When
a lens is focused on infinity, the depth of field extends from infinity
towards a point nearer the camera lens. This distance from the camera
to the near limit of sharp field is called the Hyperfocal distance.

If
you then focus on the Hyperfocal distance, the depth of field extends
from half the Hyperfocal distance to infinity.

As the
above diagram shows, to exploit the ' extra ' depth of field available
for a given aperture, set your lens to infinity. Then note, via the
depth of field scale on the lens, the nearest distance still within
the depth of field for the chosen aperture. In the above example this
is 10m when using f8. This is the ' Hyperfocal ' distance for this
combination. Now refocus the lens by moving the focusing ring markings
so that the focus point is at 10m ( the Hyperfocal distance ). Depth
of field will now extend from half this distance towards infinity.
This method of focusing is often exploited in the manufacture of cheap
point and shoot camera's, but it does have uses within more advanced
photography.