The General Ophthalmology Service and Optometrists at Scheie Eye Institute offer refraction as part of a well eye exam.
Almost all people have, or will eventually develop, a refractive eye condition. The term "refractive" is used because it denotes the fact that the human eye bends and focuses light in a certain way. The eye works much like a camera. In the normal eye, light from the outside world passes through both the cornea and the lens, whose curves focus outside images onto the retina. The retina is analogous to photographic film in a camera and is located at the back of the eye. The retina then transmits the perfectly focused image to the brain via the optic nerve, enabling us to see. The lens does fine-tuning of vision, for example, during reading. Refractive problems occur when the focusing power of the cornea and lens does not match the actual length of the eye. Refractive conditions are common developmental problems occurring in children and young adults. Refractive errors also can develop in older people from the normal aging process or because of certain eye diseases, such as cataract.
Astigmatism is a common eye condition and often occurs in association with other refractive problems, such as myopia and hyperopia. Astigmatism occurs when the surface of the eye is shaped more like a football than a basketball. In other words, the surface of the eye (the cornea) does not bend light equally in all directions. Because of this irregularity, the light does not focus perfectly on the retina at the back of the eye and the image appears blurred. Astigmatism can usually be treated with glasses, contact lenses, or refractive surgery.
Hyperopia, or farsightedness, is a common eye condition. Hyperopia occurs when the light that enters the eye focuses behind the retina. In many cases, the eyeball is too short and so the focal point of light falls behind the retina. People with hyperopia are called farsighted because, in many cases, they can see better far away than up close. In reality, however, the situation is much more complicated. Young hyperopic individuals may be able to see well at all distances and not even know they are hyperopic because the lens of the eye can accommodate for the condition. In other words, the eye in younger people can compensate for modest amounts of hyperopia. However, this ability to compensate is lost with age and so hyperopia is often discovered as that person ages. People with hyperopia are at higher risk for certain types of eye conditions, including certain forms of glaucoma. Hyperopia can usually be treated with glasses, contact lenses, or refractive surgery.
Myopia, or nearsightedness, is also a very common eye condition. People with myopia find it difficult to see people and objects at a distance. Myopia occurs when the images that enter the eye focus in front of the retina. In many cases, the eyeball is too long and so the focal point of light falls in front of the retina. The cause of myopia is not precisely known, but it may be caused by a combination of environment and genes. The prevalence of myopia is increasing worldwide, and seems to be more common in industrialized countries. Environmental influences are likely of greater impact than family history. Family history may define a risk to an environmental factor, for instance. People with myopia are at higher risk for some eye diseases, such as retinal detachment, retinal degeneration, glaucoma and certain forms of cataract. The blurred vision from myopia can be corrected with glasses, contact lenses, or refractive surgery, but these treatments are not known to reduce the risks of future eye disease.
Presbyopia is a universal condition of human aging. All individuals eventually become symptomatic with presbyopia, usually between the ages of 38 and 42.
Presbyopia occurs as the lens of the eye loses its ability to bend and flex. This loss of accommodation begins very early in life but usually becomes symptomatic around age 40. The eye focuses light much as a camera focuses light. As the eye loses its ability to focus at near items, people with presbyopia will need reading glasses or bifocals to see near objects.
Note that people with both myopia and presbyopia may still be able to see up close without their glasses, but they will need glasses with bifocals to use their glasses to see both far away and close up. Hyperopic people will need glasses with bifocals to see both distant and near.
People with presbyopia will need glasses or contact lenses. There are a few new types of surgery that address presbyopia, but, in general, presbyopia is a difficult condition to eliminate surgically.