The purpose of standard eyeglass prescription is to give a person 20/20 vision, based on an arbitrary measurement developed by Hermann Snellen in 1862. It is much too strong. Except for students looking at the blackboard, when was the last time you needed to see inch letters 20 feet away? For daily living and most tasks 20/30 to 20/50 is enough. Automobile drivers with licenses legally require only 20/40. Only airline pilots require 20/20.
What is 20/20 vision? It means that when you stand 20 feet away from the Snellen eye chart and read the letters on the 20/20 line correctly; your vision is 20/20, which is considered normal. If you have 20/30 vision, it means that when you stand 20 feet away from the eye chart you can only see what a person with 20/20 vision can see at 30 feet. If you have 20/40 vision, it means that when you stand 20 feet away from the eye chart you can see what a person with 20/20 can see at 40 feet.
From the above explanation, we can see that the first number is always 20 which is the distance to the eye chart while the second number is the distance at which a person with 20/20 could read the line. The lowest line that you can read on the eye chart is your visual acuity. The larger the second number is, the worse your vision would be.
The Snellen eye chart is the standard on which most eyeglass prescriptions are based because it is considered scientific and infallible. However, in 1862 Hermann Snellen simply based his 20/20 vision measurement on the vision of an assistant whom he thought as a person with good eyesight, not very scientific as it is.
Nearsighted eyeglass check are focused on and prescribed for 20 feet because all readings of the Snellen eye chart are done at 20 feet. They are distance vision eyeglasses suited for looking 20 feet and farther because eyes do not have to focus at this distance. However, these distance vision eyeglasses are unsuited for looking closer than 20 feet because the eyes must focus to see objects closer than 20 feet.
By wearing 20/20 eyeglasses, the nearsighted eyes must adjust focus to see objects closer than 20 feet and must further adjust focus to overcome the eyeglasses prescribed for 20 feet. The eyes unnaturally doubly strain to adjust focus and these results in more nearsightedness that eventually requires bifocal eyeglasses.
The Snellen eye chart, upon which most eyeglass prescriptions are based, really measures one visual skill – our distance vision. This is only one small part of our vision. There is much more to our vision: near vision, peripheral vision, eye coordination, color perception, light sensitivity, endurance, etc. Basing our eyeglass prescriptions on 20/20 vision is unnecessary and ultimately harms our eyes.