The Eye

                                                       National Eye Institute, National Institute of Health How the Eye Works
  return to top

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?

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.

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 20/20 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, the use of ocular medication
or other treatment may be needed. If necessary, referral will be
undertaken if an eye disease is found which warrants further investigation.
 

Astigmatism
 return to top

If you experience a distortion or blurring of images at all distances --
nearby as well as far -- you may have astigmatism. Even if your vision is
fairly sharp, headache, fatigue, squinting and eye discomfort or irritation
may indicate a slight degree of astigmatism. A thorough eye examination,
including tests of near vision, distant vision and vision clarity, can
determine if astigmatism is present. Astigmatism is not a disease nor does
it mean that you have "bad eyes." It simply means that you have a
variation or disturbance in the shape of your cornea.

Astigmatism is one of a group of eye conditions known as refractive
errors. Refractive errors cause a disturbance in the way that light rays are
focused within the eye. Astigmatism often occurs with nearsightedness
and farsightedness, conditions also resulting from refractive errors.

What causes astigmatism?

Astigmatism usually occurs when the front surface of the eye, the cornea,
has an irregular curvature. Normally the cornea is smooth and equally
curved in all directions and light entering the cornea is focused equally on
all planes, or in all directions. In astigmatism, the front surface of the
cornea is curved more in one direction than in the other. With the
cornea's shape more like that of an American football or rugby ball than a
basketball, the light hitting the more curved surface comes to a focus
before that which enters the eye through the less curved surface. Thus,
the light is focused clearly along one plane, but is blurred along the other
so only part of anything being looked at can be in focus at any time.

This abnormality may result in vision that is much like looking into a
distorted, wavy mirror. The distortion results because of an inability of
the eye to focus light rays to a point.

Why are corneas shaped differently?

Not all corneas are perfectly curved, just as sets of teeth are seldom
perfectly aligned. The degree of variation determines whether or not you
will need corrective eyewear. If the corneal surface has a high degree of
variation in its curvature, light refraction may be impaired to the degree
that corrective lenses are needed to help focus light rays better.

The exact reason for differences in corneal shape remains unknown, but
the tendency to develop astigmatism is inherited. For that reason, some
people are more prone to develop astigmatism than others.

Of interest to parents and those who work with children, astigmatism
may contribute to poor schoolwork but is often not detected during
routine eye screening in schools.

How is it treated?

If the degree of astigmatism is slight and no other problems of refraction,
such as nearsightedness or farsightedness, are present, corrective lenses
may not be needed. If the degree of astigmatism is great enough to cause
eyestrain, headache, or distortion of vision, prescription lenses will be
needed for clear and comfortable vision.

The corrective lenses needed when astigmatism is present are called
"Toric" lenses and have an additional power element called a cylinder.
They have greater light-bending power in one axis or direction than in the
others. Precise tests will be made during your eye examination to
determine the ideal lens prescription.

Astigmatism may increase slowly over time. Regular eye examinations
can help to ensure that proper vision is maintained.

Farsightedness (hyperopia)
 return to top

If you can see objects at a distance clearly but have trouble focusing well
on objects close up, you may be farsighted.

Farsightedness or long-sightedness is often referred to by its medical
names, hypermetropia or hyperopia. Hyperopia causes the eyes to exert
extra effort to see close up. After viewing near objects for an extended
period, you may experience blurred vision, headaches and eyestrain.
Children who are farsighted may find reading difficult.

Hyperopia is not a disease, nor does it mean that you have "bad eyes." It
simply means that you have a variation in the shape of your eyeball. The
degree of variation will determine whether or not you will need corrective
lenses.

What causes farsightedness?

Hyperopia most commonly occurs because the eyeball is too short; that
is, shorter from front to back than is normal. In some cases, hyperopia
may be caused by the cornea having too little curvature. Exactly why
eyeball shape varies is not known, but the tendency for farsightedness is
inherited. Other factors may be involved too, but to a lesser degree than
heredity.

How does farsightedness affect sight?

Our ability to "see" starts when light enters the eye through the cornea.
The shape of the cornea, lens and eyeball help bend (refract) light rays in
such a manner that light is focused into a point precisely on the retina.

If, as in farsightedness, the eyeball is too short, the "point of light" focuses
on a location behind the retina, instead of on the correct area of the
retina, known as the fovea. As a result, at the point on the retina where a
fine point of light should be focused, there is a disk-shaped area of light.
Since light is not focused when it hits the retina, vision is blurred. Convex
lenses are prescribed to bend light rays more sharply and bring them to
focus on the retina.

How is it diagnosed and treated?

Hyperopia is seldom diagnosed in school eye-screening tests, which
typically test only the ability to see objects at a distance. A
comprehensive eye examination that checks both near and far vision is
necessary to diagnose farsightedness. In some cases it may be necessary
for the practitioner to use drops during the examination to relax the eye
muscles and ensure that the full degree of hyperopia is detected. This is
necessary because the muscles which focus the eye are so accustomed to
being used to compensate for the hyperopia that the muscles go into
"spasm" and cannot relax without being forced to do so.

Corrective convex lenses (positive powers) are usually prescribed. They
bend light rays more sharply and bring them to focus on the retina. If you
do not have other vision problems such as astigmatism, you may only
need glasses for reading or other tasks performed at a close range.

To determine the best avenue of treatment, questions about your lifestyle,
occupation, daily activities and general health status may be asked. For
instance, you may be asked whether or not you frequently need near
vision. Providing candid, considered answers to the questions will help
assure that your corrective lenses contribute to clear sight and general
comfort.

A comprehensive eye examination at the recommended intervals will
ensure that minor changes in vision are diagnosed and treated so that
your vision will remain as clear and comfortable as possible.
 

Nearsightedness (myopia)
 return to top

If you can see objects nearby with no problem, but reading road signs or
making out the writing on the board at school is more difficult, you may
be near- or shortsighted.

This condition is known as myopia, a term that comes from a Greek
word meaning "closed eyes." Myopia is not a disease, nor does it mean
that you have "bad eyes." It simply refers to a variation in the shape of
your eyeball. The degree of variation determines whether or not you will
need corrective eyewear.

What causes nearsightedness?

Myopia most often occurs because the eyeball is too long, rather than the
normal, more rounded shape. Another less frequent cause of myopia is
that the cornea, the eye's clear outer window, is too curved. There is
some evidence that nearsightedness may also be caused by too much
close vision work.

How does myopia affect sight?

Our ability to "see" starts when light enters the eye through the cornea.
The shape of the cornea, lens and eyeball help bend (refract) light rays in
such a manner that light is focused into a point precisely on the retina.

In contrast, if you are nearsighted, the light rays from a distant point are
focused at a place in front of the retina. As the light will only be focused
in that one place, by the time it reaches the retina it will have "defocused"
again, forming a blurred image.

Myopia usually occurs between the ages of 8 to 12 years. Since the eyes
continue to grow during childhood, nearsightedness almost always occurs
before the age of 20. Often the degree of myopia increases as the body
grows rapidly, then levels off in adulthood. During the years of rapid
growth, frequent changes in prescription eyewear may be needed to
maintain clear vision. It is important to bear in mind that the frequent
changes in prescription are not making the eyes "weaker". During the
growth period that occurs during the teen years the eye is also growing
rapidly and hence the degree of blur is also increasing. As the growth
cycle slows the prescription changes slow correspondingly.

How is myopia diagnosed and treated?

Myopia is often suspected when a teacher notices a child squinting to see
a blackboard or a child performs poorly during a routine eye screening.
Further examination will reveal the degree of the problem.

A comprehensive eye examination will detect myopia. Periodic
examinations should follow after myopia has been discovered to
determine whether the condition is changing, and whether a change in
prescriptive eyewear is needed. Eye exams also help to ensure that vision
impairments do not interfere with daily activities.

Corrective concave (minus) lenses are prescribed to help focus light
more precisely on the retina, where a clear image will be formed.

Depending on the degree of myopia, glasses or contact lenses may be
needed all of the time for clear vision. If the degree of impairment is
slight, corrective lenses may be needed only for activities that require
distance vision, such as driving, watching TV or in sports requiring fine
vision.

Nearsightedness in children:

School age children may have vision problems ranging from mild to
severe. When problems are suspected, it is important that the child have
a comprehensive eye health examination to determine the nature of the
problem and to rule out serious eye diseases. When vision conditions are
treated properly, the child will enjoy the best possible sight.

To help a child cope with nearsightedness:

     Avoid referring to the child's eyes as "bad eyes;" instead tell the
     child that his or her eyes just bend light differently and corrective
     lenses are needed to help focus light rays.
     Ensure that they understand that nearsightedness rarely disappears
     and that wearing spectacles may be necessary in the long-term,
     but that this is not a disease.
     Use illustrations and simple explanations to help the child
     understand how a differently-shaped eyeball may result in his or
     her being nearsighted.
     Make the occasion of selecting new frames for lenses a fun time.
     Consider contact lenses as an option.
     Do not restrict the child's activities because of poor vision.
     Include the child in discussions about his or her eyesight.
     Encourage the child to verbalize concerns about the adjustment to
     rapidly changing vision.

 return to top