472
UNIT THREE
F
or many people, after the age of forty-F
ve,
it seems as if the print in magazines and
on medicine bottles suddenly becomes
too small to read. The problem is not in the print,
but in a lessening of the elastic quality of the lens
capsule. In the condition presbyopia, or farsight-
edness of age, eyes remain focused for distant
vision. Eyeglasses or contact lenses can usually
make up for the eye’s loss of refracting power.
Other visual problems result from eyeballs
that are too long or too short for sharp focusing.
If an eyeball is too long, light waves focus in front
of the retina, blurring the image. In other words,
the refracting power of the eye, even when the
lens is ±
attened, is too great. Although a person
with this problem may be able to focus on close
objects by accommodation, distance vision is
invariably poor. ²or this reason, the person is said
to be
nearsighted
. Eyeglasses or contact lenses
with concave surfaces that focus images farther
from the front of the eye treat nearsightedness
(myopia).
If an eye is too short, light waves are not
focused sharply on the retina because their point
of focus lies behind it. A person with this condi-
tion may be able to bring the image of distant
objects into focus by accommodation, but this
requires contraction of the
ciliary muscles at times
when these muscles are
at rest in a normal eye. Still
more accommodation is
necessary to view closer
objects, and the person may
su³
er from ciliary muscle fatigue, pain, and head-
ache when doing close work.
People with short eyeballs are usually unable
to accommodate enough to focus on the very
close objects. They are
farsighted
. Eyeglasses or
contact lenses with
convex
surfaces can remedy
this condition (hyperopia) by focusing images
closer to the front of the eye (F
gs. 12E and 12²).
Another refraction problem,
astigmatism,
re±
ects a defect in the curvature of the cornea or
the lens. The normal cornea has a spherical curva-
ture, like the inside of a ball; an astigmatic cornea
usually has an elliptical curvature, like the bowl of
a spoon. As a result, some portions of an image
are in focus on the retina, but other portions are
blurred, and vision is distorted.
Without corrective lenses, astigmatic eyes
tend to accommodate back and forth re±
exly in
an attempt to sharpen focus. The consequence
of this continual action is often ciliary muscle
fatigue and headache.
12.6
CLINICAL APPLICATION
Refraction Disorders
Light waves
Uncorrected
point of focus
Corrected
point of focus
Concave lens
Uncorrected
point of focus
Corrected
point of focus
Convex lens
(a)
(b)
FIGURE 12F
Corrective lenses. (
a
) A concave lens corrects
nearsightedness. (
b
) A convex lens corrects farsightedness.
Light waves
Light waves
Light waves
Cornea
Lens
Retina
(a) Eye too long (myopia)
(b) Normal eye
(c) Eye too short (hyperopia)
Point
of focus
Point
of focus
Point
of focus
FIGURE 12E
Point of focus. (
a
) If an eye is too long, the focus
point of images lies in front of the retina. (
b
) In a normal eye, the
focus point is on the retina. (
c
) If an eye is too short, the focus point
lies behind the retina.
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