The Mechanism of Accomodation

Viewing Objects at Varying Distances

The lens is responsible for accommodation, the adjustment of the eye for observing near or far objects. It does this by changing its shape. This changes the focal length of the lens, and permits it to focus the image formed on it by the cornea. The angle of the rays coming from near objects will be different than those from distant ones, and the lens' shape change makes allowances for this. Beyond a certain object distance, all the rays are essentially parallel to each other as the reach they eye; in this condition the object is said to be at "infinity." The lens is suspended from the muscular ciliary body by a number of delicate zonule fibers attached to its equator. In the normal relaxed condition (that is, with no tension placed on the fibers by contraction of the ciliary muscle) the lens' shape is such as to refract rays from distant objects, and the eye is focused at infinity. Accommodation for close vision requires tension to be exerted to deform the lens. Contraction of the muscles of the ciliary body tugs at the fibers and changes the shape of the lens.

By having long-distance vision as the "default" condition, the amount of exertion needed for normal vision is minimized. Most of the objects in a person's environment are effectively at infinity, when they are far enough away so that the parallax effect involved in stereoscopic vision is insignificant. When an object is close enough so that the distance between the two eyes is a significant fraction of object distance, tension is placed on the zonule fibers by contracting the ciliary muscles, and the lens' shape changes. This system makes a great deal of sense for an organ whose natural role is as a "long-range warning system": most predators (and prey) spend their time scanning the horizon. Individuals involved in close work are constantly required to make accomodation through muscular effort, and "eyestrain" is the result. It is fatigue in the ciliary muscle caused by excessive use.

One reason for the deterioration of eyesight with advancing age is that older lenses become stiffer and less deformable, and more effort is needed to make accomodation for near objects. Close vision becomes more difficult, and corrective lenses are usually needed, even in individuals who had perfect eyesight when younger. Wild animals usually don't live long enough for accommodation problems to become serious; but the situation does arise in domestic species (especially dogs) whose protected environments make it possible for them to live lives much longer than would be the case if they lived in the "natural" condition, without any care.