Optical discrepancies between the various components of the
eye cause a majority of the
human population to have some form of
refractive error. People who are unable to focus on near objects are
farsighted, or
hyperopic. Hyperopia can be caused by the
eyeball being too short or the
refracting system being too weak. Even with the lens in its most
rounded up state, the image is out of focus on the
retinal surface (focusing at some point behind it).
Even people with normal vision as young adults eventually experience difficulty focusing on near objects. One of the consequences of aging is that the lens loses its elasticity; as a result, the maximum curvature the lens can achieve when the ciliary muscle contracts is reduced. The near point (the closest point that can be brought into clear focus) recedes, and it becomes necessary to hold objects farther and farther away in order to focus them on the retina. At some point, usually during early middle age, the accommodative ability of the eye is so reduced that near vision tasks like reading become difficult or impossible. This condition is referred to as presbyopia, and can be corrected by convex lenses for near-vision tasks, or by bifocal lenses if myopia is also present.
Neuroscience, Sinaur Associates (QP355.2.N487 1997)