If you photograph a subject with a tele lens and want it to have the same size on the film or sensor when photographing it with a wide angle lens, you would have to move closer to the subject. Because this would cause the perspective to change, lenses with different focal lengths are said to "have" a different perspective. Note however that changing the focal length without changing the subject distance will not change perspective, as shown in the example below.
Images B and C show that changing the focal length while keeping the subject distance constant has—just like cropping—no effect on perspective.
Image D shows that changing the subject distance while holding the focal length constant will change perspective.
Images C and D show that a tele compresses perspective (makes subjects look closer to one another), while a wide angle exaggerates perspective (makes subjects look more separated) compared to the "normal" way we see things with the naked eye. As mentioned earlier, this change in perspective is a direct consequence of the change in subject distance and thus only an indirect consequence of the change in focal length. Indeed, a wide angle lens allows you to capture subjects from nearby, while a tele lens allows you to capture distant subjects.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
Click here to visit 123di.com