Common disorders
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More common disorders
Aniseikonia
Aniseikonia, or image size imbalance, occurs when and image is larger on the
retina of one eye than on the other. This makes it difficult to fuse the
two retinal images into a single image. A condition like this may occur when
wearing eyeglasses which have lenses that are significantly different from
each other.
The change in retinal image size caused by wearing eyeglasses and
contact lenses of various powers (Bennett, 1949).
Figure 26 shows the relationship between the magnitude of correction and the
resulting change in the size of the retinal image for both eyeglasses and contact
lenses. The effect is more noticeable for eyeglasses because they are located
much further from the principal plane of the eye than are contact lenses.
Presbyopia
Presbyopia normally is produced when the lens loses its elasticity due to aging.
This prevents the lens from accommodating, thus limiting the range over which
objects may be brought into focus. The correction for presbyopia is to wear
glasses --- either reading glasses or multifocal glasses. The workstation
may also be laid out to minimize refocusing requirements.
Average magnitude of available accommodation by age, measured subjectively
and objectively (Hamasaki, et al, 1956).
Figure 27 shows the relationship between age and the magnitude of available
accommodation.
In Figure 27, the subjective and objective measurements also indicate that
there is a reasonable depth of field for the eye, which would suggest that
minor changes in the visual distance do not require a precise readjustment
of the focusing mechanism. Accommodation may be measured subjectively by first
determining the far point setting of the eye, then moving a target inward until
the person is still able to see it without any detectable blur. The objective
method of measuring the magnitude of accommodation uses an optometer to measure
the change in shape of the lens as objects come closer to the eye. There is
about a 2 diopter difference in results between the two measurement methods,
also shown in Figure 27.
Presbyopia may be one of the main contributors to visual discomfort while
doing close work. The problem is that there is a steady loss of accommodation
with aging even during a person's relatively early years. For example, a 15
year-old person normally should have about 10 diopters of accommodation, or
be able to focus at 1/10 of a meter. That same person at age 40 will probably
have less than one-third of that accommodation. That loss may be compensated
for to some extent by applying more force to the muscles that control accommodation.
That additional muscular activation, however, may produce visual discomfort.
A muscle system should not be expected comfortably to maintain a long period
of static force greater than 15% of its capacity.
The percentage of the maximum available force the ciliary muscles
must exert to maintain accomodation (For a reading distance of 28cm) (Fisher,
1977).
Figure 28 shows the percent of maximum force required at different ages to
maintain a 3.5 diopter level of accommodation, a level appropriate for a reading
distance of about 28 cm.
Although a person may be able to accommodate for a 28 cm viewing distance
even after the age of 40, the effort required to do so for a long period of
time may result in discomfort. For the ciliary muscles of the eye not to have
to maintain a static force of more than 15% of their capacity, reading glasses
or multifocals may be required for office-type tasks after about age 30. The
degree of correction depends on the preferred viewing distance and the state
of refraction of the person's eye.
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