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Our ability to "see" starts when light reflects off an object
at which we are looking and enters the eye. As it enters the
eye, the light is unfocused. The first step in seeing is to
focus the light rays onto the retina, which is the light sensitive
layer found inside the eye. Once the light is focused, it stimulates
cells to send millions of electrochemical impulses along the
optic nerve to the brain. The portion of the brain at the back
of the head interprets the impulses, enabling us to see the
object.
Light, refraction and its importance.
Light entering the eye is first bent, or refracted, by the cornea
-- the clear window on the outer front surface of the eyeball.
The cornea provides most of the eye's optical power or light-bending
ability. After the light passes through the cornea, it is bent
again -- to a more finely adjusted focus -- by the crystalline
lens inside the eye. The lens focuses the light on the retina.
This is achieved by the ciliary muscles in the eye changing
the shape of the lens, bending or flattening it to focus the
light rays on the retina. This adjustment in the lens, known
as accommodation, is necessary for bringing near and far objects
into focus. The process of bending light to produce a focused
image on the retina is called "refraction". Ideally, the light
is "refracted," or redirected, in such a manner that the rays
are focused into a precise image on the retina.
Most vision problems occur because of an error in how our eyes
refract light. In nearsightedness (myopia), the light rays form
an image in front of the retina. In farsightedness (hypermetropia),
the rays focus behind the retina. In astigmatism, the curvature
of the cornea is irregular, causing light rays to focus to more
than one place so that a single clear image cannot be formed
on the retina, resulting in blurred vision. As we age, we find
reading or performing close-up activities more difficult. This
condition is called presbyopia, and results from the crystalline
lens being less flexible, and therefore less able to bend light.
Since changing the apparent refraction of the eye is relatively
easy through the use of corrective spectacle or contact lenses,
many of the conditions that contribute to unclear vision can
be readily corrected.
How do we make sense of light?
Sensory interpretation
Even with the light focused on the retina, the process of seeing
is not complete. For one thing, the image is inverted, or upside
down. Light from the various "pieces" of the object being observed
stimulate nerve endings -- photoreceptors or cells sensitive
to light -- in the retina.
Rods and cones
Two types of receptors -- rods and cones -- are present. Rods
are mainly found in the peripheral retina and enable us to see
in dim light and to detect peripheral motion. They are primarily
responsible for night vision and visual orientation. Cones are
principally found in the central retina and provide detailed
vision for such tasks as reading or distinguishing distant objects.
They also are necessary for color detection. These photoreceptors
convert light to electrochemical impulses that are transmitted
via the nerves to the brain. Millions of impulses travel along
the nerve fibers of the optic nerve at the back of the eye,
eventually arriving at the visual cortex of the brain, located
at the back of the head. Here, the electrochemical impulses
are unscrambled and interpreted. The image is re-inverted so
that we see the object the right way up. This "sensory" part
of seeing is much more complex than the refractive part -- and
therefore is much more difficult to influence accurately.
What is 20/20 Vision?
You may be pleased to hear that you have 20/20 vision and think
you have perfect vision. But do you? Not necessarily. 20/20
only indicates how sharp or clear your vision is at a distance.
Overall vision also includes peripheral awareness or side vision,
eye coordination, depth perception, focusing ability and color
vision. 20/20 describes normal visual clarity or sharpness measured
at a distance of 20 feet from an object. If you have 20/20 vision,
you can see clearly at 20 feet what should normally be seen
at that distance. If you have 20/100 vision, it means that you
must be as close as 20 feet to see what a person with normal
vision can see at 100 feet.
Is 25/25 vision better than 20/20?
No. 25/25 means normal sharpness of vision, or visual acuity,
at 25 feet just as 20/20 indicates normal vision at 20 feet.
Why do some people have less than 20/20?
The ability to see objects clearly is affected by many factors.
Eye conditions like nearsightedness, farsightedness, astigmatism
or eye diseases influence visual acuity. Most people with vision
slightly below 20/20 function very well, whereas some people
who have better than 20/20 vision feel that their vision is
not satisfactory. Everybody's visual expectations are different
and satisfactory vision is far more complex than just being
able to see 20/20.
If my vision is less than optimum, what can I do?
A comprehensive eye examination will identify causes that may
affect your ability to see well. We may be able to prescribe
glasses, contact lenses or a vision therapy program that will
help improve your vision. If the reduced vision is due to an
eye disease, we will use an ocular medication or other treatment
as needed.
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