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Kennett Square, PA Eye Exams: Exploring the Vision Chart

Ever question why 20/20 is the standard for ''perfect'' eyesight and what it truly represents? The term 20/20 vision describes a normal level of sharpness of eyesight (visual acuity) calculated from 20 feet away from the object. In other words an individual with such visual acuity will be able to see an object clearly from 20 feet away that the majority of people should be able to see from that distance.

For those who cannot see an object clearly at 20 feet away, the number is designated according to the distance at which they are able to see sharply, compared to what is normally expected. As an example, 20/100 vision indicates that you have to be as close as 20 feet to see what someone with normal visual acuity can see at 100 feet away.

It's also possible to have better than 20/20 vision. For instance someone with 20/10 vision can see clearly at 20 feet what most can only see at 10 feet distance. Members of the animal kingdom particularly birds of prey have been known to have incredibly acute eyesight compared to man. For example, hawks have been known to have 20/2 vision, enabling them to spot prey from great heights.

A typical vision test is done with the use of a vision chart most commonly the classic Snellen eye chart developed by Dutch eye doctor, Herman Snellen in the mid-1800's. While today there are a number of variations, the chart generally shows 11 lines with capital letters which get smaller in size as one looks toward the bottom. The chart begins with the uppercase letter - ''E'' with the addition of more letters as they get smaller. During the vision screening, the eye doctor will assess the line with the smallest lettering you can read. Each line is given a distance, with the 20/20 line usually being ascribed the eighth row. In instances where the patient isn't able to read, such as young children or handicapped individuals, a different version of the chart is used called the ''Tumbling E''. At the same scale as the regular Snellen chart, this version portrays only the uppercase letter E in different spatial orientations. The eye doctor tells the person being tested to point to the right, left, top or bottom according to the direction the E is pointing. In order for the results to be accurate the chart should be placed at a distance of 20 feet from the patient's eyes.

Despite what many think, 20/20 visual acuity does not show someone sees perfectly but merely that they see adequately at a distance. ''Perfect'' vision includes a number of other necessary skills such as peripheral vision, perception of depth, color vision, near vision and focusing and coordination between the eyes to name a few.

It's important to remember that even though a vision screening with a Snellen chart will conclude if you need a visual aid to see far away it doesn't provide the optometrist a full perception of your complete eye health. It's recommended that you still schedule an annual comprehensive eye exam which can diagnose vision-threatening diseases. Contact us now to book an eye exam in Kennett Square, PA.

 

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