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Refractive errors

The eye with normal refractive power is

called emmetropia. 

 Any abnormality in normal refractive power called refractive errors.

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ERRORS OF REFRACTION

There are mainly four type of errors of refraction

1. Ametropia

-myopia or short sightedness

-hypermetropia or long sightedness

2. Anisometropia

3. Astigmatism

4. Presbyopia

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1 AMETROPIA

 Any deviation in the refractive power from

normal condition resulting in inadeqate

focusing on ratina is called ametropia.

There are two type of ametropia :

• Myopia

• Hypermetropia

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Myopia

Definition• Myopia is the defect characterized by theinability to focus on distance object.

• It is also called short sightedness because the

person can see near objects clearly but not thedistant object.

• Cause• It is due to the anteroposterior diameter of 

eyeball is abnormally long. Therefore the imageis brought to a focus a little infront of ratina.

• Incase of lens defect the lens the refractivepower of lens is increase.

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Characteristic features

• Person cannot see distance object called near 

sightedness or short sightedness.

• Far point of vision is at a definite distance from

the eye. It may be one meter in severe myopia.it

is normally 6 meteres.• Near object is clearly visible.

Correction

• in can be corrected by the concave lens.

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HypermetropiaDefinition

• Hypermetropia is the defect characterized bythe inability to focus on near object.

• It is also called long sightedness because the

person can see far objects clearly but not thenear object.

Cause

• It is due to decrease the anteroposterior of the

eye ball.

• Incase of lens defect the lens the refractiv power 

of lens is decrease.

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Characteristic features

• Person cannot see near object called far 

sightedness or long sightedness.• Far object is clearly visible.

Correction

• in can be corrected by the convex lens.

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2. Anisometropia

It is the condition in which the two eye have

unequal refractive power.

Correction

It is corrected by the separate lens biconcave or biconvex.

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3. Astigmatism

Definition:

   Astigmatism is the condition in which the light

rays not brought to a sharp point upon retina.

• This defect present in all eyes, but it is moderate it is

know as physiological astigmatism.Causes

• The light rays passes through all meridians of a lens in

a normal eye. Lens has approx. same curvature in all

meridians. So, the light rays are Refracted almostequally in all meridians and brought to focus.

• If curvature is different meridians vertical,horizantal and

oblique the refractory power Is also different in different

meridians.

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Correction

It is corrected by the cylindrical glass lens.

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4. PresbyopiaDefinition

Presbyopia is this condition caused by progressivediminished ability of eye to focus on near object with age.It is due to the gradual reduction of the amplitude of accommodation.

Causes• Decrease the elasticity of lens is because of the physical

changes in lens and its capsule during old age. So,theanterior curvature is not increase during near vision.

• Decrease convergence of eye ball due to the concomitantweakness of ocular muscle in old age.

• Denaturation of lens protein.

Correction 

It is corrected by the biconvex lens 

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