What is 20/20 Vision?
By Sandra Duke
20/20 vision is normal visual acuity. The numbers refer to feet and in countries which use metric measurements, it is expressed as 6/6, meaning six meters. Visual acuity is the ability to see symbols and objects clearly from a distance. Typically it tested with the Snellen eye chart, with those rows of letters which become smaller towards the bottom of the chart. Each of these lines is assigned a different lower number, ranging from 20/10 to 20/200.
What do the numbers mean in 20/20?
- The top number represents the distance a person being tested stands from the eye chart.
- The bottom number represents how far away a person with normal vision can be from the chart and still read the letters in that line.
So a person with normal vision can read the 20/20 line from 20 feet away, the 20/200 line from 200 feet away, and the 20/10 line from 10 feet away. A person with 20/10 vision has better-than-normal vision. He can read from 20 feet what a person with normal vision must move up to 10 feet away to read. Conversely, a person with 20/200 vision has poorer-than-normal vision. He must be closer to the chart – he must be only 20 feet away to clearly see what a normal person can read from 200 feet away. The eye chart at your doctor’s office has been scaled down to fit the room, so while you may not actually stand 20 feet away, the ratios mean the same thing.
The Snellen eye chart
It was developed in 1862 by Dutch ophthalmologist Dr. Herman Snellen. It is the original and most commonly-used eye chart. Other tests have been developed, such as the Tumbling E, the Broken Wheel, and the ETDRS tests. The Tumbling E and Broken Wheel tests were developed for small children and adults who cannot read letters.
- The Tumbling E chart features a capital E facing up, down, left, and right. The person being tested is asked to respond with direction the E is facing.
- The Broken Wheel test uses cards with pictures of cars, some with whole wheels and some with broken wheels.
- The ETDRS test was originally developed for the Early Treatment of Diabetic Retinopathy Study. It uses letters, and is considered more accurate than the Snellen chart.
20/20 May Not Mean Perfect Overall Vision
Since these tests measure only visual acuity and not other eye conditions, you can have 20/20 vision and still have vision problems. With 20/20 vision you may have trouble seeing close objects, poor depth perception, difficulty focusing, or poor peripheral vision. Eyes are tested separately and together. Many people have a different score for each eye.
What is best corrected vision?
Your best corrected vision is the best visual acuity you can achieve with corrective lenses. But you may well achieve better vision than that after a Wavefront-guided LASIK surgery. That is because a Wavefront-guided vision correction can correct subtle imperfections in the vision system which corrective lenses cannot correct. They are known as the Higher Order Aberrations.
What vision scores can mean to you
A person with 20/200 best corrected vision or worse (higher lower number) is considered legally blind. 20/40 is the worst vision score you are allowed to have to obtain an unrestricted driver’s license. 20/50 is the limit for many professions.
Attaining 20/20 vision
Some people find that glasses and contact lenses are too uncomfortable and inconvenient, or they simply do not want to be dependent on these things. Others find that corrective lenses are not enough and can benefit from a combination of LASIK surgery and corrective lenses. If you do not have 20/20 vision, or if you have 20/20 vision but still experience vision problems, your eye doctor can help you decide if corrective lenses or LASIK surgery would be the best option for you.