What is 20/20 Vision?
If you have what has been determined as humanity's anormala eyesight, you have 20/20 vision. In countries which use metric measurements, it is called 6/6 vision, referring to meters instead of feet. 20/20 vision is not aperfecta vision, but it is clear at all distances, needing no glasses or contact lenses. Vision can be better than 20/20, so let's look at that further down this article.
The Snellen Eye Chart
Most of us are familiar with that eye chart which has a single large letter at the top (usually an
aEa) and rows of progressively smaller letters below it. There are some variations on the most common version of this chart. In all versions, one line is designated as the anormal visiona line and the others test your vision relative to that anormala line.
When your eye doctor tests your vision for a glasses or contact lens prescription, the eye chart is seen through an ophthalmic device called a phoropter. You rest your chin and forehead against smooth surfaces and look with one eye at a time at the lighted chart inside the aperture.
There are also printed charts that can hang on the wall, and these can test your vision too, but the phoropter is more precise. Instead of you having to physically stand a certain distance away from it, distance is simulated by the phoropter's optics.
From Twenty Feet Away
Normality is defined as a clear perception of the designated anormala line of letters at 20 feet from the eye chart. That is the first a20a. The second number, which may or may not be a20a is a comparison between the tested eye and a anormala eye. The higher the second number is, the more vision impairment your eyes have.
If a normal eye can see from 40 feet away what you must stand only 20 feet away to see, your vision is described as a20/40a.
If a normal eye can see from 100 feet away what you can only see if you stand 20 feet away, your vision is a20/100a.
If your vision is described as a20/200a, you are legally blind.
To describe this another way, imagine you moving back and forth from the eye chart, trying to see the anormala line clearly. If you can see it clearly from 20 feet away, your eyesight is 20/20. If you must edge closer to the chart to see it, your vision is impaired and you are nearsighted.
But ophthalmologists don't usually measure those closer distances in defining your visual acuity. They talk in terms of further distances and postulate that the tested eye is always at 20 feet from the chart. They then compare that amount of acuity with what a normal eye has at varying distances. When the normal eye can see from 80 feet what you see at 20 feet, your vision is 20/80.
Vision Better Than aNormala
If your vision is 20/16 you can see from 20 feet away what a anormala eye can only see from 16 feet. That is better visual acuity than what we consider to be normal.
After a Custom LASIK surgery many people can see with 20/16 clarity. Their vision has been made better than humanity's normal vision. Custom LASIK is based on a more sophisticated and accurate diagnosis than the original Traditional LASIK was. A Custom LASIK procedure does more than just correct nearsightedness, farsightedness and/or astigmatism (that is, the three Lower Order Aberrations).
It also corrects the tiny irregularities called Higher Order Aberrations. A phoropter is not used to diagnose your eyes for Custom LASIK. Instead, a newer technology is used called Wavefront technology. It shines a light into each eye and measures the slight changes in it as it reflects back from your eye into the Wavefront system. From the resulting colored 3-D maps of each eye's contours, your LASIK surgeon can plan a more precise treatment.
When the Custom LASIK laser smoothes out these microscopic irregularities in the eye's cornea (clear front part), your vision improves beyond what glasses or contact lenses can achieve. This is how a person can achieve 20/16 vision - better than normal. The Higher Order Aberrations cause visual distortions which show up in dim light, such as halos around light sources, decreased contrast between light and dark, and faint duplicate images. When these are corrected, your vision is clearer in all lighting conditions.