The lens is the transparent, flexible structure behind the pupil that focuses light onto the retina. By changing shape it allows the eye to form a sharp image of objects at different distances.

Accommodation is the process of adjusting the lens. For near objects the ciliary muscles contract, allowing the lens to become thicker and more curved; for distant objects the muscles relax and the lens flattens. The ability to accommodate declines with age as the lens hardens, producing the condition known as presbyopia. Cataracts—clouding of the lens—are a common cause of visual impairment in older adults and can be treated by surgical replacement with an artificial lens.

The lens contributes most of the eye’s variable focusing power; the cornea provides the majority of the fixed refractive power. Together they form a clear image on the retina under a wide range of viewing distances. Optical imperfections of the lens, such as spherical and chromatic aberration, are partly compensated by the neural processing that follows.

When you shift your gaze from a distant horizon to a nearby page, the lens is the structure that keeps both images sharp.