paper to produce black-and-white prints. Amateur and Professional Color Films Much of the color film used in the Navy is manufactured by the Eastman Kodak Company. Kodak markets color films for both professional and amateur photographers. Color films intended for use primarily by professionals are identified by the word professional in the name - for example, Kodak Vericolor III Professional Film, Type S (VPS). Both professional and amateur films have similar color quality, sharpness and granularity characteristics. They also have emulsions made up of many different chemicals that tend to change slowly with time. From the day they are made, all color films begin to change, and as the films age, their color balance changes. Amateur films are manufactured to age and reach a peak color balance much later than professional films. The manufacturer allows for the time amateur film will be in storage, on the store shelf, and in the camera before it is developed. The ISO speed assigned is adequate for calculating exposure for normal picture-taking situations. Professional films are manufactured so they are very near their optimum color balance at the time they are shipped from the factory. These films should be kept refrigerated or frozen until shortly before use. Refrigeration keeps the film near the optimum point until used and provides the photographer with confidence in consistent results. Precise film speeds are provided for professional films. The film is intended for prompt processing to prevent any significant shift in color balance after exposure. |
 
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