Accommodation

Accommodation is the automatic process by which the ciliary muscles of the eye adjust the curvature, and hence the focal length, of the flexible crystalline eye lens, so that objects at very different distances from the eye can all be brought to a sharp focus on the retina in turn. To view a distant object, the ciliary muscles relax, allowing the eye lens to flatten out into its thinnest, least strongly curved, longest-focal-length shape; to view a nearby object, the ciliary muscles instead contract, allowing the elastic eye lens to bulge into a more strongly curved, shorter-focal-length shape, increasing its converging power just enough to bring the much closer object's rays to a focus on the same fixed retina. This adjustment is not unlimited: the farthest point at which a normal, relaxed eye can focus an object sharply, called the far point, is at infinity (very large distances, for practical purposes); the nearest point at which the eye, with its ciliary muscles at their maximum possible contraction, can still focus an object sharply is called the near point (or the least distance of distinct vision), which for a normal young adult eye is conventionally taken as $25\ \text{cm}$, denoted $D$. An object brought closer to the eye than its near point cannot be brought into sharp focus at all, however hard the ciliary muscles try to contract further, because the eye lens has reached the limit of how …