The term “20/20 vision” refers to normal vision – the visual acuity that most people have who can see well without any eyewear. The two numbers refer to distance in feet. If you have ever had your vision tested you will be familiar with the eye chart that has lines of letters getting smaller towards the bottom of the chart. Our eye care providers all use it.
This is the Snellen chart, named after the Dutch eye doctor who devised it in 1862. In past years it has been used as a chart on the wall but in our high-tech era it is digitalized and you look at it through an aperture in the vision-testing equipment called a phoropter.
Counting the large letter at the top of the chart (typically an “E”) as the first row, the eighth row of letters indicates 20/20 vision if you can see it clearly from 20 feet away.
The First “20” This “20” is the number of feet you are from the chart when your eyes are being tested. It is always set at 20 feet. The phoropter’s optics are set to create the equivalent of 20 feet between the aperture you look through and the digital chart. This first “20” is a constant and is the same in each prescription.
The Second “20” A high second number indicates some degree of nearsightedness. If you are fairly nearsighted, you might be “20/80”, for example. That means that you can see clearly from 20 feet away, a normal eye can see clearly from 80 feet. Vision of 20/40 is close to normal. Legal blindness is defined as 20/200, and that is not total blindness; it is just very severe myopia such that the person has a severe handicap in daily living.
A hawk is thought to have 20/2 vision. For a bird of prey, that is extremely acute vision, because the bird can see its next meal from 20 feet away whereas a person with “normal” eyesight would have to be only two feet away to see it. That keeps the hawk well-fed.
A prescription of 20/2 would be severe farsightedness. For a human that is considered a defect, since most of our activities require clear close vision. A farsighted person might more typically have a prescription of 20/16 or 20/10.
If you wear glasses or contacts you probably have 20/20 “best-corrected vision”. That means that whatever your natural vision is like, you can see normally using your eyewear.
To learn more about your eyesight, please contact our LASIK offices today for a personal consultation.
