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How is my vision measured?

Laura C. Fine, MD
Ophthalmology
That familiar chart with rows of letters and numbers that diminish in size to test your eyesight is the Snellen chart. A doctor or technician uses this chart to test your visual acuity -- the sharpness of your central vision. Think of central vision as your eye's "vital sign," much like blood pressure and heart rate are the heart's vital signs. If you wear corrective lenses, the doctor tests your vision while you wear your glasses or contact lenses and also looks at your glasses through a device called a lensometer to determine their exact prescription.

Your exam score indicates how well you see compared with someone whose vision is normal. For instance, if you have 20/20 vision -- considered the standard for normal -- that means you can see at a distance of 20 feet what another individual with normal vision sees at 20 feet. However, if your vision is 20/40, you see at a distance of 20 feet what a person with normal vision would be able to see at 40 feet; in other words, you need to stand closer to the object to see it as clearly. In general, the higher the second number, the worse your vision.

If the test indicates a need for corrective lenses or a prescription adjustment, the doctor measures the eye's refraction, or focusing accuracy, using instruments that contain a combination of corrective lenses. To confirm that reading, you will be asked to look through a variety of lenses to ascertain which one gives you the best sight.

The doctor will also evaluate your peripheral vision. Typically you'll be asked to cover one eye and fix the other eye on a point straight ahead. The doctor will shift an object, such as a pen, back and forth at the outer edges of your visual field and ask you to note when you see it moving.

In certain circumstances, your color vision may also be measured with special color pictures. In addition, depth perception can be determined with a series of three-dimensional images. This is especially important if anyone in your family has had strabismus (eye muscle imbalance).

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.