In myopia, or nearsightedness, curvature of the cornea and/or the long length of the eye cause light rays to focus in front of the retina. The optical power of a myopic eye is too strong, resulting in near images appearing clear and distant images appearing blurred.
In hyperopia, or farsightedness, flat curvature of the cornea and/or the short length of the eye cause the point of focus of the eye to fall beyond the retina. The optical power of the hyperopic eye is inadequate or weak, making objects up-close appear blurred. Sometimes even objects in the distance can be blurred.
In astigmatism, either the cornea or the lens have an irregular curvature or shape. This can cause light to focus on more than one point of the eye and distort vision.
In an astigmatic cornea, instead of having a spherical curve like the shape of a basketball, the curvature is more oval, like the shape of a football. Most corneas affected by astigmatism have two curves, a steeper curve and a flatter curve, which generate the multiple points of focus. Astigmatism often occurs in combination with myopia or hyperopia. The result is blurred, distorted vision at near or far distance.
Presbyopia is a natural occurrence that generally affects individuals beginning in their 40s. As we age, the natural lens of the eye loses flexibility and the ability to change its focusing distance, known as accommodation. When presbyopia develops, close-range activities such as reading a book or viewing a computer screen may become increasingly difficult.
It is estimated that 90 million Americans currently have or will develop presbyopia in the next five years. In the past, reading glasses or bifocals were the only means available to correct vision in people with presbyopia. Today, however, patients have many more options, ranging from glasses and contacts and surgical correction with an intraocular lens.