What is 20/20 Vision?

The term 20/20 vision is used by eye doctors for vision they have decided to consider as normal. Over the decades, a great many people have had their eyesight tested and these results are the basis for defining normal vision. The two twenties in 20/20 are measurements of distance in feet from an eye chart.

The Snellen Chart

The eye chart is usually some form of the Snellen Eye Chart, which has one large letter at the top followed by rows of letters. Each row has smaller letters than the row above it and therefore more letters. Often the top big letter is “E”, although there are charts where it is “A” or “F”. There is also a version for those who cannot read, where the rows consist of “Es” facing in different directions.

There are eleven lines counting the big “E” and the eighth row down is designated the 20/20 line. The letters on each row are called optotypes and the rate at which they grow smaller is constant from row to row. They shrink in thickness as well as height.

This chart was invented by a Dutch eye doctor called Herman Snellen in 1862. Until recently it was in the form of a paper chart on a wall. But modern ophthalmologists use a digital form viewed through an aperture in the phoropter used to test visual acuity. That digital chart is not physically 20 feet away, but is created so as to be visually equivalent to a chart at 20 feet.

20/20 Vision vs. Myopia

If you have 20/20 vision, you can read the letters on the chart from 20 feet away as well as most people can who need no glasses. The first twenty refers to being 20 feet from the chart.

The second twenty is a comparative number.

If you are mildly myopic (nearsighted), you have to stand only 20 feet away to see what a “normal” person can see from 40 feet. That is 20/40 vision.
If you have 20/100 vision, you are very myopic, as someone with normal vision can see from 100 feet what you must be only 20 feet away to see.
If you have 20/200 vision, you are legally blind.

20/20 vision vs. Hyperopia

For a farsighted person the second number is lower than 20 and is written as a positive diopter number on your prescription.

If you are mildly hyperopic you can see from 20 feet away from the chart what a normal person must stand closer to see. For example, 20/16 is hyperopic vision – normal eyes must be only 16 feet from the chart to see the detail you can see from 20 feet.
If you are more severely hyperopic, someone with normal vision must be only 10 feet from the chart to see what you can see at 20 feet. That would be 20/10 vision.

Hyperopia is regarded as a negative condition and is corrected with glasses, contact lenses, or LASIK. That is because humans need good close-up vision for most activities other than sports. A hyperopic person has blurry near vision. Looking at hyperopia another way, it can be seen as better-than-normal vision because you do not have to be so close to see the object clearly.

We think of birds of prey as having superbly acute eyesight. Based on how very dense the retinal cells are in a hawk’s eye, it is thought that a hawk has 20/2 vision. That is very hyperopic but for a hawk it is necessary and positive.

In nations that use metric measurements, 20/20 vision is expressed as 6/6/ vision, referring to meters. Six meters is approximately the same as 20 feet.

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