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 Vision
Testing
Visual acuity is usually measured with a Snellen chart. The Snellen
chart displays letters of progressively smaller size. "Normal"
vision is 20/20. This means that the test subject sees the same
line of letters at 20 feet that a normal person sees at 20 feet.
20/40 vision means that the test subject see as 20 feet what a normal
person sees at 40 feet. Another way of saying this is that a person
with 20/40 vision has vision that is only half as good as normal
- or, objects must be at half the normal distance for him to see
them. A person with 20/20 vision is able to see letters 1/10th as
large as someone with 20/200 vision. However, 20/15 vision is better
than 20/20. A person with 20/15 vision can see objects at 20 feet
that a person with 20/20 vision can only see at 15 feet.
Levels of Vision
- 20/20 - Normal vision. Fighter pilot minimum. Required to read
the stock quotes in the newspaper, or numbers in the telephone
book.
- 20/40 - Able to pass Driver's License Test in all 50 States.
Most printed material is at this level.
- 20/80 - Able to read alarm clock at 10 feet. News Headlines
are this size.
- 20/200 - Legal blindness. Able to see STOP sign letters.
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Your Prescription
 The
refractive error of the eye can be expressed in numeric terms. The
power of the lenses necessary to correct your vision is measured
in units called diopters (see below). The first number designates
the amount of myopia (minus numbers) or hyperopia (plus numbers).
The second number (if present), indicates the amount of astigmatism.
The third number shows the axis of the astigmatism. Depending on
the sign of the second number, the axis indicates the orientation
of the steepest or flattest meridian of the cornea. The fourth number
is the "add", or the amount of magnification needed to
give clear vision at near. If this number is present, bifocals have
been prescribed to correct presbyopia.
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What is a Diopter?
A diopter is a unit of measurement of the refractive error. It may
be a negative number (myopia, nearsightedness), or a positive number
(hyperopia, farsightedness). A -1.00 diopter myope is able to see
objects at 1 meter clearly. A -2.00 diopter myope is able to see
objects at a ½ meter clearly. The greater the myopia, the
strength of the lens in diopters needed to correct the refractive
error, and the closer an object must be to be viewed clearly.
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