Have you ever asked yourself why 20/20 is the benchmark for ''perfect'' vision and what it really represents? The term 20/20 vision expresses a normal level of clarity of vision or visual acuity assessed from 20 feet away from the object. In other words an individual with such eyesight can see an object clearly at a distance of 20 feet that most individuals should be able to see from that distance.
For those who cannot see an object clearly at 20 feet away, their visual acuity score is designated according to the distance at which they are able to see clearly, compared to what is normally expected. For instance, if your acuity is 20/100 that indicates that you have to be as close as 20 feet to see clearly what someone with normal visual acuity would see at a distance of 100 feet.
One can also have better than 20/20 vision. For example a person with 20/10 eyesight can see sharply at 10 feet an object that the average person can only see at 20 feet. Members of the animal kingdom particularly birds of prey have more acute vision in comparison to what humans are capable of. A hawk for instance can have 20/2 vision, designed for locating prey from great heights.
Most eye doctors use a form of the Snellen eye chart, which was created by Hermann Snellen, a Dutch eye doctor in the 1860's, to conduct a vision screening. While there are a number of versions, the chart generally has 11 lines of capital letters which get progressively smaller as they move downward. The chart begins with one capital letter – ''E'' with the addition of more letters as they get smaller. During the eye exam, the optometrist will assess the line with the smallest lettering you can read. Every line is given a distance, with the 20/20 line usually being assigned forth from the bottom. For small children, illiterate or disabled persons who can not read or vocalize letters, the ''Tumbling E'' chart is used. Similar to the standard Snellen chart, this version shows only the uppercase letter E in different directions. The eye doctor asks the patient to indicate which rotational direction the arms of the E are pointing: right, left up or down. In order for the results to be accurate the chart must be positioned at a distance of 20 feet from the patient's eyes.
Despite common conception, 20/20 visual acuity does not indicate someone has flawless eyesight but merely that they see normally from a distance. There are a number of other necessary abilities needed to make perfect vision such as side or peripheral sight, depth perception, focus for near vision, color vision and eye coordination to name a few.
Although a vision screening with an eye chart will determine whether you need eyeglasses to see clearly at a distance it will not give the optometrist a full understanding of the overall health of your eyes and vision. You should still book an annual comprehensive eye exam which can identify any more serious conditions. Call our optometrist in Calgary today to schedule an eye exam.