FOCUSING

also called Ocular Accommodation, adjustment of the eye to see things at different distances. (The term also is applied to the adjustment of man-made lenses, as in a camera or microscope.) In fish, the eyes are focused by altering the distance between lens and retina, much as occurs in a camera, where the distance between lens and light-sensitive film is adjusted. In humans and other mammals the forward surface of the lens is made more convex for seeing closer objects, and the pupil becomes smaller; at the same time, the two eyes turn inward enough (i.e., cross) so that their gaze is fixed on the object. The capsule, or envelope enclosing the lens of the eye, is attached by ligaments to the ringlike ciliary muscle. The inside diameter of this muscle is greatest when the muscle is relaxed, smallest when it is contracted. Thus, when the gaze is fixed on a distant object, as when a camera is set at infinity, the ciliary muscle relaxes, the muscle's inside diameter is increased, more pull is exerted on the lens by the ligaments, and the front surface of the lens is flattened. When near objects are viewed the ciliary muscle contracts, the ligaments relax, and the lens, being elastic, bulges in front and gains more curvature.