DEPTH OF FIELD
Depth of field refers to the area of critical sharp-
ness within a photographic image. A lens can only
focus on one area in an image at a time, and this
area is also called the zone of focus and refers to the
distance between the closest and farthest parts of
an image that are reasonably sharp. Depth of field
varies because of a number of factors such as the
size of the aperture, the distance between the cam-
era and the subject, and the focal length of the lens.
Depth of field is used for creative control and
allows the photographer to manipulate the area of
focus in a photographic image to a small area of
selective focus, a large area of overall sharpness,
and any amount in between these two extremes.
A mechanism inside the lens called a diaphragm
creates an opening in its center named the aperture,
and the diaphragm allows the aperture to change in
size creating a variety of aperture settings. The aper-
ture setting is the most influential over depth of field;
the larger the aperture in the lens the lower the area
of critical sharpness, and the smaller the aperture in
the lens the larger the area of critical sharpness.
Aperture sizes on most lenses progress through a
set of numbers called F-stops, which are numerical
values for the opening’s area and move from the
largest to the smallest in a sequence: f/1.0, f/1.4, f/2,
f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/64. Setting
the aperture at the largest opening is sometimes
called ‘‘shooting wide open,’’ and the phrase ‘‘stop-
ping down’’ is used to indicate the opening in the
lens becoming smaller. If a lens has a variable focal
length or a zoom function, the widest aperture may
also vary as the focal length of the lens changes. On a
darkroom enlarger, the aperture setting of the lens
will also affect depth of field in a photographic print.
If a print requires more overall sharpness, using a
smaller aperture on the enlarger lens will increase the
sharpness of the photographic print slightly, while
using a large aperture on the darkroom enlarging
lens limits the overall range of sharpness projected
through the image.
The aperture affects depth of field in a photo-
graph through the formation of small, round image
elements called circles of confusion. These circles of
confusion are created as light passes through the
opening in a lens, and their size is dependent upon
the aperture setting. A larger aperture will form big
circles of confusion, and a small aperture will cre-
ate small individual image elements. Thus, using a
small F-stop will create an image made up of small
circles of confusion, and using a large F-stop will
create an image area made of large circles of con-
fusion. The smaller the circles of confusion, the
sharper an image will appear on the photograph.
The area in the image that is the sharpest is called
the plane of critical focus, and depth of field is
measured out from this plane. The larger the area
on either side of the plane of critical focus, the
wider the degree of sharpness; a large area of
depth of field is referred to as wide, and a small
area of depth of field is called shallow.
Depthoffieldcanalsobecontrolledbythefocal
length of a lens and the distance between the subject
and the lens. A lens with a longer focal length such as
a telephoto will condense space and produce a shal-
low depth of field, while a lens with a short focal
length such as a wide angle or fisheye will produce an
image with a wide depth of field. As the focal length
of a lens increases in a telephoto lens, the magnifica-
tion of the image will also increase and result in more
image distortion that decreases depth of field. A
wide angle lens has a very small amount of magnifi-
cation and thus creates a minimum amount of dis-
tortion in the image, resulting in a wider depth of
field. The distance between the subject and the lens
will also affect depth of field: The farther the object
is from the camera lens, the wider the depth of field.
As an object moves away from a camera lens, the
overall image area is increased and this allows for a
larger area of sharpness in the image. The closer an
object is to the lens, the shallower the depth of field.
Depth of field can be determined in a number of
ways on a camera, such as using the depth of field
scale on a lens, utilizing a depth of field preview
function if it is available, and setting the focal dis-
tance of the subject through zone focusing or hyper-
focal focusing on the lens. A depth of field scale is
often placed on manual lenses, and will show the area
of sharpness relative to the aperture chosen for an
image. Many cameras include a depth of field pre-
view function or show the area of critical sharpness
through the lens and the viewfinder of the camera as
the aperture, focal length, and distance to subject
vary. Zone focusing sets the area of depth of fieldDEPTH OF FIELD