Eye Anatomy

Cornea: The cornea is the crystal clear dome that covers the front of the eye. The majority (70%) of the bending (refracting) of light rays is accomplished by the cornea . The shape of the cornea does not change (with the exception of small changes that occur over a lifetime).

Lens: The crystalline lens finishes the focusing of light. It helps to "fine tune" vision, and it is able to change shape to allow the eye to focus on near objects. When it becomes cloudy, it is called a cataract.

Unlike thecornea , the lens can be made to change its shape (and, therefore, its refractive power) rapidly and voluntarily. Using its ability to change shape, the lens allows the eye to change its focal point. Changes in the shape of the lens will allow a normal eye to focus on near objects.

Iris: The iris is the part of the eye that gives it color (i.e., blue, green, brown).

Pupil: The pupil is the opening in the middle of the iris . It functions like the diaphragm in a camera, controlling the amount of light that enters the eye. The pupil is small in bright light and large in dim light.

Retina: The retina is a thin layer of nerve tissue at the back of the eye that senses light. Specialized cells called rods and cones convert light energy into nerve signals that travel through the optic nerve to the brain. The retina is analogous to the film in a camera.

Fovea: The fovea is the center of the retina that receives the focus of the object being viewed. Nerve cells are more densely packed in this area, so images that are focused on the fovea can be seen in greater detail.

Optic nerve: The optic nerve is the nerve that runs from the eye to the brain. It carries information from the retina to the brain for interpretation.

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Eye Optics

The term "refractive" refers to the way light rays reflected from objects in our field of vision are brought into focus within the eye. The eye's complex optical components intercept, focus, and process light into nerve impulses, which are sent to the brain -- that is how we "see." The shape of the cornea, the power of the crystalline lens inside the eye, and the length of the eye together determine how rays of light are focused on the retina, the photosensitive membrane that lines the back wall of the eye. These three components must be well matched for the eye to see clearly without correction ( emmetropia ). If the cornea is too steeply curved relative to the length of the eye or if the eye is too long relative to the curvature of the cornea, then myopia results. If the cornea is too flat or the eye too short, then hyperopia results. In the picture above, light entering from the left travels through the cornea, then through the lens, and is focused perfectly on the retina at the fovea . In the normal (emmetropic) eye, distant objects are focused on the retina.

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Visual Acuity and Your Prescription

What does 20/20 mean?

Visual acuity is usually measured with a Snellen chart , see image above. The Snellen chart displays letters of progressively smaller size. "Normal" vision is 20/20. This means that the test subject sees the same line of letters at 20 feet that a normal person sees at 20 feet. 20/40 vision means that the test subject sees at 20 feet what a normal person sees at 40 feet. Another way of saying this is that a person with 20/40 vision has vision that is only half as good as normal -- or, objects must be at half the normal distance for him to see them clearly. A person with 20/20 vision is able to read letters 1/10th as large as someone with 20/200 vision. However, 20/15 vision is better than 20/20. A person with 20/15 vision can see objects at 20 feet that a person with 20/20 vision can only see at 15 feet.

Levels of Vision

• 20/20: Normal vision. Fighter pilot minimum. Required to read the stock quotes in the newspaper or numbers in the telephone book.
• 20/40: Able to pass the Driver's License Test in all 50 states. Most printed material is at this level.
• 20/80: Able to read alarm clock at 10 feet. News headlines are this size.
• 20/200: Legal blindness. Able to see STOP sign letters.

Your eye functions much like a camera. In order to have normal (20/20) vision, all of the parts must work together properly and the optical system must be correctly aligned. Light rays from objects in our environment are bent (refracted) by the cornea and the crystalline lens inside the eye. When the eye is working properly, a sharp image is formed on the retina, a layer of nerve cells lining the inside of the eyeball that can sense light. When the optical system of the eye does not focus properly, we say that the eye has a "refractive error ." Refractive errors can be corrected with glasses, contact lenses, or surgery to restore clear vision.

Your Prescription

The refractive error of the eye can be expressed in numeric terms. The power of the lenses necessary to correct your vision is measured in units called diopters (see below). The first number designates the amount of myopia (minus numbers) or hyperopia (plus numbers). The second number (if present) indicates the amount of astigmatism . The third number shows the axis of the astigmatism. Depending on whether the second number is positive or negative, the axis indicates the orientation of the steepest or flattest meridian of the cornea. The fourth number is the "add," or the amount of magnification needed to give clear vision at near distances. If this number is present, bifocals have been prescribed to correct presbyopia .

What is a Diopter?

As mentioned above, a diopter is a unit of measurement of refractive error . It may be a negative number (as in   myopia , nearsightedness) or a positive number (as in   hyperopia , farsightedness). A -1.00 diopter myope is able to see objects at 1 meter clearly. A -2.00 diopter myope is able to see objects at 0.5 meters clearly. The greater the myopia, the greater the strength of the lens in diopters needed to correct the refractive error, and the closer an object must be to be viewed clearly.

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The Nearsighted Eye

Approximately one in four Americans is nearsighted, a total of 70 million people. The degree of myopia can vary from low (-1 to -3 diopters ) to high (greater than -6 diopters). Myopia is not caused by reading at an early age, prolonged reading, reading in the dark, watching TV too closely, wearing glasses too strong, wearing glasses too weak, or other similar activities. Some controversy exists over whether myopia can be made worse by prolonged near work as a child, but this has never been proven.

Some studies have correlated myopia with higher I.Q.s, but this probably has to do with the fact that myopes tend to read more since that's where they can see the best without glasses (see below). Myopia tends to run in families, so it is probably inherited -- at least to some extent. Myopia tends to begin in the early teens (earlier for high myopes), and it usually increases as the eye grows in length during puberty. It tends to stabilize after age 18.

Optics

In a nearsighted eye, the cornea is too steeply curved for the length of the eye, causing light rays to focus in front of the retina. Distant objects appear blurred or fuzzy because the light rays are not in focus by the time they reach the retina. The greater the myopia , the more the light rays converge and the more blurred distant objects are.

Near objects, viewed at the proper distance, can be seen clearly because the focus of their light rays matches the refractive error of the nearsighted eye. For example, a patient with -2 diopter correction is naturally in focus at 0.5 meters (18 inches). Having the focus point of the eye within arm's length is an intrinsic advantage to the low myope, who always has the option of seeing well at near distances without optical correction. Indeed, many myopes instinctively remove their spectacles for near work, particularly after the onset of presbyopia .

Spectacle Correction

The correction of myopia with spectacle lenses creates both advantages and disadvantages for the myope. The obvious advantage is bringing distant objects into focus. The disadvantages increase in rough proportion to the strength of the lens. Most nearsighted patients begin wearing spectacles in childhood and, therefore, incorporate the less desirable cosmetic, convenience, and optical disadvantages into their daily routine.

Among the optical drawbacks of spectacle lenses that correct myopia is minification of the image. There is about a 2% minification for every diopter of spectacle power. For example, a -10 diopter spectacle correction results in about a 20% reduction in image size. Thick lens edges and supporting frames also distort and reduce peripheral vision.

Contact Lens Correction

Contact lenses reduce the optical problems associated with spectacle lenses, but they have their own unique problems, including the need for cumbersome care solutions, corneal warpage, corneal hypoxia (lack of oxygen), corneal infections, and eyelid allergies. Some people eventually lose their ability to wear contact lenses -- particularly those who have dry or sensitive eyes. It is typical for contact lens intolerance to occur at about the age of 40.

Surgical Correction

Please see Vision Correction Surgery for details on specific techniques.

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The Farsighted Eye

Hyperopia (farsightedness) is much less common than myopia or emmetropia . It is typically in the +1.00 to +4.00 diopter range. Rarely, it can be as high as +8.00 diopters .

Optics

In contrast to myopia , hyperopia occurs when the eye is too short for the power of its optical components. In hyperopia , the cornea is not steep enough, and light rays hit the retina before they come into focus. Distant objects appear blurred, and nearby objects are even more fuzzy. Most farsighted individuals need corrective eyewear to see clearly at all distances.

Spectacle Correction

Correction of hyperopia requires a lens which is convex (thicker in the middle than the edges). This acts as a magnifier and causes objects to appear larger by about 2% per diopter. This is why hyperopes wearing spectacles appear to have "big" eyes. Optical abberations and decreased peripheral vision occur with large amounts of hyperopia.

Contact Lens Correction

Contact lenses reduce the optical problems associated with spectacle lenses, but they have their own unique problems including the need for cumbersome care solutions, corneal warpage, corneal hypoxia (lack of oxygen), corneal infections, and eyelid allergies. Some people eventually lose their ability to wear contact lenses -- particularly those who have dry or sensitive eyes. It is typical for contact lens intolerance to occur at about the age of 40.

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Astigmatism

Astigmatism (Greek: a-stigma, meaning not to a point) is a type of refractive error that prevents rays of light from being brought to a sharp point of focus on the retina.

Optics

Astigmatism is usually caused by an abnormal corneal curvature that resembles the shape of a football rather than that of a basketball. Light rays focus at different places depending on their orientation, and objects at all distances appear blurred. Astigmatism is usually accompanied by myopia or hyperopia .

For example, if vertical light beams are focused in front of the retina, while horizontal light beams are focused at or near the retina, the result is two different focal planes 90 degrees apart, instead of a sharp focal point.

Spectacle Correction

A special type of lens (called a "cylinder") is used to correct astigmatism . Soft contact lenses are not able to correct astigmatism unless they are a special type known as toric lenses. Large amounts of astigmatism often can only be treated by rigid contact lenses.

Surgical Correction

Please see Vision Correction Surgery for details on specific techniques.

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Presbyopia: The Need for Reading Glasses

Presbyopia affects 100% of the population by the time they reach the age of 50. Currently, bifocals (reading glasses) are the only successful way to treat presbyopia.

Optics

The crystalline lens allows the eye to vary its optical power, permitting individuals with normal vision to view distant objects and refocus their eyes to see near objects sharply. This ability begins to decline around the age of 40 with the onset of presbyopia (literally "old eye") when the lens starts to lose its flexibility. Presbyopic individuals with previously normal vision will need reading glasses, while nearsighted and farsighted patients need bifocals for clear viewing at both far and near distances.

People who have low myopia (3 diopters or less) often notice that they can read fine print comfortably without their glasses well past the age of 40. This is because their nearsightedness allows them to focus on near objects without the use of any additional optical power from their crystalline lens.

Spectacle Correction

Bifocal lenses allow the user to view distant objects through the top portion of their glasses, and near objects with magnifiers added to the bottom portion of their glasses. Recently, these lenses have been blended together to produce a "progressive add" or "no line" bifocal.

Special Refractive Surgery Considerations

It is important that you understand that refractive surgery DOES NOT PREVENT the age-related loss of the eye's ability to vary its focusing power. If you are over 40 and have both your eyes fully corrected for distance vision, you will need reading glasses for near distance work. As an alternative to this, you may elect to leave one eye slightly nearsighted. This is an outcome called monovision . There are many other new treatments for presbyopia at InView today. These alternatives should be discussed thoroughly with your surgeon.

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