The lens focuses on objects at different distances by changing its curvature

What it is

The eye brings objects at different distances into focus by actively changing the curvature of its lens — a process called accommodation. For a near object the lens becomes more convex (steeper curvature, higher refractive power); for a far object it flattens (lower power). This differs fundamentally from a camera zoom lens, which changes focal length by physically moving lens elements: the eye's "focusing" happens through the shape change of a single elastic lens, with no moving element position.

Why it happens

The ciliary muscle, a ring of smooth muscle surrounding the lens, pulls on the lens edge through the zonular fibers. Looking far away, the ciliary muscle relaxes and the zonular fibers tighten, stretching the lens flat. Looking near, the ciliary muscle contracts, slackening the zonular fibers; the lens then springs back toward its own more convex, elastic resting shape, its radius of curvature shrinks, refractive power rises, and the eye refocuses on the nearer object. The switch between near and far is driven entirely by changes in muscle tension, not by repositioning an optical element — this is the key structural difference from a camera lens.

Studying it

Accommodation is commonly measured with dynamic retinoscopy or an autorefractor while the subject fixates targets at different distances, yielding a curve of accommodative response against accommodative stimulus (the dioptric demand implied by target distance). This curve typically falls slightly short of a perfect one-to-one match, a gap known as accommodative lag. Another common method moves a target progressively closer until the subject reports blur, giving the amplitude of accommodation (in diopters, D) and the near point distance. Typical independent variables are target distance / dioptric demand; typical dependent variables are measured refraction, response latency, and amplitude.

Where it stops holding

Accommodation has a physiological range, bounded jointly by the near point (the closest distance that can still be brought into focus) and the far point — it cannot keep focusing on ever-closer objects indefinitely. This entry describes the accommodation mechanism itself in an emmetropic (normally refracting) eye: myopia and hyperopia shift where the unaccommodated eye's resting focal point sits, but they do not change the muscle-tension-and-elastic-recoil mechanism itself. This entry also covers only monocular focusing — aiming both eyes at the same point relies on a separate rotational movement driven by the extraocular muscles, which occurs simultaneously with accommodation during natural viewing but through a different structure and mechanism.

Related

• Same group: A1.21.2 Accommodative amplitude declines with age, the physiological basis of presbyopia · A1.21.3 Focusing, pupil constriction, and binocular convergence occur together during natural near viewing · A1.21.4 Displays that pair a fixed focal depth with variable vergence break this linkage
• Nearby: A1.15 Visual Fatigue and Accommodative Load · A1.20 Pupil Adjustment and Light Intake
• Search terms: accommodation · ciliary muscle · dynamic retinoscopy · accommodative lag