Depth of Field

The depth of field defines the object distance over which the image is in focus.

Features of Depth of Field

Smaller apertures (higher F numbers) give greater depth of field.
Shorter focal lengths (smaller mm numbers) give greater depth of field.
Greater subject distances give greater depth of field, unless a telephoto is being used.

The lens aperture does more than control the amount of light—it also controls the depth of field.
The more the lens is adjusted to a higher F number ("stopped down"), the greater the depth of field. The more the lens is adjusted to a lower F number (which is not called "stopped up"), the lower the depth of field.
Changes in focal length also affect the depth of field.
As focal length decreases (moves toward the wide angle direction), depth of field increases.
As focal length increases (moves toward the telephoto direction), depth of field decreases.
Since a wide-angle lens has a greater depth of field, it can give both a sharp foreground and a sharp background.
At the same distance a telephoto lens will have less foreground and background in focus.