Clear vision is the result of light entering the cornea (clear window of the eye), passing through the lens inside the eye and coming to a single point of focus on the retina. The most common types of visual distortions are nearsightedness, farsightedness and astigmatism.
How the Normal Eye Works
The human eye functions much like a camera, with a lens in front and a light-sensitive screen, the retina, in the rear.
Images enter the eye through the cornea, a transparent domed window at the front of the eye. In normal eye, the cornea bends (refracts) incoming images, causing the images to focus on the retina.
occurs when the cornea comes too steep or the eye is too long. This causes light rays entering the point of focus before they reach the retina. People who are nearsighted have difficulty seeing distant objects.
occurs when the cornea too flat or the eye is too short. This prevents light rays entering the eye from bending enough to come to a point of focus on the retina. People who are young and farsighted can often see at distance but have difficulty seeing close objects.
occurs when the cornea is shaped like a American football , steep in one direction and flat in the other direction. This causes light rays entering the eye to focus in multiple points within the eye. People who are nearsighted or farsighted also have astigmatism.