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.
| Contents |
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.
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.
Photographic paper is placed into a special holder, called an easel, designed to hold the paper perfectly flat. Some easels are designed with adjustable overlapping flat steel "blades" to crop the image on the paper to the desired size while keeping an unexposed white border about the image. Larger sheets of paper are sometimes placed directly on the table or enlarger base, and held down flat with metal strips.
As the image size is changed it is also necessary to change the focus of the lens, unless the enlarger is equipped with a mechanical linkage to change focus with each adjustment in height, as in Leica's "Autofocus" enlargers. Usually, the enlarger is focused by moving the lens closer to or further from the negative holder by adjusting the length of a light-tight bellows with a geared rack-and-pinion mechanism.
Large horizontal enlarger structures are used when high quality, enormous large format enlargements are required such as when photographs are taken from aircraft for mapping and taxation purposes.
There are practical limits to the enlargement ratio determined by the the quality of the enlarging lens used and the quality of the negative being enlarged. Negative quality is determined by a number of factors, the most critical being grain density and critical focus, followed by the amount of camera shake and the quality of the lens used to create the negative.
The enlargement is made by first focusing the image with the lamp on and the lens wide open and the easel empty, usually with the aid of a grain focuser. The lamp is turned off, or in some cases, shuttered by a light-tight mechanism. The photo-paper is placed into the easel, and the lens stopped down to a reasonable working aperture. The aperture is selected by balancing the duration of exposure with both depth-of-field to compensate for the minute focusing and alignment error inherent in even the finest mechanisms, and also with regard to which stop the enlarging lens produces the best optical results at. The color correction is dialed in, if it's a color print, or the variable-contrast filter selected if using variable-contrast black and white paper. While there are exposure and color meters available for the darkroom, most printers use the judgement of experience and comparative tests to find the optimal settings for each image.
The enlarger's lamp or shutter mechanism is controlled either by an electronic timer, or by the operator, who marks time with a luminescent clock or metronome, shuttering or turning off the lamp when the exposure is complete. The exposed paper can be developed right away, or placed in a light-tight contaner for later processing.
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.
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 interrupting 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. His book trilogy, The Camera, The Negative and The Print are essential reading for those who would like to learn more about printing and enlarging photographs.
As with other photographic printing methods, it 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.
See also
- Contact printer for a non-enlarging method of producing photographic prints.
- Projector for a directory of projector types.de:Vergrößerungsgerät