Posted by: juliegriffey | May 22, 2011

What does it mean to have 20/20 vision?

Almost everyone has heard the expression: “I have 20/20 vision,” but how many people know exactly what this means?  Even if you don’t know what the 20/20 represents, you probably know that 20/20 vision is good… maybe even perfect.

However, having 20/20 vision does NOT mean that the subject has perfect vision; it simply means the subject has “normal” vision. But what is normal?  Over the years, eye doctors have examined many subjects and defined “normal” vision as being able to read a specific size of letter (9 mm) from a distance of 20 feet. This letter size is standardized on a Snellen eye chart (line 8 on the chart).

The Snellen chart not only helps an examiner determine whether the patient has 20/20 vision.  It also helps the practitioner assess whether the patient has vision better or worse than 20/20 and to what degree.  For example, if a patient can only read at 20 feet away from the chart what “normal” patients can read at 40 feet, the patient is said to have 20/40 vision.  Conversely, if a patient can distinguish letters at 20 feet away that people with “normal” vision can see only at 15 feet away, then the patient has 20/15 vision.


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