Sarah J. Blake is a person with a visual impairment who has also studied blindness and eye conditions. She is not an eye care professional, and information on this site is not a substitute for face-to-face consultation with an eye care professional. This site is designed solely to assist patients and doctors in building cooperative relationships.
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Many parents whose blind child is getting glasses for the first time want to know how much the glasses will help. Some look at the number on the prescription and begin asking about what a child who wears that prescription will be able to see. Often, a parent will say something like, "My child sees -4.5."
-4.5 is actually not a representation of what the child sees. This number represents the child's refractive error. The images a person sees are made up of patterns of light which pass through several structures in the eye and brain before reaching the visual cortex, where they are interpreted as colors and recognizable images. The structures of the eye cause the waves of light to be bent in a certain way so that they are focused directly on the retina. This process is called refraction. In a normally functioning eye, the bending of light is measured at 0 diopters. When the eye is shaped abnormally, or when a part of the eye is damaged, the refraction process is altered. The changes that result are called refractive error and result in a change to the number of diopters--and thus a change in the prescription needed in order to bring the refraction back to 0, allowing images to reach the back of the eye in best focus.
If any of the structures in the eye are damaged, abnormally shaped or have been removed, the light will not bend correctly. Images will be focused in front of or behind the retina rather than directly on it. This is called refractive error. A person with a refractive error will be able to see some things more clearly than others. Whether he sees nearby or far away objects better depends on the type of refractive error he has.
Refractive errors can be corrected with glasses or contact lenses. In some cases, a person can have surgery to correct the refractive error by changing the shape of the cornea.
When a refractive error has been corrected and the person has no additional eye problems, the person will have fairly normal vision. However, if the person has a condition which causes damage to the structures of the eye or part of the brain which is involved in seeing, vision may still be poor. Damage to the eye structures may cause an image to be distorted or keep a person from seeing many things at one time. This is why corrective lenses cannot necessarily make a person see normally.
Correcting refractive errors is important for people with visual impairments because it enables them to maximize the use of the vision they have and to see things as accurately as possible. Additional features added to the corrective lenses, such as colored filters, may enable the person to see more clearly by enhancing contrast.
A person who is near-sighted wears a prescriptive lens with a minus number, taking away extra refractive power from the eye. The number represents only the strength of the lens and does not indicate how well the person will be able to see while wearing the lens. Minus lenses may cause images to appear smaller. Because of this effect, some people who already have difficulty seeing small objects because of other eye conditions may prefer not to wear their lenses while doing certain things.
People who are far-sighted wear lenses with plus numbers, adding power to the eye which does not have enough natural refractive power. People who have had their lenses removed because of cataracts or, in rare cases, for other reasons, and who do not have an intraocular lens implant (IOL) may also wear very strong plus lenses. These lenses may cause objects to appear magnified.