How Perception Creates Depth of Field
The very fact that you have to focus a camera
implies that at some point things in front of the cam-
era will be out of focus. In other words, focus must
end somewhere. In fact, for the best of lenses there
is only a single, spherical plane—one that sur-
rounds the camera like an invisible soap bubble—
where any point in the photograph is as sharp as
possible. Everything else in front of or behind that
plane is out of focus. Then why don’t more things in
a photo look out of focus? Don’t we even speak of
depth of field as a range before and behind
whatever we’re focused on where other objects are
also in focus? Well, yes, but you see, it’s really just
an illusion. It’s a trick by an imperfect eye that
makes the brain accept one standard of fuzziness
as sharp focus and another degree of fuzziness as
just fuzzy. Fortunately, it’s an illusion that obeys
some rules and that a photographer
can control.An object being pho-
tographed will be in focus
when it lies on the plane
of critical focus. It’s
actually a sphere, rather
than a plane, at the center
of which is the lens, but con-
vention uses the term plane.
The location of this plane or
sphere is determined by the
optics of the lens, the lens’s
focal length, and how the
photographer has adjusted
the lens’s focus.Rays of light from a point on some object
in that plane—the red point in our illustra-
tion—extend to all points on the front of
the lens, which bends them to form a
cone. At the tip of the cone, all the light
rays converge on a single point at the
focal plane of the camera. This is
where the digital image sensor or film is
mounted. (To make everything easier to
understand, we’ve exaggerated the size of
the digital sensor to something that resem-
bles a drive-in movie screen.)1 2
Other points
from objects
in front of and
behind the plane
of critical focus—
such as the green
and blue points—are
also resolved by the
lens to individual
theoretical points. The
only difference is that
these focused points are
not on the focal plane.
The cones of light from these
spots meet at focal points that
fall either behind or in front of
the focal plane. That plane,
instead, is intersected by a section
of the cones created by the light
beams converging on their points.
The digital sensor records not a point of
light or even a least circle of confusion as
occurs at the focal plane. Instead, the sensor
records an everyday, common circle of
confusion, what we would ordinarily call a blur.
The farther from the focal plane that the point
is actually focused, the bigger the blurred circle.4
These circles of confusion, however, are often still so
small that our eyes perceive them as a focused point.
That’s why an equal distance behind and in front of the
plane of critical focus appears to also be in focus. This
area is called the depth of field. The depth, or size,
of the depth of field is determined by the focal length of
the lens, the design of its optics, the size of the aperture
(the opening light passes through when the shutter is
snapped), and the distance from the lens to the critical
focus plane. The size of the film or sensor compared
to the size of the print made from it is another set of
factors in which subjective judgment, eyesight, and
the distance from a print to someone looking at it all
play a part.5
(^60) PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES