The phrase 20/20 vision means normal vision. It is the measure of visual acuity which is the sharpness or clearness of the vision. Dr. McKinney, an ophthalmologist and glaucoma specialist, while explaining 20/20 vision says;
In the term 20/20, the first 20 refers to the distance between you and the eye chart which in this case is 20 feet. While the second 20 refers to your ability to read the letters written on the eye chart. Each row of letters is of a specific size determined by mathematical studies about normal vision. If you have 20/20 vision then you can easily read the fourth line from the bottom of the chart.
But having a 20/20 vision doesn’t mean that your eyesight is perfect. Many factors count in determining the perfect vision and having a 20/20 vision alone can’t determine that your eyesight is perfect.
Firstly, the eye chart is hung on a wall and it is stationary and you also look at the chart while sitting or standing still. So, if you can see or read the letters on the eye chart in these conditions does not mean that you can also read those letters when you are moving or the chart is moving. One factor that counts in having a perfect vision is to focus on objects and words when they are in motion. So, having a 20/20 vision in stationary conditions does not conclude that you have perfect vision.
Secondly, the words written on the eye chart are in high contrast i.e., letters written in black against a white background to check your vision acuity while in real life everything isn’t black and white. Having a 20/20 vision doesn’t mean that you can focus on things in any contrast or dim conditions like at night.
Thirdly, some people do have a vision more than 20/20 like 20/15, which is sharper than average. A Person having 20/15 vision can focus on a word on the eye chart being 20 feet away that an average person can focus only when he is 15 feet away. While a person having 20/200 vision is legally blind.
Studies show that only 35% of adults have 20/20 vision without glasses, contact lenses, or eye surgery. While with the aid of glasses, contact lens or eye surgery about 75% of adults have 20/20 vision.
So, having 20/20 vision isn’t enough to determine how perfect your eyesight is. Other tests are also needed to determine the correctness of your vision. The physiology of the eyes should be checked with scans and photos. These tests would examine other factors that affect a person’s vision. An eye chart alone cannot give information about all the factors affecting a person’s eyesight and vision.