Eyeglasses are generally worn to correct refractive defects, or faulty focusing. The conditions that can cause this are usually near-sightedness, when near objects are more closely focused and distant objects are fuzzy, or far-sightedness, when close-up vision is blurred. Another common problem is astigmatism, when the |cornea| or |lens| is uneven and causes a distortion. Eyeglasses can be single |lens|, the entire |lens| is one prescription, bifocal, when two prescriptions are found on the |lens|, or even trifocal, with three prescriptions on the |lens|. Reading glasses are commonly used by people who have developed presbyopia ("old |eyes|"), when the |lens| of the |eye| becomes less flexible and less able to change shape and focus on close objects.
