OPHTHALMOLOGY

Q:

I have always heard that 20/20 was normal vision. What exactly is normal vision and how is it determined or measured?

A:

In the United States, 20/20 is used to express the normal standard for visual acuity, that is, the clarity or sharpness of your vision measured at 20 feet. Testing visual acuity is most often done with the Snellen eye chart and is the most common clinical measurement of visual function. In the notation of 20/20, the numerator or top number represents that distance from the patient to the eye chart in feet. The denominator or bottom number represents the smallest size of letters or figures that the patient can identify. Therefore, the higher the bottom number, the worse the eyesight. Visual acuity is evaluated one eye at a time and is measured with and without correction (glasses or contact lenses). Having 20/20 vision does not necessarily mean you have perfect vision; it simply indicates your visual acuity at a distance. The fact is, you can have normal visual acuity such as 20/20 but still have problems with your vision. There are several other important visual skills that contribute to you overall visual ability including peripheral or side vision, eye coordination, depth perception, focusing ability, and color vision. To get a complete picture of your eye and vision health, a comprehensive eye exam is recommended. During this exam, your eye doctor will perform a series of tests and assessments that include the following: 1. Your eye and health history including vision problems, any corrective methods you have (glasses or contact lenses), your overall health, family history, and current medication. 2. A refraction to see if glasses are needed and to measure your visual acuity. 3. Pupil examination. 4. Dilation. 5. Slit lamp exam of the front part of your eye. 6. Fundus exam of the back (inside) of your eye. 7. Intraocular pressure. Other testing may include visual field testing, ocular motility, and corneal curvature measurements among others depending on need. If the results of your eye exam are normal, this means that your doctor detected nothing abnormal during your exam; you have normal vision, no signs of glaucoma, no abnormalities with the optic nerve, retina, or eye muscles, and no signs of eye disease or conditions. Even if your eyes are normal and you have good vision, they can change over time like any other part of the body. This is why it’s important to have regular, comprehensive eye exams. Many eye and vision problems have no obvious signs or symptoms. Early diagnosis and treatment of eye and vision problems are important for maintaining good vison and eye health, and as possible, preventing vision loss. Talk with your eye doctor about any questions or concerns you have about your eyes and what is normal for you.