Exposure uses shutter speed to control the timing and aperture (f-number) to control the size of the opening. These settings must balance the film speed with the illumination of scene, therefore they must be based on metering.

Besides the timing control, shutter speed has the freezing effect. Freezing effect is the relative result between shutter speed and the movement of subject. The taken image will have a stiff appearance when shutter speed is faster than the subject movement, a blur appearance when shutter speed is slower than the subject movement.

Besides the control of opening, Aperture (f-number) affects the Depth of Field and the quality performance of lens. When Aperture is big (a smaller f-number, e.g. f/4), the Depth of Field will be shorter. When Aperture is small (a higher f-number, e.g. f/16), the Depth of Field will be longer. Depth of Field means the distance of visual sharpness of a scene, from a nearer to a farther distance of the focus point. It is affected by the focal length of lens, aperture and the rate of enlargement, basing on the principle of Least Circle of Confusion. Quality of lens is not at its best when the aperture is small or big. Usually the best quality of a lens can be achieved when the aperture is somewhere in the middle. There is not a fixed f-number, f/8 is the best in average.

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