Oxnard - Port Hueneme Optometry  Dr. Don Steensma 465 W. Channel Islands Blvd, Port Hueneme, CA 93041  805/486-3585

20/20

The term 20/20 means that a persons vision is such that they can distinguish a certain size letter at a distance of 20 feet.  The technical explanation is that a 20/20 letter measures 5 degrees of arc.  In reality it really means the ability to see a letter that is 0.875 cm tall at a distance of 20 feet.  The term 20/40 means that a persons vision is such that the best they can do is to distinguish a letter that is 1.75 cm (twice as large as 0.875 cm).  Likewise 20/10 means that a persons vision is such that they can distinguish a letter only 0.438 cm tall at 20 ft  (one half of 0.875).

If you doubled the test distance you need to also double the image size to maintain the same ratio.  So testing at a 40 ft  test distance the 20/20 letter would be 1.75 cm tall and could be recorded as 20/20 but a more descriptive recording would be 40/40.  In plain English that means that that person can recognize a letter at 40 ft distance that a person with 20/20 can see.

Testing visual acuity at reading distances is done in the same way.  There are several different systems of recording vision at near but they are all basically just ratios of what a person with 20/20 is able to see.  A 20/40 letter is twice as large as a 20/20 letter and a 20/60 letter is three times as large as the 20/20 letter.

Stating what the visual acuity is should always be qualified with the conditions in which it is measured.  Vision without glasses may be 20/100 but with glasses 20/20.

Most states require at least 20/40 vision with both eye open to drive a car.  A commercial license requires at least 20/40 in both eyes seperately.

Legal blindness in an eye is usually stated as worse than 20/400 vision.  Most blind people have some vision and can function with some visual tasks.  No light perception or 'black-blindness" is a lot worse than 20/400.  If you see a person with a white cane with a red tip you can assume they have some severe visual handicap.  A totally blind person uses a long white cane that they can use to probe the space around them.

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