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Basic Anatomy:
Cornea : 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 focus on near objects. When in becomes cloudy it is called a cataract.
Unlike the cornea, 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 : This is the part of the eye that gives it color (i.e. blue, green, brown). It functions like a shutter in the camera analogy, allowing more or less light into the eye.
Pupil : This is the opening in the middle of the iris.
Retina : This is a thin layer of nerve tissue 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 : This is the center of the retina that receives the focus of the object of regard. 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 : This is the nerve that runs from the eyeball to the brain. It carries information from the retina to the brain for interpretation.
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 - and that is how we "see." The shape (curve) 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 factors must be well matched in order for the eye to see clearly without correction (emmetropia). If the cornea is too steeply curved or if the eye is too long,
myopia (nearsightedness) results. If the cornea is too flat or the eye too short,
hyperopia (farsightedness) 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. To focus near objects on the retina, the lens changes its shape--that is, until the lens becomes stiff with age
(presbyopia).
Intro:
Approximately one in four Americans is nearsighted, totalling 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 folklore. Some controversy exists over whether myopia can be made worse be prolonged near work as a child, but this has never been proven.
In studies, it has been correlated with higher IQ'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. Myopia tends to start in the early teens (earlier for high myopes), and 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. In the picture above, the arrow shows the focus of the nearsighted eye while the dotted lines represent the normal eye. The greater the myopia, the more the light rays converge and the more blurred distant objects are.
Near objects when 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 myope, who always has the option of seeing well at near without optical correction. Indeed, many myopes instinctively remove their spectacles for near work, particularly after the onset of
presbyopia .
Spectacle Correction for Myopia
The correction of myopia with spectacle lenses accrues 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 myopes begin wearing spectacles in childhood and therefore incorporate the less desirable cosmetic, convenience, and optical disadvantages into their daily routine; however, this adaption varies among individuals.
Among the optical drawbacks of minus spectacles lenses is the minification of the image. There is a 2% minification for every diopter of spectacle power. For example, a -10 diopter spectacle correction results in a 20% reduced image size. Thick lens edges and supporting frames also distort and reduce the peripheral vision.
Glasses fog up under certain conditions, slide down the nose, fall off, need cleaning, and other various maladies that prevent optimum performance under all conditions.
Contact Lens Correction Contact lens correction reduces the optical and physical problems that plague spectacle lenses, but they have their own unique problems including high-maintenance care solutions, corneal warpage, aggravating dry eye, corneal suffocation, corneal infections, and eyelid allergies. Some people lose their ability to wear contact lenses - particularly individuals with dry or sensitive eyes. It appears that 20 to 30 years of contact lens wear causes a chronic dry eye syndrome.
Surgical Correction. See LASIK technique
Intro:
Hyperopia (i.e farsightedness) is much less common than myopia or emmetropia. It is typically in the +1.00 to +4.00 diopter range; rarely can it be as high as +8.00
 diopters. It is often confused with age-related farsightedness or presbyopia. The difference is a truly farsighted individual will often see 20/15 when they are younger but as they approach 40 they not only lose the ability to read up close but also the ability to see distant objects. They will find themselves having to wear bifocals in their 50s, when they never had to wear glasses at all in their 30s.
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. In the picture above the light rays are shown to focus behind the eye (arrow). Distant objects appear blurred, and nearby objects are even fuzzier. Most farsighted individuals need corrective eyewear to see clearly at all distances.
Spectacle Correction:
Correction of hyperopia requires a lens which is convex (i.e. thicker in the middle than the edges). This acts as a magnifier, and causes objects to appear bigger by 2% per diopter. For this reason hyperopes while wearing their spectacle correction, appear to have "big" eyes. Optical aberrations and decreased peripheral vision occur are likely to occur with large amounts of hyperopia.
Intro:
Astigmatism is present in nearly 100% of the population to some degree.
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 may be accompanied by myopia or hyperopia.
For example, this picture demonstrates that the vertical light beams are focused in front of the retina, while light beams oriented 90 degrees away (horizontal) 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 is needed to correct astigmatism. Soft contact lenses are not able to correct astigmatism unless they are a special type known as toric lens. Many patients complain of toric contact lenses cause momentary blur when they blink or when they lye on their side. This is because these lenses are weighted and their corrective power is dependent on their orientation. Large amounts of astigmatism often can only be treated by rigid contact lenses.
Presbyopia affects 100% of the population by age 50. Currently, bifocals or monovision are the only successful ways 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 age 40 with the onset of presbyopia (literally "old eye") when the lens starts to lose its flexibility. Presbyopic individuals with normal vision need reading glasses while nearsighted and farsighted patients need bifocals for clear viewing at both distance and near.
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 at near without the use of any additional optical power from their crystalline lens. It is extremely difficult for most intelligent people to understand, but if you have both eyes corrected for distance (with LASIK or contacts), eliminating your nearsightedness, you will no longer be able to see up close like you did before. You will become like everybody else your age that had “perfect vision” when they were young but loses the ability to read up close after age 40. This means you will have trouble putting on make up or shaving, seeing your watch, reading a menu or looking up phone numbers.
Spectacle Correction:
Bifocal lenses allow the user to view distance objects through the top portion of their glasses, and near objects with magnifiers added to the bottom portion of their glasses. These lenses can be 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 work. As an alternative to reading glasses for near work, you may elect to leave one eye slightly nearsighted; an outcome called
monovision.
Nearsighted people over 40 who are accustomed to removing their glasses for close work need to give extra thought to vision correction surgery. Because their eye gradually becomes a single-focus optical system which can view EITHER near objects OR distant objects clearly (but not both), they will definitely need glasses to read if they have surgery to focus both eyes for distant objects. For some, this may be an advantage, but for others, it may not.
It is possible to correct one eye for distance and leave the other slightly nearsighted for reading. This technique, called monovision, may give presbyopes the best chance of eliminating corrective eyewear entirely. If you are in the presbyopic age range, you must determine if your goal is to achieve best vision correction for distance in both eyes or to leave one eye slightly nearsighted for reading.
If you are over 40 and a contact lens wearer, you may have already experienced monovision by wearing one contact lens that has slightly less power. With refractive surgery, you may have monovision by leaving one eye slightly nearsighted. About one-third of patients who try monovision adapt to it successfully and the other two-thirds elect to have both eyes corrected optimally for distance and wear simple "dime-store" reading glasses for near work. If you try monovision and donot like it, your under corrected eye can be enhanced to the full correction allowing you to see clearly at a distance with both eyes.
Please discuss monovision with your doctor so that an appropriate surgical plan can be made.
Learn more about eye disorders on www.improveyourvision.com
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