The four most common vision problems are nearsightedness, farsightedness, astigmatism and presbyopia. They are usually caused by focusing problems of the cornea or the lens, or by an abnormal shape of the eye.
1. Nearsightedness (or myopia)
Also known as shortsightedness, it is the most common eye problem in the world. This could be caused by the cornea protruding too far away from the eye or from an excessive elongation of the eye structure itself.
People with this condition will notice they can see nearby objects more clearly than the distant objects.
Nearsightedness can be corrected with concave lenses (which are thinner in the middle than at the edge) on eyeglasses or contact lenses. There are procedures like refractive surgery to reduce the curvature of the cornea.
2. Farsightedness (or hyperopia)
Also known as long-sightedness, it is a vision problem which can arise from having a short eyeball or a lens that is inflexible to the point of maintaining a stretched shape under natural condition of focusing.
People with this condition will notice that as objects comes nearer, the power of the cornea and lens are not enough to keep the image on the retina and the image appears blurred.
Farsightedness can be corrected with convex lenses (which are thicker in the middle than at the edge) on eyeglasses or contact lenses. There is a surgical option too, but it is not as frequently performed as that for correcting myopia because of its complexity.
Astigmatism is a condition in which the bending of light occurs to different degrees in different planes. This is because the cornea of an astigmatic is often not spherical, and so the tilt of the cornea bends light to a larger degree in one plane compared with that in a perpendicular plane. As a result, the image produced may be directed on the retina in one plane, but in another plane, the image may be in behind or in front of it.
People with this condition will thus notice they have difficulty in making out the fine details of a viewed object.
Serious astigmatism can be corrected with cylindrical lenses that counteract the uneven curvature of your cornea, and this can be made with the same pair of lenses to correct nearsightedness or farsightedness. The surgical option is also available to reshape the uneven curvature of the cornea.
Presbyopia occurs as the eyes ages and becomes less able to focus light especially for nearby objects through the natural focusing mechanism. This is most widely believed to be due to the loss of elasticity of the lens, or the curvature of the lens may change as well when the muscles holding the lens weaken.
People with this condition, and usually between the ages of 40 to 50, will find difficulty in reading fine prints under poor lighting or feel the eyestrain after prolonged period of reading.
Presbyopia can be corrected with bifocal or progressive addition lenses (PALs). They provide two points of focus: the main part comprises a prescription for distant vision, while the lower portion contains the stronger prescription for close work. You can wear reading glasses, usually worn just during the close work.
Another correction, known as monovision, is to wear a different strength of contact lenses on each eye, one for distant vision (usually the dominant eye) and the other for close vision. It will usually continue to worsen, and so require periodic changes to the prescription for glasses or lenses. There are procedures to treat presbyopia, and it is performed on one eye only for a monovision correction.
Fortunately, most problems arising from focusing can be corrected with eyeglasses, contact lenses or surgery that adjusts the curvature of the cornea. When adjusted, the cornea and lens adjust the point of focus so that an image falls sharply defined onto your retina.