What does the measure of vision (like 20/400) mean?

Please help me. My eye doctor tells me I have 20/400 vision. What does this mean?! (Signed) Blind & Confused.

Well, B&C, the short form is that you, my friend, are blind. At least you are according to the World Health Organizationís International Classification of Impairments, Disabilities and Handicaps, circa 1980. Specifically you fall into the category of "profound low vision", characterized as 5% vision, or 20/400 on a Snellen Chart. Iím pleased to report that in the good old US of A, you may not be legally blind, as defined by the federal Social Security Act. That would require visual acuity of 20/200 or less in the better eye with best correction.snellen chart

The Snellen Chart to which I keep referring is the standard tool for the measurement of visual acuity. It displays letters of progressively smaller size. Normal vision is 20/20. This means that the test subject sees the same line of letters at 20 feet that a normal person sees at 20 feet. 20/40 vision means that the test subject see as 20 feet what a normal person sees at 40 feet.20/X vision, therefore represents a simple ratio.. A person with 20/20 vision is able to see letters 1/20th as large as my beleaguered questioner, Blind & Confused.

Some handy benchmarks for corrected vision are:

20/20 - Normal vision. Fighter pilot minimum. Required to read the stock quotes in the newspaper, or numbers in the telephone book.
20/40 - Able to pass Driver's License Test in all 50 States. Most printed material is at this level.
20/80 - Able to read alarm clock at 10 feet. News Headlines are this size.
20/200 - Legal blindness. Able to see STOP sign letters.

B&Cís vision is at least better than that of a newbornís. A two-week-old who will have 20/20 vision by the age of five begins its life outside the womb with 20/800 vision at the age of two weeks. By six months, acuity will improve to 20/70.

Most of the information for this reply came from the most excellent website of the Emory Vision Correction Center of Atlanta, Georgia.

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