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The primary function of your eye is to focus light. You need glasses or contact lenses when your eye cannot precisely focus rays of light onto the retina. The cornea, at the front of your eye, provides most of the eye's focusing power. The lens inside your eye provides the fine tuning of light, contributing to your ability to read. If you wear eyeglasses or contact lenses, it is probably because one of the following common vision problems:
This nearsightedness occurs when your eye is too long, compared to the curvature of your cornea. Objects that are close to you are seen more clearly than distant objects.
This farsightedness occurs when your eye is too short, compared to the curvature of your cornea. Distant objects are seen more clearly than objects that are close to you.
Astigmatism occurs when your cornea is shaped like an oval, instead of round. This causes more than one focal point within the eye, which distorts what you see.
Between the ages of 40 and 60, the lens of your eye becomes less flexible and presbyopia (difficulty in reading small print) occurs.
Myopia, hyperopia, astigmatism and presbyopia are measured in units called diopters. The number of diopters represents the amount of correction needed to make vision normal. The more correction that is needed, the higher your prescription will be in diopters.