Focusing the Eye


In the figure below, first mouseover the ciliary muscle to see where it is. Next, select whether you wish to focus on something "far" or something "close". Observe where the object is in focus. Then click the "contract" button in order to contract the ciliary muscle.

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As an object moves closer to the eye, does the focal point move towards or away from the front of the eye?

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Does contracting the ciliary muscle move the focal point towards or away from the front of the eye?

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Myopia

In myopia, the eyeball grows too long.

Can someone with myopia, who is not wearing glasses, see a very close object better than someone with normal vision?

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As a myopic without glasses walks towards a distant object, is a point reached at which the object is in focus without any contraction of the ciliary muscle?

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Hyperopia

In hyperopia, the eyeball is too short.

Can someone with hyperopia, who is not wearing glasses, bring an object into focus at a distance?

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Is "farsighted" an accurate term for hyperopia? In other words, can someone with hyperopia see distant objects better than someone with normal vision?

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By their early 50s, everyone develops presbyopia. This is caused by the lens stiffening. As a result, it does not change shape during accommodation for close-up vision.

Does the normal lens become more curved or less curved during accommodation?

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Suppose one of your parents puts on reading glasses. Does this move the focal point of the image forward in the eye or backward in the eye?

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Astigmatism refers to an asymmetry in the cornea such that the curvature is different on different radial axes. The solution is to wear glasses or contact lenses that have a different correction on the different corresponding axes.

The lens is more accurately called the crystalline lens (because there is an even more powerful "lens" in the eye, the cornea). The crystalline lens is made up of cells, which are surely the most unusual cells in the body. Of course, there are no capillaries in the lens (or cornea), since they would distort the image. Likewise, the cells lose their nuclei for the same reason. They are filled with an extremely unusal type of proteins called crystallins (crystalline protein). The cells are extremely elongated six-sided prisms that stretch from the anterior to posterior of the lens. But they are curved, with the curvature increasing from the center to the periphery of the lens. Apparently new lens cells are slowly created even during adulthood.

Normally, the lens is clear. But if a region becomes cloudy, it is called a cataract. As a cataract develops the vision becomes cloudy with a loss of visual accuity. Distortion of light by the cataract tends to produce glare, so that a halo around lights is often seen. The most common cause is aging, with about half of people developing cataracts in their 50s or 60s. By their late 70s and early 80s over 90% have developed cataracts.

While advancing age is the principle cause of cataracts, other risk factors include increased UV light, diabetes mellitus, and heavy use of glucocorticoid drugs. The genetic predisposition also varies. There are also rare genetic causes of cataracts at much earlier ages.

Cataracts can be completely cured by an outpatient operation in which a small slit is made in the side of the cornea, a probe is inserted that emulsifies and removes the lens, and finally a replacement lens (which is usually flexible) is inserted. Before the operation, the length of the patient's eyeball is measured and the curvature of the replacement lens is selected based on this measurement. Any pre-existing myopia or hyperopia is automatically corrected in the process. A more expensive replacement lens can also correct for astigmatism.