2. • Astigmatism : Incident parallel rays of
light are brought to a line of focus instead
of a single point focus due to inequality in
curvature of different meridians.
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5. 1. Simple astigmatism – Here one img is located
in the retinal plane and based on the location of
the other img it may be:
a) Simple myopic astigmatism : the other img is
located in front of the retina.
b) simple hypermetropic astigmatism : the other
img is located behind the retina.
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6. 2. Compound astigmatism : here both the images
are either in front of the retina or behind the
retina & designated as:
a) Compound myopic astigmatism
b) Compound hypermetropic astigmatism
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7. 3. Mixed astigmatism : here one img is formed in
front of the retina & other img is located behind
the retina.
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8. Based on relative position of image of distant
object on the retina,
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9. Based on angle b/w the two principal
meridians of max & min curvature
a. Regular astigmatism : here two principal
meridians of max & min curvature are
positioned at right angles,
b. Irregular astigmatism : here two principal
meridians of max & min curvature are
positioned to each other at angles other than
90°.
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11. Based on Aetiology
• Astigmatism with the rule : usually vertical
corneal meridian is more curved than
horizontal,
• Astigmatism against the rule : here the corneal
curvature in horizontal meridian is greater than
vertical.
• Oblique astigmatism : here the ROC of max &
min curvature are aligned at 90° to each other
but the two principle meridians are neither near
horizontal nor near vertical.
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13. • Lenticular astigmatism : it is due to,
curvature
Refractive index
Displacement of the refractive element of
Crystalline lens,i.e.subluxation.
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14. STURM'S CONOID
• It is an optical condition in which refractive
power of cornea and lens is not the same in all
meridians , therefore instead of single focal
point there are two focal points separated by
focal interval, this is called sturm's conoid.
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19. References,
• Bhattacharyya B, Bhattacharyya D. textbook of
visual science & clinical optometry,2009.
• National eye institute of health.
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