Visual Acuity Test – Measures a person’s ability to see fine detail with central vision. It tells the examiner how well a person sees in comparison to how well someone with “normal” vision sees. Vision recorded as 20/20 means that the person tested sees the same small object at 20 feed as a “normal” person sees at 20 feet. Vision recorded as 20/200 means the person tested has to be 20 feet to see the same object as a normal person sees at 200 feet.
Myopia – If the eyeball is too long for the focusing system, the focused light rays – and the clearest image – will fall in front of the retina. People with a longer eyeball might not be able to read a street sign from half a block away, but they would have no trouble reading a book held close to their eyes. This type of refractive error is called myopia (my-OH-pee-uh), or nearsigtedness.
Hyperopia – If the eyeball is too short for the focusing system, light rays focused by the cornea and lens form a clear image that will fall behindthe retina. This condition is known as hyperopia (hy-per-OH-pee-uh) Because people with hyperopia see better at a distance than they do up close, the term farsightedness is often used to describe the condition.
Astigmatism – A third kind of refractive error occurs when the cornea is not perfectly round and smooth. This kind of cornea scatters light rays to different points and prevents the rays from focusing on the retina. The word astigmatism (uh-STIG-muh-tizm) is used to describe this condition. It comes from Greek words meaning “no spot of focus.” With astigmatism, vision is blurred and objects viewed seem distorted, broader, or longer than they really are. Astigmatism can occur alone or in combination with farsightedness or near sightedness.
Presbyopia – As people age, many parts of the body change and lose their flexibility. The eyes are no exception. In younger people, the eye’s lens can easily change its shape to help us focus on objects at different distances. Over time, the lens slowly begins to lose this ability. Starting at about age 40, many people who never needed glasses before find that they now need them to read or do other close work. The name for this refractive error is presbyopia (prez-bee-OH-pee-uh). It comes from Greek works meaning “old sight.”