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