| Have you ever wondered how your eyes actually | | | | it does get to the retina it has diverged again and gives |
| work? There are many delicate parts to the eye that | | | | you blurry vision. However, it is only relatively distant |
| work together almost like a camera, including the: | | | | objects which look blurry while the book print is clear. |
| • Pupil, the "black dot" in the center of the eye | | | | That is because light from close objects needs to be |
| • Cornea, the clear covering of the eye | | | | refracted more than light from distant objects. So your |
| • Crystalline lens, behind the iris | | | | eyes are doing well for close objects and need to be |
| • Ciliary muscles, controlling the lens curvature, | | | | corrected for far objects. |
| known as accommodation (ability to focus at all | | | | 2. Farsightedness (hyperopia) A hyperopic eye |
| distances) | | | | refracts light too little. The light is focusing behind the |
| • Retina, a light-sensitive membrane covering the | | | | retina (except that light cannot travel through this |
| back of the eye | | | | dense tissue, so in effect it is not being focused at all |
| • Optic nerve, running from the retina to the brain | | | | within the eyeball. This is fine for light coming from far |
| How the Eye Works Light passes through the cornea | | | | objects, since it needs less refracting. This light does |
| and the pupil, then through the lens. The image is | | | | focus on the retina. Light from near objects needs to |
| focused on the retina, where sensory cells known as | | | | be more sharply refracted. But the hyperopic eye |
| rods and cones translate the image to electrical | | | | cannot do that so these images look blurry. |
| energy and pass it to the tiny optic nerve fibers in the | | | | 3. Astigmatism An astigmatic eye has an oval-shaped |
| retina. They converge in a large nerve which exits the | | | | cornea. It has a steeper curvature on its shorter side |
| back of the eye and carries the image data to the | | | | and a flatter curvature on its longer side. It refracts light |
| brain to be interpreted. | | | | in two ways, causing some light to focus on the retina |
| The cornea is clear and curved and acts like a lens. It | | | | and some to focus in front of or behind the retina. This |
| bends (refracts) light as it enters the eye. The lens is | | | | gives blurry vision at all distances. You can be both |
| also clear and curved and refracts the light further. In a | | | | astigmatic and myopic or astigmatic and hyperopic. |
| 20/20 eye, their combined light refraction focuses the | | | | 4. Presbyopia Presbyopia is age-related. Everyone |
| light rays on the retina. | | | | starts to become presbyopic after the age of about |
| When light travels from a distant object it needs less | | | | 40 or 50. You may have 20/20 eyes and still be |
| refraction than light coming from a close-by object. | | | | presbyopic, which means that book print is blurry while |
| The ability of the lens to change its curvature | | | | intermediate and distant objects are clear. |
| "accommodates" that by bending incoming light at a | | | | Causes of presbyopia are not fully understood, but it is |
| greater or lesser angle. The corneal curvature is fixed. | | | | related to the lens being less able to change its |
| Four Types of Refractive Error Your eyes have a | | | | curvature. It becomes fixed within a narrower range of |
| refractive error if: | | | | curvatures which maintain clear distance vision but |
| • Book print looks blurry (you are farsighted or | | | | progressively give blurry vision at closer distances. |
| presbyopic) | | | | Subjectively it is like being farsighted but the cause is |
| • You cannot recognize a person standing a block | | | | different. |
| away (you are myopic) | | | | Diagnosing Refractive Errors |
| • Things look a bit blurry at all distances (you are | | | | Any eye doctor can test your vision to determine your |
| astigmatic) | | | | eyes’ refractive errors. For glasses or contact |
| In those scenarios your eyes are not refracting | | | | lenses, an instrument called a phoropter is used. It is a |
| (bending) light well enough to give you clear vision at all | | | | device that positions a variety of different corrective |
| distances. | | | | lenses in front of your eyes. You read an eye chart |
| 1. Nearsightedness (myopia) A myopic eye refracts | | | | while looking through the lenses, and the eye doctor will |
| light too much. The cornea and the lens between them | | | | switch between different lenses until you can see the |
| are focusing light before it gets to the retina. So when | | | | eye chart clearly. |