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Chapter 1 - Why People Wear Glasses Below is an example of a patient prescription written by a doctor of optometry. It is the primary role of the dispensing optician or eye care professional (ECP) to fill prescriptions as written by the doctor to best correct the patient’s eyesight. In order to provide the best lens choice and patient service, it is important to understand what the prescription is trying to correct. If you are to be a competent ECP, it is fundamentally important that you understand how and why eyeglasses (spectacle lenses, ophthalmic lenses) make it possible for people to see well.

Question: Why do people wear glasses? Answer: So they can see clearly.

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Basic Eye Anatomy Question: How do ophthalmic glasses help people see clearly? Answer: Light is the energy source that makes sight possible. Our eyes are constantly bombarded by billions of rays of light. These rays come directly from light sources (light bulbs, the sun) and are reflected off objects in our surroundings. The reflected rays provide us with the colors and dimensional details about the objects we are viewing. The human eye is actually a system of lenses and focusing devices. In simple terms, light entering the eye will pass through the cornea, the crystalline lens, and the vitreous humor, then focus on a specific spot on the retina, called the fovea, that allows our brains to interpret the rays of light as clearly focused objects. The retina contains special receptor cells, known as rods and cones, which send the light as nerve impulses to the brain for interpretation. The brain, in turn, tells us what we are looking at. It is important to remember that we actually see with our brains, not with our eyes. The perfect human eye is said to be emmetropic, or to have the condition of emmetropia. It requires no correction of any kind to see objects clearly. The Emmetropic Eye

The Emmetropic Eye Notice that in the emmetropic eye all the rays of light entering the eye all focus on the retina right where they need to be to provide crisp sight without the need of corrective lenses. For others, the eye system is flawed, so those billions of rays of light are not passing through the eye and focusing on the fovea. Instead, the rays are getting lost along their path to the fovea, creating difficulties for the brain in interpreting what it is seeing. In some cases, all the rays fail to reach their intended goal. In others, only some fail to reach it. When all the rays are not landing

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where they need to be objects being viewed and interpreted may seem blurry, out of focus, tilted, or out of place in relation to their actual position. When an individual suffers from these wandering rays of light and cannot see clearly, we must use ophthalmic lenses to redirect the rays of light back to their intended path, so they all focus on the fovea. Then, the brain can interpret the rays of light correctly. When an individual suffers from these wandering rays, we say the individual has a refractive error. Refraction is the bending of light that occurs in a lens. The Eight Common Refractive Errors of the Human Eye There are eight common simple refractive errors of the human eye. There are also combinations of refractive errors, complex refractive errors and a wide range of medically based problems that can cause sight loss. As an optician working with spectacle lenses, you are primarily concerned with the following eight refractive errors: –~~~~~~~~~~~~–

Simple Myopia Simple Myopia: “Simple” because all the rays of light entering the eye focus at the same spot, it is the wrong spot, but they all meet at the same place. This is because the eye’s cornea is spherical or perfectly round, but the person has an eyeball that is long in shape or has a spherical cornea that is too steeply curved. The retina is further back from the cornea than in an emmetropic eye, so the rays fail to reach the back of the eye and the retina. Persons with myopia are nearsighted; they are capable of seeing things at “near” distances, or up very close to their eyes. They can read fine print, thread a needle, and work with tiny objects. They cannot see a street sign down the road or a bird high in a tree, without correction.

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Myopia is corrected with minus lenses (see below). It is easy to remember: Just think my-opia and mi-nus lenses. A prescription for a person with simple myopia would be written like one of these examples: -1.00 Sphere -2.50 Sphere

Simple Hyperopia Simple Hyperopia: In “simple hyperopia” all the rays of light entering the eye focus at the same spot, it is the wrong spot, but they all meet at the same place. This is because the eye’s cornea is spherical, or perfectly round, but the person has an eyeball that is short in shape or has a spherical cornea that is too shallowly curved. The retina is further forward toward the cornea than in an emmetropic eye, so the rays are hitting wide of the back of the eye and the retina. They are trying to focus on an imaginary point beyond the back of the eye. Persons with simple hyperopia are farsighted; they are capable of seeing things in the distance or far off. They can easily see a street sign half a mile down the road and a bird high up in a tree. They cannot see fine print, thread a needle, or do detail work without correction. Simple hyperopia is corrected with plus lenses (see below). A prescription for a person with simple hyperopia would look like one of these examples: +1.00 Sphere +2.50 Sphere

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Simple Myopic Astigmatism Simple Myopic Astigmatism: In “simple myopic astigmatism,” some of the rays of light entering the eye fall short of their intended spot on the retina, but some fall directly on the fovea, where they need to be. The eye’s cornea is not perfectly round, but instead has two different curves. This forces some of the light rays to deviate from their intended path, and others to remain on target. People with myopic astigmatism are still nearsighted, but due to the variable distribution of light rays, they may even have trouble clearly seeing things up close. Depending on the degree of astigmatism (the degree to which the cornea is misshapen), the individual may see objects as bent or distorted in shape as well as blurred. Simple myopic astigmatism is corrected using lenses with variable powers. They are known as sphero-cylinder lenses, toric lenses, or sometimes compound lenses. One focus point of the eyeglass lens will provide no correction, or have 0.00 power, for those rays that are falling where they are needed. Another focus point of the lens will have power for the rays that need to be redirected to the correct place on the retina. –~~~~~~~~~~~~– A prescription for a person with simple myopic astigmatism would look like one of these examples: 0.00 -0.50 X 45 -0.50 + 0.50 X 135 0.00 -2.00 X 130 -2.00 +2.00 X 40 After working through modules 2 and 3 you will be able to tell that all these prescription examples have 0.00 or no power in one meridian (point on the lens), the meridian where the rays are hitting where they need to, and minus power in the other. The + or – sign in front of the middle number does not indicate power.

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Simple Hyperopic Astigmatism Simple Hyperopic Astigmatism: In “simple hyperopic astigmatism,” some of the rays of light entering the eye focus on a spot beyond the retina, but some fall directly on the fovea where they need to be. The eye’s cornea is not perfectly round, but instead has two different curves. This forces some of the light rays to deviate more from their intended path than others. People with hyperopic astigmatism are still farsighted, but due to the variable focus of the rays of light, may have trouble seeing things in the distance. Depending on the degree of astigmatism (the degree to which the cornea is misshapen), the individual may see objects as bent or distorted in shape as well as blurred. Hyperopic astigmatism is corrected using sphero-cylinder lenses. One focus point of the lens will provide no correction, or have 0.00 power, for those rays which fall where they should. Another focus point of the lens will have power for the rays that need to be redirected to the correct place on the retina. A prescription for a person with a hyperopic astigmatism would look like one of these examples: +1.50 -1.50 X 45 0.00 +1.50 X 135 +2.50 -2.50 X 130 0.00 + 2.50 X 40 After working through modules 2 and 3 you will be able to tell that all these prescription examples have 0.00 or no power in one meridian and plus power in the other. The + or – sign in front of the middle number does not indicate power (More about this in Modules 2 and 3).

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Compound Hyperopic Astigmatism Compound Myopic Astigmatism: This condition is no longer simple, because the rays of light entering the eye do not all meet at the same place. They all fall short of their intended spot on the retina, but some fall closer than others. The eye’s cornea is not perfectly round, but instead has two different curves on it. This forces some of the light rays to deviate more than others from their intended path than others. People with myopic astigmatism are still nearsighted but due to the variable focus may even have trouble clearly seeing things up close. Depending on the degree of astigmatism (the degree to which the cornea is misshapen) the individual may see objects as bent or distorted in shape as well as blurred. Individuals with astigmatic correction are prescribed sphero-cylinder lenses. –~~~~~~~~~~~~– A prescription for a person with a myopic astigmatism would look like one of these examples: -1.00 -0.50 X 45 -1.50 +0.50 X 135 -2.50 -2.00 X 130 -4.50 + 2.00 X 40 It may not be clear to you yet but all these prescription examples have minus power in all meridians.

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Compound Hyperopic Astigmatism Compound Hyperopic Astigmatism: This condition is no longer simple, because not all the rays of light entering the eye meet at the same place. They all focus on a spot beyond the retina, but some come closer to the fovea than others. The eye’s cornea is not perfectly round, but instead has two different curves. This forces some of the light rays to deviate more from their intended path than others. People with hyperopic astigmatism are still farsighted, but due to the variable focus may have trouble seeing things in the distance. Depending on the degree of astigmatism (the degree to which the cornea is misshapen) the individual may see objects as bent or distorted in shape as well as blurred. Hyperopic astigmatism is corrected using prescribed sphero-cylinder lenses. A prescription for a person with a hyperopic astigmatism would look like one of these examples: +1.00 – 0.50 X 45 +0.50 +0.50 X 135 +2.50 – 2.00 X 130 +0.50 +2.00 X 40 It may not be clear to you yet but all these prescription examples have plus power in all meridians.

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Mixed Astigmatism Mixed astigmatism: In the eye with mixed astigmatism some rays fall ahead of the retina while others try to focus on a spot beyond the retina. People with mixed astigmatism are neither nearsighted nor farsighted, but instead will have poor vision in all areas. Mixed astigmatism is corrected using sphero-cylinder lenses. A prescription for a person with a mixed astigmatism would look like one of these examples: +1.00 -2.00 X 45 -1.00 + 2.00 X 135 +2.00 -2.25 X 67 -0.25 + 2.25 X 157 It may not be clear to you yet but these prescription examples have both minus and plus powers on the same lens. –~~~~~~~~~~~~–

Presbyopia Presbyopia: Presbyopia is an entirely different type of refractive error from the previous ones. In fact it may be considered a refractive condition rather than a refractive error. Everyone will suffer from presbyopia in their lifetime. Presbyopia makes us unable to read fine print, thread a needle, or do fine work without the aid of magnification.

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The crystalline lens, shown here in blue and red, is a powerful variable focus plus lens (a magnifying lens) that provides us with plus power, so we can see things that are small and close to our eyes. When we are young as we bring an object closer to our eye the crystalline lens becomes more convex in shape like the red area above. This provides us with more plus power to help magnify the object or image so we can make out its details. As we age, the crystalline lens loses its ability to puff up (expand or become more convex in shape) and relax and instead becomes stiff and inflexible. It can no longer create the plus power needed to aid our vision. It remains in the less powerful position shown in blue. The onset of presbyopia generally happens at around age forty-five and will steadily worsen for a period of about five years before reaching its maximum loss. People with presbyopia complain that newspaper print is getting smaller, and that their arms are too short to hold the print far enough away to read it. Presbyopia is corrected using a lens with additional plus power, to give us back what we have lost. Prescriptions for people with presbyopia will have an “add power” noted on their prescription form, or the notation “Near Vision Only.” Prescriptions for presbyopia will show corrections for distance, if required, and the additional notation of an add power as in one of these examples: Add +1.25 Add +2.50

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why people wear glasses