The human eye gives us the sense of sight, allowing us to learn more about the surrounding world than any of the other five senses. The eye allows us to see and interpret the shapes, colors and dimensions of objects by processing the light they reflect or give off.
The cornea (the clear window on the front of the eye) and the lens of the eye (the transparent structure inside the eye) are both critical to normal vision. The goal of these two lenses is to focus light onto a layer on the back of the eye known as the retina. As light enters into the eye it is focused by the cornea and the lens so that images appear clearly on the retina. The retina then transmits these images to the brain where they are processed.
If the images focus perfectly on the retina, this results in 20/20 vision; focusing in front or behind the retina results in nearsightedness or farsightedness. If the cornea is shaped like a football instead of a sphere, this is called astigmatism. When any of these conditions occurs, images are perceived by the brain as being blurry. This is due to "refractive error" which means the eyeball is not the right size or the cornea does not have the right curve. Presbyopia is a vision condition in which the lens loses its flexibility, making it difficult to focus on close objects.