Common disorders
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Common disorders
Few people have perfect vision. Many people compensate for minor defects by
adjusting their viewing distance, tilting their head, using high light levels
or avoiding situations where a certain type of visual activity is required
over a sustained period of time. If an awkward position is assumed in order
to compensate for a visual defect, it may over time produce some negative
physical symptoms. It may even cause some visual defects to become more noticeable.
This section covers some of the more common visual defects and conditions.
Hyperopia
Hyperopia, or farsightedness, is a condition where the refractive power of
the eye is not strong enough to bring the image into focus on the retina.
Instead, it focuses the image behind the retina. Hyperopia may be compensated
for to some extent by the eye's ability to increase the refractive power
of the lens (accommodation). The person may also move away from near objects,
and in that way, reduce the amount of refractive power needed by the eye
to bring the image into focus. As a consequence, the mildly hyperopic person
sometimes adopts the habit of sitting further away from the visual surfaces
of the workstation.
Myopia
Myopia, or nearsightedness, may cause more problems than hyperopia because
the refractive power of the eye is too strong. The incoming light rays are
bent too much and come into focus in front of the retina. The myopic person
may help focus an object simply by getting closer to it. The closer a person
is to an object, the more the eyes must bend the light rays to bring them
into focus. This close position may lead to discomfort, since it requires
more movement of the head and other parts of the body to perform different
tasks.
Phoria
Phoria is an imbalance of the external muscles that makes it more stressful
to converge the eyes for one distance setting than for another. Phoria is
detected by presenting different images to each eye. The degree of phoria
is indicated by how the visual system integrates independent images into
a perceived single image. Most people have some degree of phoria at varying
distances, but it usually is not bothersome.
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