Errors of Refraction
The eye with normal refractive power is called emmetropic eye and the condition is called emmetropia.
Ametropia is of two types: -
Myopia or Short Sightedness-
Myopia is the eye defect characterized by the inability to see the distant object. It is otherwise called short sightedness because the person can see near objects clearly but not the distant objects.
- In myopia, the refractive power of lens is usually normal. But, the anteroposterior diameter of the eyeball is abnormally long.
- Therefore, the image is brought to focus a little in front of retina. Light rays, after coming to a focus, disperse again so, a blurred image is formed upon retina.
The myopic eye is corrected by using a biconcave lens. Light rays are diverged by the concave lens before entering the eye.
Hypermetropia or Long Sightedness-
Hypermetropia is the eye defect characterized by the inability to see near object. It is otherwise known as long sightedness because the person can see the distant objects clearly but not the near objects. It is also called hyperopia.
- Hypermetropia is due to decreased anteroposterior diameter of the eyeball. So, even though the refractive power of lens is normal,
- the light rays are not converged enough to form a clear image on retina, i.e. the light rays are brought to a focus behind retina.
- It causes a blurred image of near objects. Hypermetropia occurs in childhood, if the eyeballs fail to develop the correct size. It is common in old age also.
Hypermetropia is corrected by using biconvex lens.
|Type of error||Cause||Correction|
|Myopia||Increase in anteroposterior diameter of the eyebal||Biconcave lens|
|Anisometropia||Difference in refractive power of both eyes
||Separate lens (biconcave or biconvex) for each eye as required
|Astigmatism||Refractory power of lens is different in different meridians||Cylindrical lens|
|A-Regular astigmatism||Refractory power of lens is unequal in different meridians but uniform in one single meridian||Cylindrical lens|
|B-Irregular astigmatism||Refractory power of lens is unequal in different meridians as well as in different points in same meridian||Cylindrical lens|
|Presbyopia||Loss of elasticity in lens and weakness of ocular muscles due to old age||Biconvex lens|
|Hypermetropia||Decrease in anteroposterior diameter of the eyebal||
Astigmatism is the condition in which light rays are not brought to a sharp point upon retina. It is the common optical defect. This defect is present in all eyes. When it is moderate, it is known as physiological astigmatism.
- Light rays pass through all meridians of a lens. In a normal eye, lens has approximately same curvature in all meridians. So, the light rays are refracted almost equally in all meridians and brought to a focus.
- If the curvature is different in different meridians, vertical, horizontal and oblique, the refractive power is also different in different meridians.
- The meridian with greater curvature refracts the light rays more strongly than the other meridians. So, these light rays are brought to a focus in front of the light rays, which pass through other meridians. Such irregularity of curvature of lens causes astigmatism.
Astigmatism is of two types:
1. Regular astigmatism
2. Irregular astigmatism.
1. Regular Astigmatism -In regular type of astigmatism, the refractive power is unequal in different meridians because of alteration of curvature in one meridian. But, it is uniform in all points throughout the affected meridian.
2. Irregular Astigmatism-In irregular type of astigmatism, the refractive power is unequal not only in different meridians, but it is also unequal in different points of same meridian.
Astigmatism is corrected by using cylindrical glass lens having the convexity in the meridians.
Presbyopia is the condition characterized by progressive diminished ability of eyes to focus on near objects with age. It is due to the gradual reduction in the amplitude of accommodation.
- Decreased elasticity of lens is because of the physical changes in lens and its capsule during old age. So, the anterior curvature is not increased during near vision.
- Decreased convergence of eyeballs due to the concomitant weakness of ocular muscles in old age.
Presbyopia is corrected by using biconvex lens.