Because digital equivalents vary widely, we often use the more familiar 35mm focal lengths in this book. In the upper left corner are shown some image sensor sizes. The larger rectangle is the size of a frame of 35mm film. 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, you have a wide-angle of view that captures a wide expanse of a scene. As you zoom in, the field of view narrows and you can isolate small portions of the scene without moving closer to the subject. 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.