Visual Acuity: What is 20/20 Vision?
Visual acuity is only one in a series of factors
that evaluate one's vision.
20/20 vision is a term used to express normal
visual acuity (the clarity or sharpness of
vision) measured at a distance of 20 feet. 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.
20/20 does not necessarily mean perfect vision.
20/20 vision only indicates the sharpness or
clarity of vision at a distance. There are other
important vision skills, including peripheral
awareness or side vision, eye coordination,
depth perception, focusing ability and color
vision that contribute to your overall visual
ability.
Some people can see well at a distance, but are
unable to bring nearer objects into focus. This
condition can be caused by hyperopia
(farsightedness) or presbyopia (loss of focusing
ability). Others can see items that are close,
but cannot see those far away. This condition
may be caused by myopia (nearsightedness).
A comprehensive eye examination by a doctor of
optometry can diagnose those causes, if any,
that are affecting your ability to see well. In
most cases, your optometrist can 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 used.
Amblyopia (Lazy Eye)
Lazy eye, or amblyopia, is the loss or lack of
development of central vision in one eye that is
unrelated to any eye health problem and is not
correctable with lenses. It can result from a
failure to use both eyes together. Lazy eye is
often associated with crossed-eyes or a large
difference in the degree of nearsightedness or
farsightedness between the two eyes. It usually
develops before the age of 6, and it does not
affect side vision.
Symptoms may include noticeably favoring one eye
or a tendency to bump into objects on one side.
Symptoms are not always obvious.
Treatment for lazy eye may include a combination
of prescription lenses, prisms, vision therapy
and eye patching. Vision therapy teaches the two
eyes how to work together, which helps prevent
lazy eye from reoccurring.
Early diagnosis increases the chance for a
complete recovery. This is one reason why the
American Optometric Association recommends that
children have a comprehensive optometric
examination by the age of 6 months and again at
age 3. Lazy eye will not go away on its own. If
not diagnosed until the pre-teen, teen or adult
years, treatment takes longer and is often less
effective.
Astigmatism
Astigmatism is a vision condition that causes
blurred vision due either to the irregular shape
of the cornea, the clear front cover of the eye,
or sometimes the curvature of the lens inside
the eye. An irregular shaped cornea or lens
prevents light from focusing properly on the
retina, the light sensitive surface at the back
of the eye. As a result, vision becomes blurred
at any distance.
Astigmatism is a very common vision condition.
Most people have some degree of astigmatism.
Slight amounts of astigmatism usually don't
affect vision and don't require treatment.
However, larger amounts cause distorted or
blurred vision, eye discomfort and headaches.
Astigmatism frequently occurs with other vision
conditions like nearsightedness (myopia) and
farsightedness (hyperopia). Together these
vision conditions are referred to as refractive
errors because they affect how the eyes bend or
"refract" light.
The specific cause of astigmatism is unknown. It
can be hereditary and is usually present from
birth. It can change as a child grows and may
decrease or worsen over time.
A comprehensive optometric examination will
include testing for astigmatism. Depending on
the amount present, your optometrist can provide
eyeglasses or that correct the astigmatism by
altering the way light enters your eyes.
Laser surgery is also a possible treatment
option for some types of astigmatism. It changes
the shape of the cornea by removing a small
amount of eye tissue. This is done using a
highly focused laser beam on the surface of the
eye.
Hyperopia (Farsightedness)
Farsightedness, or hyperopia, as it is medically
termed, is a vision condition in which distant
objects are usually seen clearly, but close ones
do not come into proper focus. Farsightedness
occurs if your eyeball is too short or the
cornea has too little curvature, so light
entering your eye is not focused correctly.
Common signs of farsightedness include
difficulty in concentrating and maintaining a
clear focus on near objects, eye strain, fatigue
and/or headaches after close work, aching or
burning eyes, irritability or nervousness after
sustained concentration.
Common vision screenings, often done in schools,
are generally ineffective in detecting
farsightedness. A comprehensive optometric
examination will include testing for
farsightedness.
In mild cases of farsightedness, your eyes may
be able to compensate without corrective lenses.
In other cases, your optometrist can prescribe
eyeglasses or contact lenses to optically
correct farsightedness by altering the way the
light enters your eyes.
Myopia (Nearsightedness)
Myopia is a vision condition in which the
crystalline lens of your
eye loses its
flexibility, which makes it difficult for you to
focus on close objects.
Presbyopia may seem to occur suddenly, but the
actual loss of flexibility takes place over a
number of years. Presbyopia usually becomes
noticeable in the early to mid-40s. Presbyopia
is a natural part of the aging process of the
eye. It is not a disease, and it cannot be
prevented.
Some signs of presbyopia include the tendency to
hold reading materials at arm's length, blurred
vision at normal reading distance and eye
fatigue along with headaches when doing close
work. A comprehensive optometric examination
will include testing for presbyopia.
To help you compensate for presbyopia, your
optometrist can prescribe reading glasses,
bifocals, trifocals or contact lenses. Because
presbyopia can complicate other common vision
conditions like nearsightedness, farsightedness
and astigmatism, your optometrist will determine
the specific lenses to allow you to see clearly
and comfortably. You may only need to wear your
glasses for close work like reading, but you may
find that wearing them all the time is more
convenient and beneficial for your vision needs.
Because the effects of presbyopia continue to
change the ability of the crystalline lens to
focus properly, periodic changes in your eyewear
may be necessary to maintain clear and
comfortable vision.
Laser procedures are also a possible treatment
for nearsightedness in adults. They involve
reshaping the cornea by removing a small amount
of eye tissue. This is accomplished by using a
highly focused laser beam on the surface of the
eye.
For people with higher levels of
nearsightedness, other refractive surgery
procedures are now available. These procedures
involve implanting a small lens with the desired
optical correction directly inside the eye,
either just in front of the natural lens (phakic
intraocular lens implant) or replacing the
natural lens (clear lens extraction with
intraocular lens implantation). These procedures
are similar to one used for cataract surgery
patients, who also have lenses implanted in
their eyes (intraocular lens implants).
Presbyopia
Presbyopia is a vision condition in
which the crystalline lens of your eye loses its
flexibility, which makes it difficult for you to
focus on close objects.
Presbyopia may seem to occur suddenly, but the
actual loss of flexibility takes place over a
number of years. Presbyopia usually becomes
noticeable in the early to mid-40s. Presbyopia
is a natural part of the aging process of the
eye. It is not a disease, and it cannot be
prevented.
Some signs of presbyopia include the tendency to
hold reading materials at arm's length, blurred
vision at normal reading distance and eye
fatigue along with headaches when doing close
work. A comprehensive optometric examination
will include testing for presbyopia.
To help you compensate for presbyopia, your
optometrist can prescribe reading glasses,
bifocals, trifocals or contact lenses. Because
presbyopia can complicate other common vision
conditions like nearsightedness, farsightedness
and astigmatism, your optometrist will determine
the specific lenses to allow you to see clearly
and comfortably. You may only need to wear your
glasses for close work like reading, but you may
find that wearing them all the time is more
convenient and beneficial for your vision needs.
Because the effects of presbyopia continue to
change the ability of the crystalline lens to
focus properly, periodic changes in your eyewear
may be necessary to maintain clear and
comfortable vision.
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