Ever wonder why 20/20 is the standard for ''perfect'' vision and what it actually represents? 20/20 vision is a term used to describe a normal level of sharpness of eyesight or visual acuity measured from 20 feet away from the object. That is to say that an individual with 20/20 visual acuity will be able to clearly see an object from 20 feet away which is deemed the norm to see at that distance.
In cases of individuals that cannot see at 20 feet away, their visual acuity score is designated according to the first point at which they are able to see sharply, compared to what is normally expected. As an example, 20/100 acuity means that you must be as close as 20 feet to see what a person with normal eyesight can see at a distance of 100 feet.
It's also possible to have vision that is better than 20/20. For example someone with 20/10 vision can see sharply at 20 feet an object that most can see only at 10 feet. A number of animals particularly birds of prey have more acute eyesight compared to humans. For example, hawks have been known to have 20/2 vision, designed for spotting prey from high in the air.
A typical eye screening is performed with the use of an eye chart usually the classic Snellen eye chart developed by Dutch eye doctor, Herman Snellen in the mid-1800's. While there are now many versions, the chart generally has eleven lines with capital letters which get progressively smaller as one looks downward. The top of the chart usually shows the capital letter - ''E'' with letters being added subsequently as you move down the chart. During the eye exam, the eye doctor will examine which is the smallest line of letters you can see clearly. Your score is determined since each row is assigned a distance, with the 20/20 row usually being assigned the eighth row. In instances where the patient can't read, such as young children or disabled individuals, a variation of the chart is used called the ''Tumbling E''. At the same scale as the regular Snellen chart, this variation shows only the uppercase letter E in different rotations. The person being tested uses their hand to mimic which direction the arms of the E are facing: right, left top or bottom. In order for the results to be accurate the chart must be positioned at a distance of 20 feet from where the patient is viewing it.
Despite common conception, 20/20 vision does not indicate someone has perfect eyesight but merely that they are able to see as expected at a distance. Total eyesight includes a number of other necessary competencies such as peripheral vision, depth perception, focus for near vision, color vision and coordination between the eyes to name a few.
Although a vision screening using a Snellen chart will establish if you need eyeglasses to see far away it will not give the optometrist a complete perception of your total eye health. Make sure you still go in for an annual comprehensive eye exam which can diagnose potential conditions. Call us today to schedule a Clarksville, IN eye exam.