Refractive Errors

Question 1

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. They meet in the same place because the eye’s cornea is spherical or perfectly round. They fail to reach the back of the eye, or the retina, because the eye itself is long or because the cornea is too steeply curved.

Persons with “ “ 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.

” “ is corrected with minus lenses. It is easy to remember.

Answer 1

Simple Myopia : 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

Question 2

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. They meet in the same place because the eye’s cornea is spherical or perfectly round. They reach an imaginary point beyond the back of the eye because the eye itself is short or because the cornea is too shallowly curved.

Persons with “ “ 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.

” “ is corrected with plus lenses.

Answer 2

Simple Hyperopia: A prescription for a person with simple hyperopia would look like one of these examples:
+1.00 Sphere
+2.50 Sphere

Question 3

Some of the rays of light entering the eye fall short of their intended spot. However, some fall directly on the retina 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 “ “ are still nearsighted. However, 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 “ “ 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.

Answer 3

Simple Myopic Astigmatism : 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
1. 00 -2.00 X 130
   - 2.00 +2.00 X 40

Question 4

Some of the rays of light entering the eye focus on an imaginary spot beyond the retina. However, others fall directly 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 “ “ are still farsighted. However, due to the variable focus of the rays of light, may have even 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.

” “ is corrected using sphero-cylinder lenses. One focus point of the lens will provide no correction, or have 0.00 power. Another focus point of the lens will have power for the rays that need to be redirected to the correct place on the retina.

Answer 4

Simple Hyperopic Astigmatism : 
A prescription for a person with a simple hyperopic astigmatism would look like one of these examples:
\+1.50 -1.50 X 45
0.00 +1.50 X135
\+2.50 -2.50 X 130
0.00 +2.50 X 40

Question 5

All the rays of light entering the eye 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.

People with “ “ are still nearsighted. However, 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.

” “ is corrected using prescribed sphero-cylinder lenses.

Answer 5

Compound Myopic astigmatism: 
A prescription for a person with a compound 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

All these examples have minus power in all areas of the lens.

Question 6

All the rays of light entering the eye focus on an imaginary spot beyond the retina. However, some come closer to the retina 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 “ “ are still farsighted. However, 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.

” “ is corrected using prescribed sphero-cylinder lenses.

Answer 6

Compound Hyperopic Astigmatism :

A prescription for a person with a compound 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
All these examples have plus power in all areas of the lens.

Question 7

Some rays fall ahead of the retina while others try to focus on a spot beyond the retina. People with “ “ are neither nearsighted nor farsighted, but instead will have poor vision in all areas. Their cornea is misshapen in a way that makes them require both minus and plus power to see clearly.
“ “ is corrected using sphero-cylinder lenses.

Answer 7

Mixed Astigmatism:
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
In each of these prescriptions one area of the lens has minus power while another area of the lens has plus power.