The normal human eye can form a sharp image of objects from a near point of about 25 cm to a far point at effectively an infinite distance from the eye. Many eyes, however, cannot see objects clearly at this full range of distances.
Eyes that can focus on distance objects but have difficulty focusing as the objects get closer suffer from hyperopia (farsightedness). Eyes that can focus on very close objects but have trouble focusing on the objects as they get further away suffer from myopia (or nearsightedness). To correct such vision defects, one can either use corrective lenses such as eyeglasses or contacts, or have surgery such as laser refractive surgery which effectively reshapes the curvature of the cornea.
Before we consider the correction of hyperopia or myopia with corrective lenses, letís first consider the factors that determine the focal length of a lens.