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An enlarger is a specialized transparency projector used to produce photographic prints from film or glass negatives. Prints made with an enlarger are known as enlargements. more...

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Typical enlargers are used in a darkroom, an enclosed space from which extraneous light may be excluded; some commercial enlargers have an integral dark box so that they can be used in a light room.

Principles of operation

A condenser enlarger consists of a light source with mirrored reflector and a condensing lens. Alternatively, a cold light enlarger has a fluorescent light tube masked by translucent glass as its light source. The directional light then passes through a film holder, which may hold sheet or roll stock photographic negatives and transparencies, which have been previously exposed in a camera and developed. Color enlargers often have an adjustable filter mechanism between the light source and the negative for color correction with controls for the amount of cyan, magenta and yellow light reaching the negative. The negative image is then projected through an adjustable iris aperture and focusing lens to a flat surface upon which is mounted the sensitised paper to be exposed. By adusting the ratio of distance from film to lens to the distance from lens to paper, various degrees of enlargement may be obtained, with the physical enlargement ratio limited only by the structure of the enlarger and the size of the paper. The parts of the enlarger includes baseboard, enlarger head, elevation knob, filter holder, negative carrier, glass plate, focus knob, girder scale, timer, bellows, and housing lift.

Image enlargement limits

The practical amount of enlargement (irrespective of the enlarger structure) will depend upon the grain size of the negative, the sharpness (accuracy) of the both the camera and projector lenses, blur in the negative due to subject motion and camera shake during the exposure of the negative, and the intended viewing distance of the final product. As an example of the intended viewing distance, a 5 by 7 inch image to be viewed in a scrapbook at 18 inches may be unsuitable for use as an 8 by 10 inch image to be hung on a hallway wall to be viewed at the same distance , but usable at a larger 5 feet by seven feet (twelve times larger) on a billboard to be viewed no closer than eighteen feet (twelve times more distant).

Enlarger physical arrangements

Smaller units are usually mounted with the head pointing downward and adusted up or down to change the size of the image projected onto the enlarger's base, or a work table if the unit is mounted to the wall. Most models consists of the head, or the assembly containing the light source, filters, film holder, focusing system and lens, mounted to a single post, which may have gearing for precise height adjustment. Other models are comprised of a trestle, with the head mounted on crossbars between two or more posts for extra stability. Large horizontal enlarger structures are used when high quality, when large format enlargements are required such as when photographs are taken from aircraft for mapping and taxation purposes.

Read more at Wikipedia.org