Refractive errors
Refractive Errors
Refractive errors, ametropia, astigmatism and presbyopia
Refractive Errors
Refractive error is defined as an inability of eye to focus the image of objects accurately on retina.
Emmetropia
Emmetropia is the vision with lens of eye having normal refractive power.
An eye with normal refractive power is called emmetropic eye.
Ametropia
Any deviation in refractive power from normal condition resulting in inadequate focusing on retina is called ametropia and the eye is called ametropic eye.
The defect is due to change in shape of the eyeball.
Types of Ametropia
- Hypermetropia
- Myopia
Hypermetropia or Long Sightedness
Hypermetropia is the eye defect characterized by inability to focus on near object.
Hypermetropia is otherwise known as long sightedness because the person can see distant objects clearly, but not the near objects.
Causes of Hypermetropia
Hypermetropia is due to decreased anteroposterior diameter of the eyeball.
So, even though refractive power of lens is normal, the light rays are not converged enough to form a clear image on retina, i.e., light rays are brought to a focus behind retina.
It causes blurred image of near objects.
Hypermetropia occurs in childhood, if the eyeballs fail to develop to the correct size.
It is common in elders also.
Correction of Hypermetropia
Hypermetropia is corrected by using biconvex lens.
Light rays are converged by convex lens before entering the eye.
Myopia or Short Sightedness
Myopia is the defect characterized by inability of the eye to focus on distant object.
It is otherwise called short sightedness, because the person can see near objects clearly, but not the distant objects.
Far Point in Myopia
Normally, in emmetropia, the far point is infinite.
In myopia, the near vision is normal, but the far point is not infinite, i.e., it is at a definite distance.
In extreme conditions, it may be only a few centimeters away from the eye.
Causes of Myopia
In myopia, refractive power of the lens is usually normal.
But, anteroposterior diameter of the eyeball is abnormally long.
Therefore, the image is brought to a focus a little in front of retina.
In other words, refractive power of lens is too strong for the length of eyeball.
Light rays, after coming to a focus, disperse again, so a blurred image is formed upon retina.
Correction of Myopia
In myopic eye, in order to form a clear image on the retina, light rays entering the eye must be divergent and not parallel.
Thus, myopic eye is corrected by using biconcave lens.
Light rays are diverged by concave lens before entering the eye.
Anisometropia
Anisometropia is the condition in which two eyes have unequal refractive power.
It is corrected by using different appropriate lens for each eye.
Astigmatism
Definition
Astigmatism is a condition in which the light rays are not brought to a sharp point upon retina.
It is the common optical defect.
When it is moderate, it is known as physiological astigmatism.
When it is well marked, it is considered abnormal.
For example, the stars appear as small dots of light to a person with normal eye.
But in astigmatism, the stars appear as radiating short lines of light (a = not; stigma = point).
Cause of 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, refractive power is different in different meridians.
Meridian with greater curvature refracts the light rays more strongly than 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.
Types of Astigmatism
Astigmatism is of two types:
- Regular astigmatism
- Irregular astigmatism
Regular Astigmatism
In this type of astigmatism, the refractive power is unequal in different meridians of lens because of alteration of curvature in one meridian.
But it is uniform in all points throughout the affected meridian.
Irregular Astigmatism
In irregular astigmatism, the refractive power is unequal not only in different meridians of lens but it is also unequal in different points of same meridian.
Correction of Astigmatism
Astigmatism is corrected by using cylindrical lens having the convexity in the meridian corresponding to that of lens of eye having a lesser curvature.
i.e., if horizontal curvature of lens is lesser, the person should use cylindrical glass lens with convexity in horizontal meridian.
Refractive Errors: Cause and Correction
| Type of Error | Cause | Correction |
|---|---|---|
| Myopia | Increase in anteroposterior diameter of the eyeball | Biconcave lens |
| Hypermetropia | Decrease in anteroposterior diameter of the eyeball | Biconvex 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 |
| Regular astigmatism | Refractory power of lens is unequal in different meridians but uniform in one single meridian | Cylindrical lens |
| 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 |
Presbyopia
Definition
Presbyopia is a condition characterized by progressive decrease in the ability of eyes to focus upon near objects with age.
It is due to the gradual reduction in the amplitude of accommodation.
Presbyopia starts developing after middle age and progresses as the age advances.
Vision in Presbyopia
In presbyopia, distant vision is unaffected.
Only the near vision is affected.
Near point is away from eye.
In presbyopia, the anterior curvature of lens does not increase during near vision.
So, light rays from near objects are not brought to a focus on retina.
Causes of Presbyopia
- Decreased elasticity of lens is because of the physical changes in lens and its capsule in elders. So, the anterior curvature is not increased during near vision.
- Decreased convergence of eyeballs due to the concomitant weakness of ocular muscles in elders.
Correction of Presbyopia
Presbyopia is corrected by using biconvex lens.
Quick Revision
Refractive Errors
- Hypermetropia
- Myopia
- Anisometropia
- Astigmatism
- Presbyopia
Hypermetropia
Decreased anteroposterior diameter of eyeball → image behind retina → corrected by biconvex lens.
Myopia
Increased anteroposterior diameter of eyeball → image in front of retina → corrected by biconcave lens.
Anisometropia
Unequal refractive power of two eyes → different appropriate lens for each eye.
Astigmatism
Light rays are not brought to a sharp point upon retina → corrected using cylindrical lens.
Presbyopia
Progressive decrease in ability to focus upon near objects with age → corrected using biconvex lens.