Ever wonder why 20/20 is the standard for ''perfect'' eyesight and what it actually means? 20/20 vision is a term to express a normal level of clarity of eyesight or visual acuity assessed from a 20 feet distance. In other words an individual with such visual acuity can see an object clearly at a distance of 20 feet that most people are expected to be able to see from such a distance.
For those who don't have 20/20 visual acuity, their visual acuity score is assigned based on where they begin to see clearly in comparison to what is normally expected. For example, 20/100 acuity indicates that you must be as close as 20 feet to see clearly what someone with normal vision can see at 100 feet away.
One can also have better than 20/20 vision. For instance a person that has 20/10 eyesight can see sharply at 20 feet what the average person can only see at 10 feet distance. Certain animals particularly birds of prey have more acute eyesight in comparison to man. For example, hawks have been known to have 20/2 eyesight, enabling them to locate prey from high in the air.
An average vision screening is performed by using an eye chart such as the classic Snellen eye chart designed by Hermann Snellen, a Dutch eye doctor in the mid-1800's. While there are now many versions, the chart typically shows 11 lines with uppercase letters which get smaller in size as they move toward the bottom. The top of the chart usually shows one uppercase letter - ''E'' with the addition of more letters as they get smaller. During the vision screening, the eye doctor will look for the line with the smallest lettering you can see clearly. Your score is determined since each row is assigned a distance, with the 20/20 line usually being ascribed the eighth row. In cases where the patient isn't able to read, such as young children or handicapped individuals, a variation of the chart is used called the ''Tumbling E''. Similar to the traditional Snellen chart, the ''Tumbling E'' is composed of only the capital letter E in different spatial orientations. The patient uses their hand to mimic which direction the ''fingers'' of the E are facing.. In order for the results to be accurate the chart needs to be positioned 20 feet away from the patient's eyes.
While 20/20 visual acuity does indicate that an individual sees at the norm from a distance this measure on its own does not indicate that a person has perfect vision. Complete eyesight includes a number of other necessary skills such as side or peripheral vision, depth perception, focus for near vision, color vision and coordination between the eyes amongst others.
It's important to remember that even though an eye exam using a Snellen chart can conclude if you require a visual aid to correct for distance vision it doesn't give the eye doctor a full perception of your complete eye and vision health. Make sure you still go in for an annual comprehensive eye exam which can identify vision-threatening conditions. Contact us now to schedule a San Francisco, CA eye test.