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We are, in fact, only
seeing light - light
that bounces off
objects
far or near
and into our eyes.
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: eye doctor Brentwood
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Patient Educational Materials -
Understanding Normal Vision
Introduction
We "see" things every day, from the moment we get up in the morning
until we go to sleep at night. Sight is our primary sense for maneuvering around
in our environment. It keeps us safe and allows us to find wherever we wish to
be. Through sight, we are able to appreciate the graceful human form, a kids'
crayon drawing, a fine oil painting, swirling computer graphics, gorgeous sunsets,
blooming gardens, rainbows and so much more. We rely on mirrors to make ourselves
presentable, signs to give us direction, and sparkling gemstones to show affection.
But, did you ever stop
to think that when we see any of these things, we are not directly connected
to them? And that we are, in fact, only seeing light - light that bounces off objects
far or near and into our eyes. Light is all our eyes can really see.
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How it works
For people with normal vision, the following sequence takes place:
- Images start out as light rays bouncing off objects and reflected into your
eye. This is why you cannot see in absolute darkness.
- At an amazing speed, the reflected light enters the front of the eye through
a clear, dome-shaped surface called the cornea. The rounded shape
of cornea bends the light towards the center of the eye. This bending
is called refraction.
- From the cornea, light is bent through the dark central opening in the
middle of your eye called the pupil. The size of the pupil limits
the amount of light passing through and is controlled by the pigmented (colored) muscular
part of your eye called the iris. The iris contracts or expands in response to the brightness of available light.
- Past your pupil, the light goes through a transparent,
double-convex structure - like this: ()
called the crystalline lens. This lens further bends (refracts)
the light rays to form a sharper focal point and is controlled
by tiny muscles that make the lens fatter or thinner depending on the
distance of the object you are viewing. This flexing of the lens is called accommodation and
allows you to change your focus from near objects to far objects and
back again. As you grow older this flexing ability diminishes, which
is why you see people over the age of forty using glasses to read.
- The more highly focused light passes through a clear, jelly-like substance that fills
the center of your eye called the vitreous humor.
- As its final destination, the light reaches the incredibly sensitive, nerve-dense
area on the back of your eye, called the retina.
- The nerves of the retina transform light rays into electric signals.
- Bundled together at the optic nerve, all of these signals are sent to your brain.
- A special area in your brain, called the visual cortex, translates the signals
into images called vision. Thus, your brain is the actual organ of sight!
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Limiting factors
In normal vision, 20/20 vision light focuses on
the center of the retina.
Where the light is focused on the retina is determined by the length and shape of the eyeball:
Read more on these topics by clicking on a link.
- If the eyeball is too long in length,
then the focused image will be in front of the retina. This condition is called
myopia
(or nearsightedness).
- If the eyeball is too short in
length, the focused image will be behind, or beyond, the retina. This condition is called
hyperopia
(or farsightedness).
- If the eyeball is out of round,
the focused image will be in more than one place on retina. This condition is
called
astigmatism
.
- In the case of presbyopia, the crystalline lens
loses its flexibility. The ability to focus is limited to one distance.
When the brain perceives out-of-focus images the result is called a refractive
error (that is to say, when the light came through the eye there was an "error"
made in bending it correctly).
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20/20 vision
Vision or visual acuity is primarily tested using a Snellen Eye Chart. By examining
lots of people, eye doctors have decided what is normal for human being to be
able to see when standing 20 feet away from the eye chart.
If you have 20/20 vision, it means that when you stand 20 feet away from the
chart you can see what the majority of people can see at that same distance.
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If you have 20/40 vision, it means that if a normal person were standing 40 feet
away from the chart, and you were standing only 20 feet away from the chart,
you and the normal person would see the chart with the same level of clarity.
20/100 vision means that when you stand 20 feet from the chart you can only see
what a normal person sees when standing 100 feet away.
20/200 vision and over is the qualification of legal blindness in the United
States.
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Better than 20/20
You can also have vision that is better than normal. If a person has 20/15 vision
they would be seeing at 20 feet what a normal person sees when standing 15 feet
away from the chart.
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Hawks, owls and other birds of prey have much more acute vision than
humans. Although a hawk has a much smaller eye than a human being, it has
more cones
packed into that space and two foveas. A hawk's vision is eight times more acute
than a human's: a hawk might have visual acuity of 20/2! |
However, with today's Wavefront technology,
it is possible for human eyes to achieve 20/15 or better (read more).
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Normal Vision