1
Q
Astigmatism Definition
A
= difference in power between two principal (perpendicular) meridians
= variation in focusing distance with meridian
2
Q
Toric Surfaces
A
Have different curvatures along two perpendicular meridians
3
Q
3 Classifications of toric surfaces
A
- ring torus
- barrel
- capstan
4
Q
The ‘Cyl’
A
- cylindrical lenses are Plano in one meridian
- axis marking is along the Plano direction
- labelled power is effective along a direction perpendicular to the cylinder axis
5
Q
WTR
A
=with the rule of astigmatism
- steepest curvature in vertical meridian
- negative cyl axis 180 +- 15
6
Q
ATR
A
=against the rule astigmatism
- steepest curvature in horizontal meridian
- negative cyl axis 90+- 15
7
Q
Effect of Astigmatism on Vision
A
- qualitative effect on vision
- effect of axis
- distortion of shapes
8
Q
Disc of Least Confusion (DLC)
A
=circular blur patch located dioptrically midway bet the astigmatic foci
- circular blur is the best approximation to point object
9
Q
Best Vision Sphere (BVS)
A
- spherical correction in the spectacle plane that places the disc of least confusion on the retina for distance vision
Visual Optics
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