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Refractive Errors in the
Eye's Optical System:
The eye's ability to refract
or focus light sharply on
the retina is based on two
main anatomic features: the
overall length of the eye
and the curvature of the
eye's surface or cornea.
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Eye Length: When
the eye is too long,
images mistakenly
focused in front of the
retina are out of focus
by the time they
actually hit the retina.
Nearsightedness or
myopia then results.
When the eye is too
short, images never have
a chance to achieve
focus by the time they
hit the retina. This
causes farsightedness or
hyperopia.
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Curvature of the
Cornea: If the
cornea is not perfectly
spherical, then the
image is refracted or
focused irregularly to
create a condition
called astigmatism. A
person can be
nearsighted or
farsighted with or
without an astigmatism.
As mentioned above, the tear
film, crystalline lens, and
internal fluids also play a
role in focusing an image
onto the retina. An
irregularly shaped natural
lens or defect in the way it
functions also can cause
focusing problems, leading
to blurry or distorted
vision.
These various defects in
focusing can cause light
rays to bend or refract at
skewed angles, which means
sharp focus cannot be
achieved. When abnormalities
of this type occur in the
optical system, they are
known as refractive errors.
More obscure vision errors,
known as higher-order
aberrations, also are
related to flaws in the way
light rays are refracted as
they travel through our
eye's optical system. These
types of vision errors,
which can create problems
such as poor contrast
sensitivity, are just now
being detected through new
technology known as
wavefront analysis.
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