The eye does not actually "see" objects. Instead, it sees the light that objects reflect. To see clearly, light striking the eye must be bent or "refracted" through the cornea--the clear window at the front of the eye that provides most of the focusing power.

Light then travels through the lens, where it is fine-tuned to focus properly on the nerve layer that lines the back of the eye--the retina--and sent to the brain via the optic nerve.  The retina acts like the film in a camera, and clear vision is achieved only if light from an object is precisely focused on it. If not, the image you see is blurred. This is called a refractive error.

Refractive errors usually occur in otherwise healthy eyes. They are caused mostly by an imperfectly shaped eyeball, cornea or lens. There are four basic types of errors:

Myopia or nearsightedness--Close objects appear sharp but those in the distance are blurred. The eyeball is longer than normal from front to back, so images focus in front of the retina instead of on it.

Hyperopia or farsightedness--Distant objects can be seen clearly but objects up close are blurred. The eyeball is shorter than normal, so images focus behind the retina.

Astigmatism--Objects are blurred at any distance. The cornea, lens, or both are shaped so that images aren't focused sharply on the retina.

Presbyopia or aging eye--The eye loses its ability to change focus due to the natural aging process. This usually occurs between ages 40 and 50.

How We See It
This graphic shows a side view of four eyes illustrating the following four visual conditions:

Normal Vision
The cornea and the lens are responsible for focusing light on the retina. With normal vision, light striking the cornea is focused directly on the central region of the retina.

Myopia
Myopia, or nearsightedness, occurs when either the cornea is too steep and/or the eye is too long, and light is focused in front of the retina. Close objects appear clear and far objects are blurry.

Hyperopia
Hyperopia, or farsightedness, occurs when either the cornea is too flat and/or the eye is too short, and light is focused behind the retina. Distant objects appear clear and close objects appear blurry.

Astigmatism
Astigmatism occurs when the surface of the cornea is oval-shaped, rather than round. Light rays have more than one focal point and can focus on different areas of the retina. Astigmatism creates double, distorted or blurry vision.