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An Overview of How the Eye
Sees:
When we have an eye
examination, we may hear
references to errors in the
way our eye refracts light.
This is because the eye's
ability to refract or "bend"
light is also its ability to
focus light, which
determines the sharpness of
our vision.
The normal eye can refract
or focus light without the
help of any other lenses
such as glasses or contacts.
If the eye cannot focus an
image sharply and requires
another lens to assist it,
then the eye is said to have
a refractive error.
The procedure to determine a
prescription for eyeglasses
or contact lenses is called
a refraction. Eye surgeons
who use vision correction
procedures are also referred
to as refractive surgeons.
But what does it really mean
when we're told that our eye
has a refractive error?
We all know that, in order
to see, we must have light.
While we don't fully
understand all the different
properties of light, we do
have an idea of how light
travels. A light ray can be
deflected, reflected, bent,
or absorbed, depending on
the different substances it
encounters.
When light travels through
water or the curved glass of
a lens, for example, its
path is bent or refracted.
Certain eye structures have
refractive properties
similar to water or lenses
and can bend light rays into
a precise point of focus
essential for sharp vision.
As the light rays are bent,
so is the image from which
they originate.
Most refraction in the eye
occurs when light rays
travel through the curved,
clear front covering
(cornea). The eye's natural
(crystalline) lens also
bends light rays. Even the
eye's tear film and internal
fluids (aqueous humor and
vitreous) have certain
refractive properties.
Light rays from an image
traveling through the eye's
optical system are refracted
and focused into a point of
sharp focus that ideally
should center on the retina.
The retina is the tissue
that lines the inside of the
back of the eye, where
light-sensitive cells
(photoreceptors) capture
images in much the same way
that film in a camera does
when exposed to light. These
images then are transmitted
through the eye's optic
nerve to the brain for
interpretation.
Just as a camera's aperture
(called the diaphragm) is
used to adjust the amount of
light needed to expose film
in just the right way, the
eye's pupil widens or
constricts to control the
amount of light that reaches
the retina. In dark
conditions, the pupil
widens. In bright
conditions, the pupil
constricts.
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.
Vision Correction for
Refractive Errors
A refraction from an eye
care practitioner helps
determine the type and
degree of refractive error,
which may be addressed with
glasses, contacts, or
refractive surgery.
An eye care practitioner has
various methods of
determining refractive
errors in an eye, including
use of this phoropter fitted
with various lenses.
Eyeglass lenses and contact
lenses are fabricated with
precise curvatures that help
offset flaws in our eye's
optical system. These lenses
intercept and bend light
rays, such that they achieve
a more precise point of
focus on our eye's retina.
Many vision correction
surgeries such as LASIK also
aim to correct refractive
errors by changing the shape
of our eye's front surface
(cornea), so that light rays
are bent into a more
accurate point of focus.
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