An image's perspective is the apparent size relationship between the objects in the picture. Why should you care? If your wife thinks her nose is twice its actual size in your pictures, you care. If your stateroom is half the size it appeared in the cruise line's brochure, you care.

Perspective is entirely controlled by distance. For example, place a 12 inch ruler one foot from the camera, and a second 12 inch ruler a foot farther away. The more distant ruler will appear half the size of the first ruler. Move the camera back (away from the rulers) one foot so the rulers are now two and three feet away respectively. The second ruler will now appear only one-third smaller. (The effect is the same for the eye).

Wide-angle and telephoto lenses are sometimes said to distort perspective. Actually, they don't distort, they just allow the camera to be moved nearer to or farther from the subject while maintaining image size.

In general, use a wide-angle to exaggerate size differences or to emphasize the closer subject. Interiors may require a wide-angle just to get most of the room in the picture. If you include a piece of furniture near the camera, the room will also appear larger than its real size (the furniture will appear very large in relation to the background which implies the background is far away).

A common misuse of wide-angle lenses is photographing scenics, such as a mountain range in the distance. Since distant objects appear smaller with a wide-angle lens, such pictures are usually disappointing. (Try shooting the same picture with different focal lengths. With digital cameras its basically free if you don't print the images, and the best way to learn photography). Such scenics can often be improved by "framing" the picture with, for example, a tree trunk and limb. The close object gives the picture a sense of depth.

Use a telephoto to minimize differences. For the mountain range scenic, mountains at different distances will seem closer together and more impressive. "Less is more" often applies to pictures. Trying to get "everything" in a picture is nearly always a mistake.

When shooting head-and-shoulder portraits, always use a focal length at least twice the normal lens (maximum telephoto on the 3X zooms common on digital cameras). With a normal lens, you must get much closer to the subject for a head-and-shoulder image than we normally use for conversation and the nose will appear over-size. Professional photographers refer to lenses that are twice normal focal length as "portrait" lenses.

After posting this sticky, I received the following email from matofthemint. He is correct:

I agree with your comments on 'photo perspective', but you left out one important factor.

The final effect of perspective is the viewing distance. If the picture is viewed at a distance of the focal length of the lens times the degree of enlargement the perspective will be 'normal'.

A foto of a given subject with the camera in a fixed position -
25 mm lens x 10 times enlargement = 10 inch viewing distance.
100 mm lens x 10 times enlargement = 40 inch viewing distance.
The pictures will be identical and appear as the eye saw the subject.
But pictures aren't usually viewed in the manner. A 10 inch print is usually viewed at about 15 inches thus the wide angle or telephoto effect.