Parts of the Eye
Cornea -- the clear front window of the eye. The cornea transmits and focuses light into the eye.
Iris -- the colored part of the eye. The iris helps regulate the amount of light that enters the eye.
Pupil -- the dark center in the middle of the iris. The pupil determines how much light is let into the eye. It changes sizes to accommodate for the amount of light that is available.
Lens -- the transparent structure inside the eye that focuses light rays onto the retina. Retina -- the nerve layer that lines the back of the eye. The retina senses light and creates impulses that are sent through the optic nerve to the brain.
Macula -- a small area in the retina that contains special light-sensitive cells. The macula allows us to see fine details clearly.
Optic Nerve -- the nerve that connects the eye to the brain. The optic nerve carries the impulses formed by the retina to the brain, which interprets them as images.
Vitreous -- the clear, jelly-like substance that fills the middle of the eye.Return to top of page
The four main vision focusing disorders of the eye are:
The units used to represent the amount of correction needed in order to correct vision for distance are called 'diopters'. The more nearsighted or farsighted you are, the higher your prescription is in diopters.
Your prescription is usually written in three numbers:
Right eye -4.25 -1.75 X 180
Left eye -5.50 -1.25 X 175
Here is the way to decipher your prescription:
The 1st number (-4.25 and -5.50 in this example) is the degree of spherical nearsightedness or farsightedness. The sign identifies whether you are nearsighted ( - sign) or farsighted (+ sign).
The second number (-1.75 and -1.25) is the degree of astigmatism. The number can be written either with a + sign or a - sign.
The 3rd number (180 and 175) is the axis, or the direction of your astigmatism. An axis of 180 degrees, for example, means the astigmatism is horizontal. Therefore, this prescription means that the patient is moderately nearsighted, with a moderate degree of astigmatism in a horizontal direction. The "add" at the bottom of the prescription is for the reading part of a bifocal glass.
It might be unusual for anyone under the age of 40 to need this. Some people only have one number written for each eye. This is when there is no astigmatism.
Myopia is the medical term for nearsightedness. The myopic eye is too long for the cornea's curvature. Light rays enter and focus in front of the retina and produce a blurred image instead of coming to a sharp focus on the retina at the back of the eye. The term 'nearsighted' means that you can see 'near' objects clearly without your glasses, but objects further in the distance are blurry. There are varying degrees of myopia (nearsightedness). The more myopic you are, the more blurred objects appear in the distance, the higher your eyeglass prescription and the thicker your glasses. Of all myopic people, about 90% have corrections less than 6.00 diopters. The table below shows the categories of severity for myopia:
- Mild Myopia Less than -3.00 diopters
- Moderate Myopia -3.00 to -6.00 diopters
- Severe Myopia -6.00 to -9.00 diopters
- Extreme Myopia Greater than -9.00 diopters
Almost everyone feels that their myopia is severe, because of how dramatically dependent they are on glasses and contact lenses. However, only one in ten myopic people are actually in the severe and extreme categories.
Many patients with myopia have some degree of astigmatism, or ovalness in their cornea. It is the most common refractive condition, and occurs to some extent in most eyes. Astigmatism occurs when the cornea is shaped more like an egg than a marble. As a result, patients with astigmatism experience distortion or tilting of images because of unequal bending of light rays entering the eye. Patients with high degrees of astigmatism have blurred vision not only for distant objects, as with myopia, but for near objects as well. Astigmatism is also measured in diopters. Of all myopic people, 50% or more have astigmatism as well. Most of these people have corrections of less than 1 diopter. The table below shows the categories of severity for astigmatism:
- Mild astigmatism Less than 1.00 diopters
- Moderate astigmatism Between 1.00 and 2.00 diopters
- Severe astigmatism Between 2.00 and 3.00 diopters
- Extreme astigmatism Greater than 3.00 diopters
Hyperopia is the medical term for `farsightedness.' It occurs when an eye is too short for the cornea's curvature. Light rays entering the eye focus behind the retina, and as a result a blurred image is produced. Farsighted individuals, however, can use their focusing muscles to 'pull' the image forward onto the retina. In a young person with severe hyperopia, or once presbyopia sets in around age 45, distant objects are seen more clearly than near objects.
Presbyopia is the normal process of aging, where the natural lens of the eye loses some of the flexibility that characterizes a younger eye. This usually occurs between the ages of 40 to 50. Everyone experiences presbyopia, some sooner, some later. Because of this normal process, nearsighted people begin to wear bifocals in their forties, and those who never needed glasses before may require reading glasses. The one advantage to mild myopia is the ability to remove your glasses after age 40 and continue to read (your myopia effectively counteracts your presbyopia).
This simple test helps determine if you are colour blind and if so which colour is effected
The Amsler Grid is a test for macular degeneration. Use it to monitor your vision between visits to an eye care professional.
- Wear the glasses you normally wear when reading.
- View the chart at arms distance and cover one eye. With the uncovered eye, stare at the white dot in the center of the grid.
- To view the chart at the proper distance, slowly move in toward your monitor until one of the two red ovals FIRST disappears.
- During the entire test, you should have one eye covered, stare at the center of the grid and only see one red oval.
- If your eye is functoning properly , you should be able to see the center white dot and the four corners and sides of the grid. The lines should appear to be straight and continuous from top to bottom and side to side.
- Now test your other eye.