The human eyes see objects in three dimensions, but a lens reproduces a view in two dimensions. The missing dimension, depth, is suggested by the relative size and position of the various objects in a picture. Perspective, which is the relationship of objects in a photograph, affects the naturalness of a picture. Good perspective represents objects as they actually appear to human eyes.

Since wide-angle lenses take in a greater area, most photographers use them to photograph in tight quarters. And they use long-focus (long focal length) lenses to bring distant objects closer. This is fine, but it is only part of the story. Lenses of different focal length are also used to control perspective.

Perspective is NOT dependent on the focal length of the lens. It is a function of camera-to-subject distance. But a choice of lenses of a different focal length does enable you to get the desired image size at the selected distance for best perspective. For example, suppose you come across a placid farm scene. A rustic rail fence is in the foreground, and a cow is munching on a haystack in the field. The cow and her lunch are 100 feet behind the fence; you are 10 feet in front of the fence. The fence is essential to your picture and you use a 50mm lens. The result! The cow is 110 feet from the camera and is too small in relation to the fence. Your picture is a flop. Now change your perspective. Back up 40 feet from the fence and use a 200mm lens. The fence at this distance, with the 200mm lens, is the same size as it was at 10 feet with the 50mm lens. The cow is now 140 feet from the camera, but her image is four times larger. In the photograph, it looks as if she were only 35 feet away or 25 feet behind the fence. The results! An interesting picture and pleasing composition. Choosing viewpoint and then selecting focal length for image size is one of the most important functions you should consider when selecting lens focal length.