Astigmatism

Definition:
It is the condition in which an optical system has different foci for rays that propagate in two perpendicular planes. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances.

Types:
A: Based on axis of the principal meridians
1. Regular
- With the rule
- Against the rule
- Oblique
- Bioblique
2. Irregular

B. Based on focus of the principal meridians
1. Simple
- Myopic
- Hypermetropic
2. Compund
- Myopic
- Hypermetropic
3. Mixed

Symptoms:
1. Blurring of vision
2. Squinting
3. Asthenopic symptoms
4. Headache

Treatment:
Astigmatism may be corrected with eyeglasses, contact lenses, or refractive surgery.
Spherical lenses correction along with a cylindrical lens for correction of the residual error is used.