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The 20/20 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.

The Snellen 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 be clear.

Snellen 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.

20/20 VisionA black 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.

A simple 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.

Your eye 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.

A few 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.”

S. 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

It is 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.

The difference 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.

It is 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 eyes.

In other words they are over-corrected for your current vision.


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