Enlarger

An enlarger is a specialized transparency projector used to produce photographic prints from film or glass negatives. It is always used in an enclosed space from which extraneous light may be excluded, called a darkroom.

Principles of operation

The enlarger consists of a light source with mirrored reflector and a condensing lens. The directional light then passes through a film holder, which may hold glass mount, sheet or roll stock photographic negatives, which have been previously exposed in a camera and developed. 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.

There are also practical limits to the enlargement ratio determined by the quality of the enlarger imaging lens and the quality of the negative. Negative quality is determined by the scene illumination, film eposure, processing, and grain size, the accuracy and rigidity of the camera body, the negative size, aperture, shutter speed used, and the quality and focus of the camera lens used to create the negative.

Styles of projectors

Smaller units are usually mounted pointing downward and may be adusted up or down to change the size of the image projected onto a table. Sensitized paper is placed on the table and held down flat with metal strips. As the image size is changed it is also necessary to change the focus of the lens. Large horizontal enlarger structures are used when high quality large format pictures are required such as when photographs are taken from aircraft for mapping and taxation purposes.

Advantages

The principle advantage is that a large print may be made from a small negative. Without an enlarger, only a contact print would be available and large images would require large size negatives and hence very large cameras.

Another advantage of using an enlarger is that it is possible to vary the exposure on the print in various areas by interupting the light with a mask - if it is large, with a hole, the process is called "burning", which will have the effect of darkening the regions with additional exposure, while the use of a small mask to reduce the exposure is called "dodging" and has the effect of lightening the regions with reduced exposure. The mask must be moved about to avoid producing a sharp edge at the region boundary. With the controls available it is possible to make significant changes in the mood or emphasis of a photographic print. The best known master of making art in the darkroom is the famous nature photographer Ansel Adams. 

As with other photographic printing methods is also possible to make composite photographs by overlaying the print with a hand-cut mask, performing an exposure, and then using the inverse of that mask to perform another exposure with a different negative. This is much more difficult to do well using photographic methods than it is now by using the methods of modern digital image manipulation.

Additional processing

After exposure of the sensitized paper it is then processed in a multi-step chemical process to develop the print. It is especially fascinating to observe the production of black and white prints using the gelatin-silver process, as the sensitized material is unresponsive to red light. It is thus possible to perform all material preparation, image adjustment, exposure, and chemical processing using a red "photographic safe light" for general ilumination of the darkroom. Equipment preparation is much easier and the development is similar to watching the development of a polaroid instant print as the image gradually appears before one's eyes. It is also possible to interrupt the process when the desired image value has been obtained by moving the print from a tray with "developer" chemicals to a tray with "stopper" chemicals (usually acetic acid, which is common distilled vinegar). After stopping, the print is moved to a tray with a fixative chemical called "hypo" that permanenty preserves the image, and then to a rinse bath, where any remaining chemical traces are removed with water. Finally, if it is desired to obtain a glossy or textured finish to the surface of the prints, they are placed face down on smooth or textured glass or metal plates for drying, otherwise they are hung to dry in the air.

When color prints are to be generated, all exposure and chemical processing must be done in complete darkness except for that provided by the enlarger during actual exposure of the paper.

Automated print machines

Automated photo print machines are comprised of the same basic elements and integrate each of the steps outlined above in a single complex machine under operator and computer control. Rather than project directly from the film negative to the print paper, a digital image may first be captured from the negative. This allows the operator to quickly determine adjustments to brightness, contrast, clipping, and other characteristics. The image is then rendered (in negative) on an appropriate display device and a built-in computer controlled enlarger projects this image to the film for final exposure.

See also

• Contact printer for a non-enlarging method of producing photographic prints.
• Projector for a directory of projector types.