Eye Tests: What 20/20 Eyesight Actually Means

Have you ever wondered why 20/20 is the benchmark for ''perfect'' vision and what it really means? The phrase 20/20 eyesight expresses a normal level of clarity of eyesight (visual acuity) calculated from 20 feet away from the object. In other words an individual with such vision will be able to see an object clearly from 20 feet away which is regarded as normal to see from that distance.

For those who cannot see at 20/20, the number is assigned based on where they begin to see clearly in relation to what is normally expected. As an example, if your acuity is 20/100 that means that at 20 feet you can only see an object that someone with normal vision can see at 100 feet away.

Someone whose eyesight is 20/200 or below is considered legally blind but can often see normally by using prescription glasses or contacts or by having laser eye surgery if they qualify.

A typical eye exam is done by using a vision chart such as the classic Snellen eye chart developed by Hermann Snellen, a Dutch eye doctor in the mid-1800's. While there are now quite a few versions, the chart typically shows eleven rows with uppercase letters which get progressively smaller as they move downward. The top of the chart usually shows one capital letter - ''E'' with the addition of more letters as they get smaller. During the vision screening, the eye doctor will examine which is the line with the smallest lettering you can make out. Your score is determined since each line is assigned a rating, with the 20/20 row typically being assigned the eighth row. For small children, illiterate or disabled persons who are not able to read or vocalize letters, a variation of the chart is used called the ''Tumbling E''. At the same scale as the standard Snellen chart, this version shows only the uppercase letter E in different rotations. The eye doctor tells the person being tested to mimic which rotational direction the arms of the E are facing.. In order for the results to be accurate the chart must be placed 20 feet away from the patient's eyes.

Despite what many think, 20/20 visual acuity does not indicate someone has flawless vision but only that they are able to see normally from a distance. Complete eyesight involves many other important competencies such as side or peripheral vision, perception of depth, color vision, near vision and focusing and eye coordination amongst others.

It's important to remember that even though an eye exam with an eye chart can conclude if you require eyeglasses to see clearly at a distance it doesn't provide the optometrist a complete understanding of the overall health of your eyes and vision. You should still schedule an annual comprehensive eye exam which can diagnose potential diseases. Contact our office today to book a Westerville, OH eye exam.