How to understand the eye test

Updated February 21, 2017

When you undergo an eye exam your ophthalmologist or optometrist will perform an eye exam to test your vision and look for any sign of eye disease. Your doctor will use a variety of instruments, aim bright lights in your eyes, and test your vision as you look through a series of lenses. The goal of the eye exam is to evaluate different aspects of your vision. Once completed, your doctor will give you an assessment of your visual acuity and, if necessary, a prescription for eye glasses or contact lenses. He or she may also talk to you about any conditions that you should be aware of. Always ask your doctor to thoroughly explain the results of your eye exam to you, and in language you can understand.

Ask for the results and meaning of your visual acuity test, which is represented by numbers such as 20/20 or 20/50. A reading of 20/20 indicates that the smallest letter you can identify is 20 feet away. The lower the bottom number is, the better the visual acuity; and the higher a bottom number, the worst the visual acuity. A person is labelled "legally blind" if their visual acuity with lenses is 20/200 or worse. In most states, you must have a visual acuity of at least 20/40 to get a driver's license. So many people need to wear eyeglasses or contact lenses in order to pass a driver's test.

Find out your PERRLA rating. A PERRLA (Pupils Equal Round Reactive to Light and Accommodation) rating shows the degree to which your pupils react to light and dark. A normal PERRLA is 2 to 4mm in diameter in light, and in dark, 4 to 8mm. Each pupil should dilate and constrict equally in size.

Determine your Diopter rating. This indicates the degree to which your eye can focus. You have a dioptre rating of 1 when you can focus at a length of 1 meter (39 inches). The higher your dioptre rating, the more magnification you need to focus. A young child might have a dioptre of 40: but this will likely decline as he or she ages. By the age of 50 most people have a dioptre of 1, meaning that they have presbyopia (where images up close are blurred). This is why many people over 50 need bifocals or magnifying glasses to read. The magnification in reading glasses is called dioptre strength.

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  • Eye tests
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About the Author

JIm Cooper is an attorney and business consultant. He serves on the board of many corporations. He is also a published writer with more than 30 years of experience. Cooper's articles have been published in "American Executive," "Men's Health" magazine, "Newsweek," "Marie Claire" and "Mademoiselle" magazines.