The cornea supplies about 40 diopters and the crystalline lens about 20. Together they give the eye its total power of roughly 60 diopters. The eye is a plus-power system. The convex cornea and the convex lens bend light inward and focus a real image on the retina.
The cornea does most of the work because it is the front curved window of the eye and is steeply curved. The crystalline lens sits behind the iris and adds the other one third of the power. The lens has a job the cornea cannot do. It can change its curvature, and so change its power. That change of focus is called accommodation. It lets the eye focus near objects while the retina stays a fixed distance behind. The eye needs this fine tuning because the screen it focuses on cannot move, but the objects a person looks at do. The retina is the light-sensing layer at the back of the eye. With age the lens stiffens and accommodation fades, which is why most people need reading glasses after about age 40.
Remember the split as 40 plus 20 equals 60. The cornea is the strongest single lens of the eye, and the crystalline lens is the adjustable one. Those two numbers explain a lot of daily practice. A small change in the corneal curve shifts the focus of the whole eye, which is why corneal surgery and contact lenses work so well. A change in the lens instead changes the focus for near work. An eye that focuses distant light right on the retina without help is in a state called emmetropia. These numbers belong to the COA optics and spectacles content area.
Incorrect Answers
- B. This reverses the two values. The crystalline lens is the weaker of the two, not the stronger, even though it is the one that can change shape.
- C. An even split is easy to guess but wrong. The cornea supplies about twice the power of the lens.
- D. The lens does not carry almost all the power. Removing the cornea's share would leave the eye far too weak to focus on the retina.
Testing Pearls
- Total power of the eye is about 60 diopters: cornea about 40, lens about 20.
- The cornea is fixed in power; the crystalline lens can change shape.
- The change of lens power for near work is called accommodation.
- Loss of accommodation with age is presbyopia, and it is treated with a reading add.
- Cornea and lens form a real, upside-down image on the retina.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 5, Optics and Refractive States of the Eye.
