Vision and Work - Fundamentals and Assessment Tools for Occupational Ergonomics - page 425

The condition of myopia or “near-sightedness”
is shown in Figure 18.2. The image of the distant
object is formed in front of the retina because the
optics of the eye have too much refractive power
and or because the eye is too long. Of course,

the rays of light do not stop where the image is
formed and are out of focus when they impinge
the retina and the image of the distant object is
blurred. As shown in Figure 18.2, myopia is cor-
rected with a negative powered lens. The lower
image in Figure 18.2 shows that, without correc-
tion, there is a near distance at which the image
is focused on the retina. The reciprocal of the dis-
tance of this near point from the eye (in meters)
is the magnitude of the myopia (in Diopters).
The condition of hyperopia or “far-sightedness”
is depicted in Figure 18.3. The refractive power of
the eye is too weak and

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FIGURE 18.2 An eye with myopia. The top figure
demonstrates improper focus because the eye is too
powerful and

or the eye is too short. A
distant object is focused behind the retina with
the accommodative mechanism relaxed. Of
course, the retina intercepts the light rays before
the image is formed and the image that is on the
retina is out of focus and blurred. A hyperope
with an adequate amount of accommodation can use accommodative effort to add refractive power
to the eye and thereby see distant objects clearly (bottom image). However, if the hyperope does not
have an adequate amount of accommodation (which eventually happens with age), then he or she
will be unable to see distant objects clearly. This
situation is even worse when viewing near
objects. If the amount of hyperopia is low and
the individual is young with a large amount of
accommodation, then near objects might be able
to be cleared comfortably. Because the ability to
accommodate decreases with age (see Section
18.2.2.1 on Accommodation), the ability to com-
pensate for hyperopia decreases with age. The
dual demands of hyperopia and a near viewing dis-
tance can exceed the accommodative abilities of
the individual resulting in blur and

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or too long. The middle figure shows
spectacle correction with a minus lens, the bottom
figure shows that a near object is properly focused
without the need for accommodation.

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or discomfort.
Hyperopia is corrected with a positive power lens
as shown in Figure 18.3.
Astigmatism is a refractive error in which the
power of the eye is different for different meridians
or orientations of a plane of light that enters the
eye. This condition can be visualized by imagining
that the front surface of the eye is shaped like a
football, in which the curvature is greater in one
orientation than in the other. Astigmatism also
results in a blurred image at all viewing distances.
Astigmatism can coexist with myopia or with
hyperopia, or it is also possible for one meridian
to be hyperopic and the other myopic.

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FIGURE 18.3 An eye with hyperopia. Top figure shows
hyperopic eye with relaxed accommodation — distant
object is unfocused because eye is too short and

or
optics are too strong, middle figure shows correction
with a plus lens. Bottom figure shows that (excessive)
accommodation can be used to focus on the retina.

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