Refraction 3 k n jha 31.08.16

1. Refractive Error 3

2. Learning Aim
• Definition of Astigmatism
• Conoid of Strum
• Types of astigmatism
• Correction of astigmatism
• Aphakia and its correction

3. Astigmatism
• In astigmatism, light rays do not focus to a single
point.
• Regular astigmatism
• Irregular astigmatism
• With-the-rule astigmatism
• Against-the-rule astigmatism

4. • Regular astigmatism: the principal meridians of the
astigmatism have constant orientation across the pupil,
and the amount of astigmatism is same at every point.
• Irregular astigmatism: The orientation of the principal
meridians or the amount of astigmatism changes from
point to point across the pupil.
• Regular astigmatism is correctable by cylindrical lenses.

5. Regular astigmatism: Types
• With the rule astigmatism : The vertical meridian is
steepest. The correcting plus cylinder is used at or near axis
90°.
• Against the rule astigmatism : The horizontal meridian is
steepest. Correcting plus cylinder is used at or near 180°.
• Oblique astigmatism: Principal meridians do not lie at or
close to 90° or 180°.

6. Conoid of Strum
• The complicated geometrical envelop of pencil
of light rays refracted by a spherocylindrical
lens is called conoid of Strum.

7. Formation of Circle of Least Confusion

8. Conoid of Strum

9. Conoid of Strum
• Compound hypermetropic astigmatism
• Simple hypermetropic astigmatism
• Mixed astigmatism
• Simple myopic astigmatism
• Compound myopic astigmatism

10. Astigmatism
• Symptoms: blurred vision , eye strain.
• Complication: Amblyopia
• Correction: Cylindrical lens
• Surgery: Astigmatic LASIK

11. Correction of Astigmatism
• Glasses
• Contact lenses
• Corneal surgeries
• Intraocular lenses

12. Aphakia and pseudophakia
• Aphakia is absence of crystalline lens from the
pupillary area.
• Pseudophakia refers to an eye implanted with
intraocular lens in place of the natural lens

14. Aphakia
• Etiology: Surgical, traumatic.
• Refractive status: highly hypermetropic.
• Correction: Intraocular lens, contact lens, and
refractive surgery, and spectacle lenses.

15. • Magnification
• Roving ring scotoma
• Jack-in-the-box phenomenon
• Problem of binocular vision
Disadvantages of correction of Aphakia
with Glasses

16. Points to Remember
• Definition, symptoms, optics, and types of
astigmatism
• Conoid of Strum
• Correction of astigmatism
• Problems of correction of aphakia with glasses