Accommodation

Accommodation is the eye’s adjustment of optical power to keep objects at different distances in focus on the retina. It occurs chiefly through changes in the curvature of the crystalline lens.

Ciliary muscle: A ring of smooth muscle in the eye that changes tension on the lens’s suspensory fibers. Its contraction initiates the lens-shape change that increases optical power for near focus.
Refraction: The change in direction of light as it passes between materials with different refractive indices. The eye focuses images by refracting light, and accommodation changes how strongly it bends.
Amplitude of accommodation: The maximum increase in optical power an eye can produce when shifting from distance to near focus. It quantifies the range of near focusing available through accommodation.
Convergence: The inward rotation of both eyes so their visual axes meet on a nearby object. It accompanies near accommodation but changes eye alignment rather than lens power.
Zonular fibers: Fibers that suspend the crystalline lens from the ciliary body and transmit changes in tension. Their reduced tension allows the lens to become rounder during near accommodation.
Focal length: The distance from an optical system’s principal plane to the point where parallel rays converge. A change in lens power changes focal length, helping the eye focus near objects on the retina.
Accommodative insufficiency: A condition in which the eye’s accommodative response is insufficient for the demands of near work. It is a clinical failure of the focusing response rather than simply a loss of distance focus.
Myopia: A refractive error in which distant objects focus in front of the retina when accommodation is relaxed. Unlike accommodation, myopia is a baseline focusing error that persists for distant viewing.
Crystalline lens: The transparent, flexible structure behind the iris that fine-tunes the eye’s focus. Its changing curvature supplies the optical power adjustment at the heart of accommodation.
Retina: The light-sensitive tissue lining the back of the eye that converts light into neural signals. Accommodation is useful when it brings an object’s image into sharp focus on this tissue.

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