[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Neurophysiology](https://durgical.com/en/physiology/#s-neuro) › Optics of the eye, accommodation and refractive errors [Physiology](https://durgical.com/en/physiology/) · [Neurophysiology](https://durgical.com/en/physiology/#s-neuro) # Optics of the eye, accommodation and refractive errors - Concept - Exam priority: medium The cornea and lens bend light onto the retina; if the eye is too long or too short, the image blurs. Updated 2026-11-01 Also called: Myopia, Hyperopia, Presbyopia **See it in 3D:** [Cornea](https://durgical.com/en/anatomy/cornea/) · [Lens](https://durgical.com/en/anatomy/lens/) · [Ciliary body](https://durgical.com/en/anatomy/ciliary-body/) · [Zonular fibres](https://durgical.com/en/anatomy/ciliary-zonule/) · [Retina](https://durgical.com/en/anatomy/retina/) · [Oculomotor nerve (III)](https://durgical.com/en/anatomy/oculomotor-nerve/) ## Key points - The cornea gives about two thirds of the eye's refractive power; the lens gives the adjustable third. - Near vision: parasympathetic fibres (CN III) contract the ciliary muscle, the zonules slacken and the lens becomes rounder and stronger. - Near triad: accommodation, convergence and pupil constriction. - Myopia: image in front of the retina (eye too long), corrected with diverging (minus) lenses. Hyperopia: image behind it, corrected with converging (plus) lenses. - Presbyopia: the lens loses elasticity with age (noticeable after about 40–45 years), so near work blurs. ## What it is The eye focuses light with two lenses: the cornea, which gives most of the power and is fixed, and the crystalline lens, whose power changes for near vision (accommodation). A sharp image needs the focal point to fall exactly on the retina. Refractive errors happen when eye length and optical power do not match. ## Key facts - Light bends most where refractive index changes most: at the air–cornea surface. That is why vision blurs under water. - Astigmatism: the cornea curves more in one meridian than another; cylindrical lenses correct it. - A small pupil increases depth of focus (pinhole effect), which is why squinting sharpens vision. - Cataract (opacity of the lens) scatters light: glare, faded colours and slowly blurred vision. ## Equations **Refractive power**P = 1 / f **P** power in diopters (D) **f** focal length in metres A +2 D lens focuses parallel light at 0.5 m. ## Approximate optical values (adult) | Parameter | Value | Note | | --- | --- | --- | | Total power of the eye | ≈60 D | relaxed for distance | | Cornea | ≈40–45 D | fixed | | Crystalline lens | ≈15–20 D | adjustable | | Accommodation range | ≈12–14 D in children, under 2 D by about age 60 | declines with age | Approximate values for a healthy adult at rest; each laboratory sets its own ranges. ## Clinical notes - Uncorrected refractive error is a leading cause of visual impairment worldwide, and of poor school performance in children. - Anticholinergic eye drops paralyse accommodation (cycloplegia) and dilate the pupil, used to measure the true refraction in children. - Cataract is the main cause of reversible blindness; surgery replaces the lens with an artificial one. ## Memory aid Myopes see near and need Minus lenses; hyperopes need Plus. Close work means 'contract and round': the ciliary muscle contracts, the lens rounds. ## Test yourself Think of the answer, then tap the question to see it. ## Why does the lens become rounder when the ciliary muscle contracts? The ciliary muscle acts like a sphincter: contracting brings it closer to the lens, the zonules slacken, and the elastic capsule lets the lens round up, increasing its power. ## A myopic eye focuses distant objects where, and what lens corrects it? In front of the retina, usually because the eye is too long. A diverging (minus) lens moves the focal point back onto the retina. ## Why does a 50-year-old need reading glasses even with no previous refractive error? Presbyopia: the lens stiffens with age and cannot round up enough to add the power needed for near objects. ## Which structure has the largest refractive power in the eye, and why? The cornea, because the biggest change in refractive index is between air and cornea; the lens is surrounded by fluids of similar index. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/optics-of-the-eye-accommodation-and-refractive-errors/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Cornea](https://durgical.com/en/anatomy/cornea/) - [Lens](https://durgical.com/en/anatomy/lens/) - [Ciliary body](https://durgical.com/en/anatomy/ciliary-body/) - [Zonular fibres](https://durgical.com/en/anatomy/ciliary-zonule/) - [Retina](https://durgical.com/en/anatomy/retina/) - [Oculomotor nerve (III)](https://durgical.com/en/anatomy/oculomotor-nerve/) ## Related topics - [The visual pathway, pupillary reflexes and eye movements](https://durgical.com/en/physiology/the-visual-pathway-pupillary-reflexes-and-eye-movements/) - [The retina and phototransduction](https://durgical.com/en/physiology/the-retina-and-phototransduction/) - [Aqueous humor and intraocular pressure](https://durgical.com/en/physiology/aqueous-humor-and-intraocular-pressure/) - [Sympathetic and parasympathetic effects on each organ](https://durgical.com/en/physiology/sympathetic-and-parasympathetic-effects-on-each-organ/) ## Related conditions - [Cataract](https://durgical.com/en/conditions/cataract/) - [Strabismus and amblyopia](https://durgical.com/en/conditions/strabismus-and-amblyopia/) [← All of physiology](https://durgical.com/en/physiology/) Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given. Sources: [14.1 Sensory Perception](https://openstax.org/books/anatomy-and-physiology-2e/pages/14-1-sensory-perception/) (OpenStax (Rice University), 2022); [Anatomy, Head and Neck, Eye Ciliary Muscles](https://www.ncbi.nlm.nih.gov/books/NBK482132/) (StatPearls (NCBI Bookshelf), 2024); [Cataract](https://www.ncbi.nlm.nih.gov/books/NBK539699/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 50). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/optics-of-the-eye-accommodation-and-refractive-errors/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/optics-of-the-eye-accommodation-and-refractive-errors/