7.2 ZOOM LENSES AND FOCAL LENGTH

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A zoom lens lets you choose any focal length within the range the lens is designed for. When you change focal lengths by zooming the lens, two important effects are immediately obvious in the lens’ angle of view and its magnifying power.

Angle of view refers to how much of a scene the lens covers. Zoomed out using a shorter focal lenth, you have a wide-angle of view that captures a wide expanse of a scene. As you zoom in using a longer focal length, the field of view narrows and you can isolate small portions of the scene without moving closer to the subject.

Here you see the angle of view of various focal length lenses. The shorter focal length lenses are on the left and the longer ones on the right.

Magnification is related to the lens’ angle of view. Since zooming out includes a wide sweep of the scene, all of the objects in the scene are reduced to fit into the image. Zooming in gives a much narrower angle of view, so objects in a scene appear larger.

Zoom lenses on digital cameras work much like those on camcorders. There are two buttons or a movable lever. Pressing one zooms in, increasing the focal length and narrowing the angle of view. Pressing the other zooms out, decreasing the focal length and widening the angle of view.

One of the best things about zoom lenses is the speed with which you can react to photo opportunities. Here, the key action in the scene on the top is lost in the large frame (left). By zooming in, this key action was isolated (right).