What are "normal" results of an eye exam? Other than 20/20, what are normal axis and sphere numbers?

In the United States we think of "normal" vision as being 20/20, although maximum human acuity is even finer at 20/16 to 20/12. Therefore, interestingly, "normal" acuity is not "perfect" vision. The common 20/20 vision standard can be considered the lower limit of normal or as a screening cutoff. To have 20/20 uncorrected vision, or vision achieved without glasses or contact lenses, you must have low "sphere" and "cyl" numbers on your prescription.

On an eyeglass or contact lens prescription a "sphere" with a "plus" value describes hyperopia or farsightedness and a "minus" indicates myopia or nearsightedness. Astigmatism is another critical factor that determines a corrective prescription and is defined by two values: "axis" and cylinder or "cyl." If astigmatism is thought of as being like an American football, the axis describes its rotation or orientation between 0 and 180 degrees, while the power or "cyl" defines the steepness of the curvature. As the power number increases, so does distortion, which requires more correction for good vision.

Finally, normal 20/20 visual acuity does not necessarily mean completely normal vision. Some people may suffer from other visual problems, such as color blindness, reduced contrast, or inability to track fast-moving objects and still have 20/20 visual acuity. Even if your eyes are normal they can change, which underscores the importance of routine screening and appropriate care with an ophthalmologist, your ophthalmologist, who can both diagnose normal versus correctible and abnormal vision while suggesting a personalized plan to best protect your vision for the future.


This question was originally answered on June 13, 2012.