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Tucson, AZ Eye Tests: Exploring the Vision Chart


Have you ever asked yourself what 20/20 eyesight truly means? 20/20 vision is a term to express a normal level of clarity of eyesight (visual acuity) measured from 20 feet away from the object. In other words an individual with such vision can clearly see an object at a distance of 20 feet which is considered the norm to see from that distance.

For those who cannot see an object clearly at 20 feet away, their visual acuity score is determined based on the first point at which they are able to see sharply, in comparison to the norm. As an example, if your vision is 20/100 that indicates that you must be as close as 20 feet to see clearly what a person with normal eyesight would see at 100 feet away.

An individual whose vision is 20/200 or below is considered legally blind but can often see normally with the use of prescription glasses or contact lenses or by having LASIK if they qualify.

Most optometrists employ a version of the Snellen eye chart, developed by Dutch eye doctor, Herman Snellen in the 1860's, to perform an eye screening. While today there are a number of variations, the chart typically has eleven lines of capital letters which get progressively smaller as they move toward the bottom. The chart begins with the capital letter - ''E'' with the addition of more letters on the lines as they get smaller. During the vision test, the eye doctor will assess the line with the smallest lettering 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. In instances in which the patient can't read, such as small children or handicapped individuals, a variation of the chart is used called the ''Tumbling E''. At the same scale as the standard Snellen chart, this version is composed of only the uppercase letter E in different directions. The optometrist asks the person being tested to point to the right, left, top or bottom to show which direction the E is facing. In order for the results to be accurate the chart must be placed at a distance of 20 feet from where the patient is viewing it.

Despite what many think, 20/20 eyesight does not show an individual sees perfectly but only that their distance vision is normal. There are a number of other essential elements to make perfect vision such as peripheral sight, depth perception, focus for near vision, color vision and eye coordination amongst others.

While an eye exam using a Snellen chart will often determine if you need a visual aid to see clearly at a distance it doesn't provide the optometrist a complete picture of your overall eye and vision health. You should still schedule an annual comprehensive eye exam to screen for any more serious diseases. Call us today to schedule an eye exam in Tucson, AZ.