Perspective

Vincent Bockaert, 123di.com

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.

A.
Scene taken with a 33mm wide angle.
B.
Cropped area indicated in image A, taken with a 33mm wide angle.
C.
Scene taken with 80mm tele, with the camera in the same position as in image A (same subject distance). Note that the perspective is the same as in image B, taken with a 33mm wide angle.
D.
Scene taken with a 33mm wide angle after coming closer to the subjects so that the width of the two front tiles covers the width of the frame, just like in image C. The perspective is clearly different and the distance between the subjects appears larger than in image C.

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