- Your eye has a refractive error if it cannot focus an image sharply. Determining the lens prescription needed to correct the error is called a refraction or refraction assessment.
- Your eye's ability to refract light depends on the overall length of the eye and the curvature of the eye's surface (cornea). If the eye is too long, images mistakenly focused in front of the retina are out of focus when they reach the retina, and you are nearsighted. If the eye is too short, images are not in focus by the time they get to the retina, and you are farsighted. If the cornea is not properly curved, you have astigmatism.
- To estimate the refractive correction for your prescription, the doctor may use a computerized refractor to measure your eyes and estimate what's needed to correct your refractive error. Or the doctor may shine a light into your eyes and measure the refractive error based on the movement of the light reflected by your retina (a technique called retinoscopy).
- Your eye doctor fine-tunes the initial refraction assessment by having you look through a Phoroptor, a device that contains wheels of different lenses, and read an eye chart to determine which combination of lenses gives you the sharpest vision.
- Surgeries to correct vision, such as LASIK, change the shape of the cornea while attempting to correct refractive errors.
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