label is a relative measurement of how well you see. If you can see a 9 mm
letter at a distance of 6 meters (20 feet) then you have 20/20 vision. If
you can see the letters designed for 6 meters (20 feet) at 12 meters (40 feet)
then you have 20/10 vision, and so on.
eye chart was developed by Dutch ophthalmologist Herman Snellen (1835
1908) using his assistant who had excellent eyesight as a base-line for normal
vision. The distance of 20 feet (6 meters) is important because the eye, for
all practical purposes accommodates only within the first 20 feet (6 meters).
If an object is clear at that distance then objects farther away will also
placed 3/8” (9 mm) letters in a row and his assistant could read this
line at 20 feet (6 meters), so it is called the 20/20 line. When all the letters
on the 20/20 line can be read with one eye then you are said to have natural
20/20 vision for distance in that eye. Larger letters on the chart correspond
to vision with acuity less than 20/20.
letter E, which is 3/8” (9 mm) in height and width, placed twenty feet
(6 meters) away, then that image occupies a 5° area of the macula. If
the three black and two white horizontal lines are equal in width and length
then a horizontal stroke or a white space occupies a 1° area in the center
of the macula, called the fovea. People with superior vision have their attention
at a very tiny central area a key to natural vision training principles.
way to measure visual acuity is the ability to distinguish between two points
when placed very close together. This method was used by Arabs when choosing
their horsemen. They chose only those who were able to distinguish the two
stars which form the second “star” in the tail of the Great
Bear constellation. A person with normal eyesight can distinguish two points
apart if they are separated by 1 mm when placed 10 meters away. The two points
would be separated by 2 µm (micro meters) on the retina. The resolution
of the eyes is limited by the size of the cone cells, which have a diameter
of 1 to 1,5 µm.
doctor determines the correction, measured in diopters needed to provide you
with 100% ability to perform visual discrimination at 6 meters (20 feet).
Your glasses will compensate for any refraction errors and provide you with
20/20 vision. However, it is important to realize that wearing contact lenses
or glasses does absolutely nothing to improve your vision.
optometrists have actually written about this:
“Spectacle lenses can create their own problems. There are frequently ignored patterns of addiction to minus lenses. The typical prescription tends to overpower and fatigue the visual system and what is often a transient condition becomes a lifelong situation, which is likely to deteriorate with time.”
Gallup, Journal of Behavioural Optometry 5(5):115-120, 1994
“I have yet to hear of a research paper confirming the beneficial effect of prescribing compensatory lenses. I am sure most optometrists will confirm the clinical observation that patients who receive compensatory lenses for full time wear are usually the ones who need a stronger prescription every year.”
J. Liberman OD, Ph.D., Journal
of the American Optometry Association, 47(8): 1058-1064,1976
interesting to note that corrective lenses were originally designed to correct
the shape of your eyeballs. First you were given one set of lenses to wear
for about three days, and then a weaker set for another three days, and so
on, until your eyes got better. Then you gave the glasses back to the eye
doctor. Somewhere along the way someone saw the commercial potential in selling
the glasses to people rather than just improving their eyesight.
between objective measurement and the subjective experience of vision vary
greatly. The equipment used by eye-care professionals calculate your visual
acuity based on an average taken from a dozen tests conducted in a short span
of time. Testing equipment has yet to reach the level where it is as sensitive
as the human eye. Also something happens when you look into a machine, binoculars
or anything you bring in close to your eyes. Testing machines compensate for
this in the formula used to calculate your vision status. The margin of error
is plus / minus 0.5 diopters, as well as the difference between machines.
normal for your eyes to vary up to 2 diopters between being fully relaxed
in the morning and tired and ready to sleep at night. So if you were to have
your eyes measured every hour, each measurement will be different. You have
probably experienced this when you pick up new glasses and they hurt your
In other words they are over-corrected for your current vision.