# Accommodation and Presbyopia > Accommodation physiology in MBBS Physiology: Helmholtz theory, the near triad, amplitude decline with age and presbyopia correction. - Canonical URL: https://prepelephant.com/topics/mbbs/physiology/accommodation-physiology - Exam / course: MBBS · Subject: Physiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Accommodation and Presbyopia", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/accommodation-physiology ## Direct answer Accommodation is the increase in the eye's refractive power — by roughly 10-14 dioptres in a young adult — that brings diverging rays from a near object to focus on the retina. By the Helmholtz theory, parasympathetic fibres of cranial nerve III contract the ciliary muscle, the circular muscle moves forward and inward, zonular tension on the lens falls, and the elastic capsule rounds the lens into a more convex, stronger shape. It is always accompanied by convergence and miosis (the near triad), and its amplitude falls steadily with age as the lens stiffens, producing presbyopia by the mid-forties. ## What you must remember - **Near triad:** accommodation + convergence of the eyes + miosis; all are yoked so that a clear single near image forms. - **Mechanism (Helmholtz):** ciliary muscle contraction relaxes zonules; the lens becomes more convex, mainly the anterior surface, which steepens from about 10 D curvature towards 15 D. - **Innervation:** parasympathetic via Edinger-Westphal nucleus, third nerve, ciliary ganglion, short ciliary nerves; blocked by cycloplegics, which is why atropine blurs near vision. - **Amplitude values:** about 14 D at age 10, roughly 10 D at 20, near 6-7 D at 40, and about 1 D by 60 — a decline of roughly 2.5 D per decade through adult life. - **Near point recession:** from about 7-10 cm in childhood to 25-30 cm or beyond in the forties; when it exceeds comfortable reading distance, presbyopia is symptomatic. - **Presbyopia correction:** convex reading addition; the near addition typically starts around +1.00 to +1.50 D at 45 years and rarely exceeds +3.00 D. - **Convergence measurement:** in metre angles — one metre angle when the eyes converge to fix an object 1 m away; the AC/A ratio links accommodation to convergence, relevant to esotropia and exotropia. ## Numbers worth knowing Track one person through life. At 10 years the amplitude is about 14 D: the near point sits at roughly 7 cm, and tiny print at the nose is crisp. At 25 years the amplitude has fallen to about 10 D and the near point to 10 cm; nothing is noticed. At 45 years only about 6 D remains, pushing the near point to 25 cm — newspapers get held at arm's length, and evening asthenopia appears because near work now consumes nearly the whole reserve. By 60 the amplitude is near 1 D, the lens is effectively rigid, and no amount of effort helps. The clinical arithmetic follows directly. A presbyopic emmetrope who wants to read at 25 cm needs a lens that makes light from 25 cm appear to come from infinity: 1/0.25 = +4.00 D of total near power is the physiologic maximum addition for close work, though most patients are prescribed less, around +2.50 to +3.00 D at a 40 cm working distance (1/0.40 = +2.50 D), because comfort beats maximal magnification. A 45-year-old myope of −2 D already has a far point at 50 cm, so he reads without glasses by simply removing them — the classic myopic presbyope shortcut. ## Where students slip The recurring error is the direction of the mechanics: many state that the ciliary muscle pulls the lens flat to focus near. It is the opposite — contraction relieves zonular tension so the lens can round out; the zonules suspend the lens in a state of stretch at rest, and accommodation is a release, not a pull. The second trap is calling presbyopia a refractive error. It is an accommodative failure of a physiologic kind, present in every eye, and it is corrected with an addition, not with a cycloplegic or surgery. In viva, the capsular elasticity (lost with age) versus ciliary muscle power (preserved) distinction — established by the classic lens-stamping experiments that showed hardened lenses accommodate poorly even when the muscle works — earns marks. ## Frequently asked questions ### Which nerves and muscles execute the accommodation reflex? Parasympathetic outflow from the Edinger-Westphal nucleus travels in the third nerve to the ciliary ganglion and short ciliary nerves to the ciliary muscle; convergence is mediated by the medial recti driven by the third nerve nuclei. ### Why does atropine blur near vision? Atropine blocks the muscarinic receptors on the ciliary muscle, producing cycloplegia — the lens is held at its resting, flattest power, so near objects cannot be focused. ### What is the amplitude of accommodation at age 40? Roughly 6 dioptres, giving a near point near 25 cm; because comfortable near work needs about half the amplitude in reserve, symptoms of presbyopia begin around this age even though some amplitude remains. ### How does a myope manage presbyopia differently from an emmetrope? A myope can simply remove the distance glasses and read at the far point set by the myopia — for −2 D that is 50 cm — so low myopes often never need reading glasses for intermediate print. ### What is the AC/A ratio and why does it matter? It is the amount of convergence produced per dioptre of accommodation; a high ratio causes accommodative esotropia in children, treatable with hypermetropic correction or bifocals rather than immediate surgery.