Have you ever asked yourself why 20/20 is the standard for ''perfect'' vision and what it really means? 20/20 vision is a phrase to express a normal level of clarity of eyesight also known as visual acuity measured from a 20 feet distance. In other words someone with 20/20 vision will be able to see an object clearly from 20 feet away that most people are expected to be able to see from that distance.
For those who cannot see an object clearly at 20/20, their visual acuity score is assigned according to the first point at which they are able to see sharply, in relation to what is normally expected. For example, if your vision is 20/100 that indicates that at 20 feet you can only see an object that the standard would see from 100 feet away.
It's also possible to have better than 20/20 vision. For example someone that has 20/10 eyesight can see sharply at 20 feet what the average person can see only at 10 feet. A number of animals particularly birds of prey have more acute vision compared to the human species. A hawk for instance can have 20/2 eyesight, enabling them to spot prey from high in the air.
Most optometrists utilize some version of the Snellen eye chart, invented by Dutch eye doctor, Herman Snellen in the mid-1800's, to conduct an eye test. While there are many variations, the chart usually has 11 rows with uppercase letters which get progressively smaller as they move toward the bottom. The chart begins with one uppercase letter – ''E'' and gradually adds more letters as they get smaller. During the vision screening, the optometrist will determine which is the smallest line of letters you can see clearly. Every line is given a rating, with the 20/20 line typically being assigned the eighth row. In cases in which the patient can't read, such as small children or disabled individuals, a varied version of the chart is used called the ''Tumbling E''. Similar to the standard Snellen chart, this variation portrays only the uppercase letter E in different spatial orientations. The optometrist asks the person being tested to show the direction the ''fingers'' of the E are facing: right, left up or down. Either chart needs to be positioned at a distance of 20 feet from where the patient is viewing it.
While 20/20 visual acuity does mean that the person is able to see at the norm from a distance this measure alone doesn't suggest that the individual has perfect vision. There are a number of other necessary elements to make perfect vision such as side or peripheral vision, depth perception, color vision, near vision and focusing and coordination between the eyes amongst others.
While an eye exam using an eye chart will conclude if you require a visual aid to see clearly at a distance it will not provide the optometrist a comprehensive picture of your total eye health. It's recommended that you still go in for an annual comprehensive eye exam to screen for potential conditions. Call us today to schedule a Winston Salem, NC eye exam.