Depth of field of camera photography has to do with front to
back sharpness in the scene you are photographing. When you focus your
lens on an object, everything at exactly that distance from the camera
is in focus, while everything not at that distance is, to some degree,
out of focus. Objects that is out of the small depth of field would
appeared blur in photography. However, focus doesn't change from sharp to blurry
suddenly; it is a smooth transition.
Starting with the point of greatest clarity, sharpness diminishes
little by little as distance from this point increases. The gradual
change occurs in both directions, in front of the object focused on as
well as behind it. Fortunately, the human eye and brain can accept a
certain amount of less than perfect focus without paying much attention
to it. Eventually, however, limits are reached beyond which sharpness
is unsatisfactory. If you measure the distance between the near limit
and the far one, you will have what is know as the depth of field.
How will you know what the depth of field will be before taking a
picture? Better still, how can you control depth of field to make it
suit your own purposes? How to control the zone of sharpness? There are several ways, the three most
important involving lens focal length, lens aperture size, and the
distance to your subject.
It is important to realize that as lens focal length increase, depth of
field gets shallower, all other conditions being equal. The normal lens
on a 35mm camera has generous depth under most circumstances. However,
the really long lenses, 500mm and over, have such limited depth that
focusing must be done very carefully to make sure you have actually
selected the exact point you had in mind. On the other hand, wide angle
lenses are more and more lavish with their depth as their focal lengths
get shorter. With a super wide angle lens depth of field is so
extensive that focusing can be dispensed with altogether under some
circumstances, such as when the lens is focused at 1 meter (3 feet) and
a medium to small f-stop is used.
Lens aperture is the second device for controlling depth of field and
is probably the most convenient. Control the aperture to create depth
of field. The application is simple: the smaller
the stop (all other things being equal), the greater the depth and the
larger the stop, the shallower the depth.
The third important factor governing depth of field is focusing
distance. With any given lens and f-stop depth can be increased by
focusing farther away or decreased by focusing closer to the camera. It
is this principle that makes close up photography such a challenge. The
very short distances between camera and subject produce extremely
limited zones of sharpness.