5The Photometry of Image Formation

The light-transmitting ability of a photographic lens, which principally determines the illuminance of the image at the film plane, is usually referred to as the speed of the lens. Together with the effective emulsion ‘speed’ or photosensor sensitivity and the subject luminance, this determines the exposure duration necessary to give a correctly exposed result. The separate problem of the determination of camera exposure is dealt with in Chapter 19. In the sections below the factors influencing image illuminance are examined in detail.

Stops and Pupils

An optical system such as a camera and lens normally has two ‘stops’ located within its configuration. The term stop originates from its original construction ...

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