The term for this is accommodation. Accommodation is the eye's power to change focus from far to near. The crystalline lens is the clear lens that sits right behind the iris. It can change its curve. When your patient looks down at the near card, the lens becomes rounder. A rounder lens bends light more strongly. That extra bending keeps the small print sharp on the retina, the light-sensing layer at the back of the eye.
The ciliary muscle drives this change. It lies in the ciliary body, a ring of tissue behind the iris. Thin fibers called zonules join that ring to the lens. When the ciliary muscle contracts, the zonules relax. The lens is then free to get rounder, and its plus power goes up. When the muscle relaxes, the zonules pull tight and the lens flattens again for distance. The whole eye bends light with about 60 diopters of power. About 40 diopters come from the cornea and about 20 from the lens.
The lens slowly hardens with age. By about age 45, most people can no longer focus on very near print. That normal change is called presbyopia, and it is why so many people start to wear reading glasses. Using plain words like these helps you explain to a patient what her eyes are doing. Teaching patients about focus and aging is a core part of the COA patient services and education content area.
Incorrect Answers
- A. Fixation means holding your gaze steady on a target. It does not change the eye's focusing power.
- B. Convergence is the inward turn of both eyes toward a near object. It moves the eyes, not the lens.
- D. Jaeger is a way to record near visual acuity, such as J2. It is a notation, not a focusing action.
Testing Pearls
- Accommodation adds plus power so near objects stay in focus.
- Ciliary muscle contracts, zonules relax, and the lens becomes rounder.
- The eye's total focusing power is about 60 diopters, roughly 40 from the cornea and 20 from the lens.
- Presbyopia is the age-related loss of accommodation, usually noticed near age 45.
- Jaeger notation records near vision; J2 matches the 20/30 distance-equivalent line.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 5, Optics and Refractive States of the Eye.
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 2, Anatomy and Physiology of the Eye.
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 8, Comprehensive Medical Eye Examination.
