Visual accommodation is the automatic adjustment process by which the lens of the eye adjusts to focus on objects at different distances. The lens is a pliant transparent elliptical structure that refracts, or bends, rays of light inward, thus focusing them on the retina. When the eye is at rest, the suspensory ligaments hold the lens firmly in a relatively flattened position. The normal resting eye is then in a far-point vision position and can focus on objects that are at least 20 feet (6 meters) distant, without any accommodative adjustment of the lens. Light rays passing through the cornea and aqueous humor then enter the pupil of the eye and pass through the lens, after which they pass through the vitreous humor and reach the retina in focus.
For near vision, closer than 20 feet, accommodation for focusing takes place: The ciliary muscles, located around and attached to the suspensory ligaments, contract. This causes relaxation of the suspensory ligaments, which then allow the flattened lens to thicken and bulge, becoming more convex, or rounded. The light rays are thus bent and fall, sharply focused, on the retina.
The ability to focus changes with age. In early childhood, children can focus on objects as close as 2.5 inches (6.3 cen timeters). As age increases, accommodation becomes less possible due to progressive hardening of the lens. By 30 years of age, near vision is usually not clear at less than 6 inches (15 centimeters) from the eye. During the 40s, visual articles usually have to be moved farther and farther away in order to be clearly seen. Presbyopia is the term given to decreasing ability to focus with advancing age. This leads to the need for near-vision-lensed eyeglasses for most senior citizens for activities requiring close vision. Hyperopia, or farsightedness, and myopia, or nearsightedness, may also be related to problems of accommodation.
Illumination level has been found to have an effect upon accommodation. There have been various theories of the physiological mechanism for accommodation. Some researchers consider the sympathetic nervous system to be responsible for a basic tonal background, through vascular innervation. The oculomotor nerve, through increased or decreased innervation, leads to positive and negative accommodation, or specific adjustment for focusing.
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