Refractive Errors

Refractive errors often are the main reason a person seeks the services of an optometrist or ophthalmologist. But what does it really mean when we’re told that our vision is blurry because we have a refractive error?

We see the world around us because of the way our eyes bend (refract) light. Refractive errors are optical imperfections that prevent the eye from properly focusing light, causing blurred vision. The primary refractive errors are nearsightedness, farsightedness and astigmatism.

Refractive errors usually can be “corrected” with eyeglasses or contact lenses, or they can be permanently treated with LASIK and other vision correction surgery (also called refractive surgery).

Causes of Refractive Errors

The eye’s ability to refract or focus light sharply on the retina primarily is based on three eye anatomy features:

• Eye length. If the eye is too long, light is focused before it reaches the retina, causing nearsightedness. If the eye is too short, light is not focused by the time it reaches the retina. This causes farsightedness or hyperopia.
• Curvature of the cornea. If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism. A person can be nearsighted or farsighted with or without astigmatism.
• Curvature of the lens. If the lens is too steeply curved in relation to the length of the eye and the curvature of the cornea, this causes nearsightedness. If the lens is too flat, the result is farsightedness.

More obscure vision errors, known as higher-order aberrations, also are related to flaws in the way light rays are refracted as they travel through the eye’s optical system.

These types of vision errors, which can create problems such as poor contrast sensitivity, are detected through new technology known as wavefront analysis.

Detection and Treatment of Refractive Errors

Your eye doctor determines the type and degree of refractive error you have by performing a test called a refraction.

This can be be done with a computerized instrument (automated refraction) or with a mechanical instrument called a phoropter that allows your eye doctor to show you one lens at a time (manual refraction).

Often, an automated refraction will be performed by a member of the doctor’s staff, and then the eye care practitioner will refine and verify the results with a manual refraction.

Your refraction may reveal that you have more than one type of refractive error. For example, your blurred vision may be due to both nearsighted and astigmatism.