Have you ever asked yourself what 20/20 eyesight actually represents? 20/20 vision is a term used to describe normal visual acuity or sharpness of vision. In other words someone with 20/20 eyesight will be able to clearly see an object at a distance of 20 feet which is considered normal to see at that distance.
In cases of individuals that don't have 20/20 vision, the number is assigned based on the first point at which they are able to see clearly, in comparison to what is normally expected. For instance, if your acuity is 20/100 that means that you must be at a distance of 20 feet to see clearly what someone with normal eyesight would see at a distance of 100 feet.
It's also possible to have vision that is above 20/20. For example a person that has 20/10 vision can see sharply at 20 feet an object that the average person can see only at 10 feet. Certain animals particularly birds of prey have been known to have incredibly acute vision compared to the human species. For example, hawks have been known to have 20/2 eyesight, enabling them to locate prey from great heights.
Most optometrists utilize some version of the Snellen eye chart, invented by Hermann Snellen, a Dutch eye doctor in the mid-1800's, to conduct an eye test. While there are many variations, the chart generally has eleven lines of capital letters which get smaller in size as one looks downward. The chart begins with the capital letter – ''E'' and gradually adds more letters as they get smaller. During the eye exam, the optometrist will assess the smallest line of letters you can see clearly. Your score is determined since each line is assigned a rating, with the 20/20 row usually being assigned forth from the bottom. For young children, illiterate or handicapped persons who can not read or vocalize letters, a different version of the chart is used called the ''Tumbling E''. Similar to the standard Snellen chart, this variation portrays only the uppercase E in different directions. The person being tested uses their hand to point to the right, left, top or bottom according to the direction the E is facing. In order for the results to be accurate the chart should be placed 20 feet away from the patient's eyes.
Despite common conception, 20/20 visual acuity doesn't show an individual sees perfectly but rather that they are able to see well from a distance. There are a number of other necessary abilities needed to make perfect vision such as peripheral vision, perception of depth, focus for near vision, color vision and eye coordination amongst others.
Although a vision screening with an eye chart can determine if you require glasses to see far away it will not provide the optometrist a full understanding of your complete eye health. You should still book an annual comprehensive eye exam to screen for any more serious diseases. Contact us now to schedule an eye exam in Nashua, NH.