An adjustable camera controls the amount of light reaching the film in two ways. It can make the Image darker or lighter by a variable aperture. positioned behind the lens. and it can control the length of time that the light reaches the film. by a timed shutter. The lens aperture consists of overlapping movable leaves which form a diaphragm.

This can be set to a range of diameters. so that the quantity of light admitted is controlled In the same way that the width of a funnel controls the quantity of water flowing Into a container. When photographing a dimly lit subject you use a wide lens aperture to admit as much light as possible; for a bright subject you can c;hange to a small aperture to reduce the amount of light. In this way the film still receives the same amount of llght.

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