Principles of Operation
The image from the negative or transparency is projected through a lens fitted with an adjustable iris aperture, onto to a flat surface bearing the sensitized photographic paper. By adjusting 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. As the image size is changed it is also necessary to change the focus of the lens. Some enlargers, such as Leica's "Autofocus" enlargers, perform this automatically.
An easel is used 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. Paper is sometimes placed directly on the table or enlarger base, and held down flat with metal strips.
The enlargement is made by first focusing the image with the lamp on, the lens at maximum aperture and the easel empty, usually with the aid of a focus finder. The lamp is turned off, or in some cases, shuttered by a light-tight mechanism.
The image is focussed by changing the distance between the lens and the film, achieved by adjusting the length of a light-tight bellows with a geared rack and pinion mechanism.
The lens is set to its working aperture. Enlarging lenses have an optimum range of apertures which yield a sharp image from corner to corner, often around f8. The lens is normally set to this aperture and any color filtration dialed in, if making a color print or one on variable-contrast black-and-white paper.
The enlarger's lamp or shutter mechanism is controlled either by an electronic timer, or by the operator - who marks time with a clock, metronome or simply by counting seconds - shuttering or turning off the lamp when the exposure is complete. The exposed paper can be processed immediately or placed in a light-tight container for later processing.
Digitally controlled commercial enlargers typically adjust exposure in steps known as printer points; twelve printer points makes a factor of two change in exposure.
Read more about this topic: Enlarger
Famous quotes containing the words principles of, principles and/or operation:
“Government ... thought [it] could transform the country through massive national programs, but often the programs did not work. Too often they only made things worse. In our rush to accomplish great deeds quickly, we trampled on sound principles of restraint and endangered the rights of individuals.”
—Gerald R. Ford (b. 1913)
“...at this stage in the advancement of women the best policy for them is not to talk much about the abstract principles of women’s rights but to do good work in any job they get, better work if possible than their male colleagues.”
—Virginia Crocheron Gildersleeve (1877–1965)
“It requires a surgical operation to get a joke well into a Scotch understanding. The only idea of wit, or rather that inferior variety of the electric talent which prevails occasionally in the North, and which, under the name of “Wut,” is so infinitely distressing to people of good taste, is laughing immoderately at stated intervals.”
—Sydney Smith (1771–1845)