YOUR EYE

	Emmetropia in the normal eye, or the Emmetropic eye, light rays focused directly on the retina result in clear vision.
	Myopia or nearsightedness, occurs when light rays have their focal point in front of the retina instead of directly on the retina. This is normally caused by the eye being either too long or the cornea too steep, resulting in blurry distance vision.
	Hyperopia or farsightedness, occurs when the eye is either too long or the cornea is too flat, resulting in light rays that would have to focus behind the retina. This inability to focus within the relaxed state of the eye, makes vision blurry at close, and sometimes blurry at distance as well.
	Astigmatism is an optical defect of the eye just as nearsightedness (myopia) and farsightedness(hyperopia), are. Astigmatism results from the cornea having a sharper curve in one direction than the remainder of the cornea. An example of this situation is the bottom of a teaspoon where the curvature from side to side is a good deal sharper than from front to back. (Of course, this example is exaggerated and the astigmatic eye appears normal.) This results in the light rays focusing on more than one point on the retina resulting in blurry or distorted vision.

Presbyopia

is not truly a refractive error, but is the natural process of the lens losing its ability to accommodate, or change its shape inside the eye. Why does the eye lose its ability to focus at near? For certain, it is not a disease or degeneration of the eye. One popular theory holds that the tiny muscles inside the eye that control the lens grow tired and lose their tone with age. Exercises were tried in an attempt to strengthen these muscles. The best information we have today regarding the gradual loss of accommodation places blame on the lens itself, which lies within its own capsular casing. Unlike the rest of the body, which stops growing by the age of twenty, the lens of the eye continues to grow throughout life. As the lens ages and grows, it becomes harder in consistency and, because it is confined within that capsular casing, it is therefore resistant to changes in shape. The result is a gradual reduction in accommodation, and more dependence on reading glasses.