Refractive errors often are the primary 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?
Refractive errors are optical imperfections that cause the image to be out of focus, resulting in blurred vision. There are three primary refractive errors referred to as nearsightedness, farsightedness, and astigmatism.
Refractive errors usually can be “corrected” with eyeglasses or contact lenses. They can sometimes be permanently treated with LASIK or other vision correction surgery.
How Light Travels Through the Eye
Light can be deflected, reflected, bent, or absorbed as it travels through the eye. When light travels through the cornea or lens, its path is bent or refracted. The eye structures have refractive properties similar to lenses and can bend light rays onto a precise point on the retina to provide clear vision.
Most refraction in the eye occurs when light rays travel through the curved, clear front surface of the eye (cornea). The eye’s natural (crystalline) lens also bends light rays.
Causes of Refractive Errors
The eye’s ability to refract or focus light sharply on the retina primarily is based on three eye anatomical features: 1) the overall length of the eye, 2) the curvature of the cornea and 3) the curvature of the lens inside the eye.
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.
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.
Detection and Treatment of Refractive Errors
Your eye doctor determines the type and degree of refractive error you have by performing a test called refraction. This can 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. An eye care practitioner performs 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 nearsightedness and astigmatism. Your eye doctor will use the results of your refraction to determine your eyeglasses prescription or for use in your contact lens fit.