Graphics Reference
In-Depth Information
with the result (if the lens is properly shaped) that the rays all converge again at
some point on the other side. If this point happens to lie on the imaging plane, we
say that the object is “in focus.” If the point of convergence is not on the imaging
plane, then instead of producing a bright point of light, the rays generate a dim
disk of light on the imaging plane. If the imaging plane is the sensor array for a
digital camera, for instance, then the point
B
appears out of focus and blurry.
The convergence of all rays to a single point depends on the
index of refrac-
tion
(see Chapter 26)—a number that describes how much light bends as it passes
from air to the lens and back to air—being independent of the wavelength of the
light. For most materials, the index of refraction
does
vary slightly with wave-
length; this can make objects of one color be in focus while those of another color
are not, which accounts for the rainbow-colored fringe on the edges of objects
when they're viewed through a magnifying glass, for instance.
Because the eyes can slightly modify their lenses' shape, the visual system can
use focus/defocus to detect distance from the eye to an object, at least for nearby
objects (defocus becomes less severe the farther away objects are). The amount
of defocus-from-depth depends on the lens diameter. For very small diameters,
there is a much larger depth range that's almost in focus (this range is described in
photography as
depth of field
); for large diameters, the depth of field tends to be
small. For an idealized pinhole camera, in which light passes through an infinites-
imal hole on its way to the image plane, depth of field is infinite; unfortunately,
the light-gathering ability of such an idealized device is zero. The human eye also
has an adjustable pupil. In low light, the pupil opens wide and gathers more light,
but at the cost of reduced depth of field; in bright light, the pupil closes, enhancing
depth of field. Contrary to common wisdom, this pupil adjustment is hardly sig-
nificant in the matter of adapting to a wide range of brightness levels—the pupil's
area changes by a factor of, at most, ten, while the largest arriving radiance in
ordinary experience is about ten orders of magnitude larger than the smallest, but
the response
is
fast, making the pupil very effective at short-term adjustments. The
longer-term adjustment is a chemical process in the receptors.
A large portion of the inner back surface of the eye, the
retina,
is covered with
cells that respond to the light that arrives at them. These are primarily in two
groups:
rods
and
cones,
which we discuss further in Chapter 28. Rods are respon-
sible for detecting light in low-light situations (e.g., night vision), while cones
detect light in higher-light situations. There are three kinds of cones, each respon-
sive to light of different wavelengths; the combination of the three responses gen-
erates the sensation of color (discussed further in Chapter 28). There are far more
rods than cones (a ratio of about 20:1), and the distribution of rods and cones
is not uniform: At the
fovea,
a region opposite the pupil, the cone cell density is
especially high. Deering [Dee05] gives detailed descriptions of these distributions,
and a computational model for the eye's response to light. There's another special
area of the retina, the
optic disk,
where the optic nerve attaches to the eye. In this
region, there are no rods or cones at all. Despite this, you do not have the sense,
as you look around, that there is a “blind spot” in your perception of the world;
this is an instance of higher-level processing masking out (or filling in) the details
of low-level information. The blind spot is very much present, but if you were to
notice it all the time, it would distract you constantly.
