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What Does 20/20 Mean?
Visual Acuity Measurements
You're probably familiar with the vision
chart that hangs in every optometrist's examining room, you know, the one
with a big "E" at the top. It's officially known as a Snellen chart and
it's been the basis for measuring what's known as visual acuity since the
late 1800's. The Snellen chart displays letters of progressively smaller
size. Think of visual acuity as a quantitative measure of your vision,
establishing where your vision is placed on a numeric scale. There are
also qualitative measures of your vision, such as your ability to perceive
subtle contrast changes, especially in low-light situations. Taken
together, these quantitative and qualitative measures determine the
overall performance of your vision. The charts are standardized too and
doctors around the world use the same basic format.
The "vision test" is the simplest component of an eye exam. In order for
eye doctors to compare results, it’s always done at a standardized
distance of twenty feet. Offices years ago had rooms that were twenty feet
long. Today, mirrors are used to reflect the image so the room can be
shorter; but the image still looks like it is twenty feet away. My exam
room is about 17 feet long but my Snellen chart is adjusted for 20 feet.
The charts are standardized too and doctors around the world use the same
basic format.
Each line of the eye chart is assigned a notation in the form of a
fraction that represents your visual acuity. The numerator (the top number
in the fraction) is the distance in feet the patient is from the eye
chart. The denominator, represents the distance an eye with “normal”
vision can read the same line. Interpreting the numbers is simple. If a
patient reads the 20/40 line, he is able to see at 20 feet what a person
with a "normal" eye could see at 40 feet. And if you can read the letters
on my "Challenge Line," your vision is 20/15! You’re above average because
you can see an object from 20 feet that a normal eye sees 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 - Minimum required acuity to be 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.
Low Vision
People with "Low Vision" are those who cannot see the "Big E" on the
Snellen chart and are measured by first by "Finger Counting" at a certain
distance (e.g. FC @ 5 feet). Those patients that are unable to count
fingers are measured by "Hand Motion", the ability of seeing a hand moving
in front of them. These patients are benefit from being fit by their
doctor with Low Vision aids, like telescopes and magnifiers, to assist
them in seeing what they need to see. Patients that are unfortunately
unable to see light at all are measured at "No Light Perception."
~RQ
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