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Why must a camera’s exposure match the brightness of a subject and the sensitivity of film?

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Diagram showing the negative-positive photographic process: a scene is captured on film in a camera, developed into a negative, projected onto paper with an enlarger, and developed again to produce a positive print. — technology of photography Figure 1: Sequence of negative–positive process, from the photographing of the original scene to enlarged print (see text).

To obtain a photograph that faithfully records the tone gradation of a scene, a camera's exposure must be matched to the subject's brightness and the film's sensitivity.1

The total photographic exposure is determined by the combination of the aperture (the size of the lens opening) and the shutter speed (the duration the shutter remains open). These controls regulate the amount of light reaching the film.21

Because different films have varying levels of speed—or sensitivity to light—they require different amounts of light to produce a specific density of silver during development. Matching the exposure to these factors ensures that the resulting image accurately captures the subject's luminance. While exposure latitude allows for some flexibility in achieving acceptable prints, precise control is necessary for optimal results. Modern cameras often use built-in light meters to measure subject brightness and automatically adjust the aperture or shutter speed to achieve a correct exposure.2314