What does 20/20 vision mean?

By Murray Bourne, 18 Oct 2010

Eye chart [source]

Tiger Woods had "severe near-sightendness" and needed to wear contact lenses before he underwent lasik surgery in 1999. He now has 20/15 vision, significantly better than most of us, where "normal" vision is considered 20/20.

What do these eye acuity fractions mean?

An eye chart consists of letters of varying sizes. (Optometrists use a symbol chart if the patient is likely to be illiterate.)

On the chart shown, the height of the 8th line (starting with "D") is scaled so it is around 1.75 mm. From a distance of 20 feet (6 m), the height of the letters subtends an angle of 1 arc minute.

The angle "1 arc minute", means 1/60 of one degree. (In turn, 1/60 of an arc minute is one arc second. We get these meaures from the Babylonian base-60 number system). To give an idea what this means, when we look at the full moon from the Earth, it subtends an angle of around 31 minutes, or just over half of one degree.

Someone with "normal" 20/20 vision will be able to read the 8th line at a distance of 20 feet.

In the fraction 20/20, the numerator (the first number) is the distance in feet between the subject and the eye chart. The denominator (the second number) is the distance from which a normal person can read the 8th line (and this gives a visual angle of 1 arc minute).

20/20 vision is the minimum for fighter pilots.

20/40 vision is half the acuity of 20/20. This means the patient can only see clearly at 20 feet what normal people can see at 40 feet. So a patient with 20/40 vision can see clearly only down to about the 6th or 7th line of the chart (text which is 3.5 mm high). This is the legal limit for driving in the United States.

On the other hand, 20/10 vision is twice normal acuity, and this would imply the patient can read down to the lowest line on the chart.

The aim for the optometrist when dealing with someone with vision difficulties is to find the prescription that will provide at least 20/20 vision.

Tiger Woods performed much better in his golf game after his lasik surgery. Some commentators argue this may have given him an unfair advantage (in the same way steroids give athletes an unfair advantage). What do you think? Is eye enhancement cheating?

9 Comments on “What does 20/20 vision mean?”

1. DEPR says:

No definitely the eye enhancement by Tiger Woods isn't cheating.

2. Jemal Kemal Nigo says:

I like the article 20/20 vision, as it describe sight/eye related problem with the aid of mathematics.It is also an article in which one realize that mathematics is a soul to all sciences.I mean it gives life to all sciences.

Hi Mr Murry, you wonderful to me, am appreciating you for developing me through your newsletters, ride on.thanks

4. april says:

Mr. murry,
every end of the school year, we, the students are required to go under physical examinations..
when it comes to the eye examination, it seems to me that in my left eye, word are more blurred than in my right eye??
is it normal? can math explain this??

5. Murray says:

Hi April. Well, physics can explain it! The lens of your left eye does not focus the light in exactly the right way, hence the blur. Sounds like you need glasses!

6. april says:

really.... thanks a lot!!!!
actually i have my glasses before but it's for astigmatism only.=)
well, i think i need an ophthalmologist. anyways, thanks again..

A 20/20 letter at 20 feet is actually 8.73 mm tall( 5*1.75mm). Each letter detail subtends 1 arcmin. A 20/20 "E" has 5 details (3 "arms" with 2 gaps between).

Draw a letter 1.75 mm tall (which is hard to do) and stand back 20 feet. Pretty small. You could see it if you had 20/4 vision.

9. Charity Arnold says:

Hey thanks for the article! I really enjoyed the math analysis. It gives me a new understanding of what 20/20 vision really is.

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