The Physics and Art of Photography, Volume 3: Detectors and the meaning of digital by John Beaver

Author:John Beaver
Language: eng
Format: epub
ISBN: 9781643273860
Publisher: IOP Publishing
Published: 2018-12-20T00:00:00+00:00

8.2 Brightness and contrast adjustments

When one makes a print from a negative, each shade of gray on the negative is ‘mapped’ to a corresponding shade of gray on the print. Of course it is a negative process, and so a light shade on the negative becomes a dark shade on the print, and vice versa. But how does one tell precisely which shade from the negative ends up exactly what shade on the print?

For making a black-and-white print in the darkroom from a black-and-white negative, this question is usually answered in two parts. The overall lightness of the print is determined by controlling the exposure of the enlarging paper to the projected image. The projection lens in the darkroom enlarger has an aperture adjustment so that one may allow either more or less light to pass through the lens. But also, the enlarger lamp is always connected to some kind of timer (see figure 8.4) that allows one to expose the print paper for a controlled length of time. Since the enlarging paper is a negative process, a longer exposure or a larger lens aperture results in an overall darker print.

Figure 8.4. The overall lightness/darkness of a black-and-white print is controlled in the darkroom by adjusting the exposure from the image projected by the enlarger. A timer (rear left) allows one to vary the exposure time. Some enlargers also allow one to vary the brightness of the projection lamp (rear right). The two sighting devices in the foreground allow one to magnify the projected image in order to achieve a perfect focus.

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