Refractive Error
20/20 vision is a term used to express normal vision measured at a distance of 20 feet. If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at that distance. If you have 20/100 vision, you must be as close as 20 feet to see what a person with normal vision can see at 100 feet. 20/20 does not necessarily mean perfect vision; it only indicates the sharpness or clarity of vision at a distance. Other important vision skills, including peripheral awareness (side vision), depth perception, and focusing ability contribute to overall visual ability.
- Nearsightedness, or myopia, is a vision condition in which near objects are seen clearly, but distant objects do not come into proper focus. Nearsightedness occurs if your eyeball is too long or the cornea has too much curvature. The light entering your eye is not focused correctly. Nearsightedness affects nearly 30 percent of the US population, and some evidence supports the theory that nearsightedness is hereditary. There is also growing evidence that nearsightedness may be caused by the stress of too much close vision work. Since the eye continues to grow during childhood, nearsightedness generally develops before age 20.
- Farsightedness, or hyperopia, is a vision condition in which distant object are usually seen clearly, but close ones do not come into proper focus. Farsightedness occurs if your eyeball is too short or the cornea has too little curvature. The light entering your eye is not focused correctly. Common symptoms include difficulty concentrating and maintaining a clear focus on near objects, eye strain, fatigue and headaches after close work, and aching or burning eyes.
- Astigmatism is a vision condition that occurs when the front surface of your eye, the cornea, is slightly irregular in shape. This irregular shape prevents light from focusing on the retina. As a result, vision may be blurred at all distances.
- Presbyopia is a vision condition in which the crystalline lens of your eye loses its flexibility, which makes it difficult to focus on close objects. Presbyopia usually becomes noticeable in the early to mid forties. It is a natural part of aging, not a disease and not preventable. Signs of presbyopia include the tendency to hold reading materials at arm’s length, blurred vision at normal reading distance, and eye fatigue along with headaches when doing close work.
