Clear vision in the normal eye is the result of light rays passing through the cornea, pupil and lens and focusing directly on the retina.
Nearsightedness occurs when the cornea is too curved or the eye is too long.
This causes light to focus in front of the retina, resulting in blurry distance vision. Farsightedness occurs when the cornea is too flat in relation to the length of the eye. This causes light to focus at a point beyond the retina, resulting in blurry close vision and sometimes blurry distance vision as well. This causes light to focus in more than one point on the retina, resulting in blurry and distorted vision.