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an optical apparatus designed for the projection of an enlarged image from a negative onto photographic paper, thus producing an enlarged print.
The simplest enlarger (Figure 1) consists of a lamphouse, negative carrier, projection lens, easel, and a vertical track. The luminous flux generated by the light source passes through the negative and strikes the enlarging lens, which forms an image of the negative frame on the photosensitive layer of the photographic paper positioned on the easel. A latent photographic image is formed in the photosensitive layer, which becomes a visible, positive image as a result of subsequent photographic processing—developing and fixing. Thus, an enlarger is designed primarily for the completion of one stage in the positive process. A diapositive is often inserted into the holder instead of a negative. Prints from diapositives are primarily used for technical purposes, such as the production of graphs and blueprints. An enlarger may also be used to project an image from a diapositive onto a screen.
The lamphouse contains a light source, usually an incandescent lamp with a frosted bulb. A spherical reflector is positioned behind the lamp to increase the luminous flux directed toward the negative. Most enlargers have a condenser, which concentrates the luminous flux emitted by the lamp, thereby increasing the illumination of the negative image. The negative carrier consists of two slats, each with a frame window and a guide for the film. The lamphouse, negative carrier, and enlarging lens are structurally combined to form a single unit, called the head assembly.
The size of the image on the photographic paper depends on the focal length of the lens as well as on the distance between the negative and the easel; the latter may be altered by shifting the head assembly relative to the easel along the vertical track. The enlarging lens may be focused manually or automatically when the size of the image is changed. Some enlargers are designed to permit rotation of the head assembly by 180° about the vertical axis (for projecting images on the floor when large-size enlargements are required) or by 90° about the horizontal axis (for projecting on the wall). In enlargers used in producing color prints, the head assembly has a pocket for the insertion of corrective light filters.
Enlargers are sometimes equipped with a slit-type rangefinder mechanism to simplify focusing. The most advanced enlargers have an exposure meter for determining or automatically setting exposures during printing, as well as a device for selecting color filters. A timer connected to the enlarger automatically switches the lamp off when the selected exposure time is reached.
REFERENCEIofis, E. A. Tekhnika fotografii. Moscow, 1973.
S. V. KULAGIN