Which one is Against the Rule astigmatism
Astigmatism:- Definition:- When the rays are focused at different points due to unequal refraction in different meridians. With the Rule Astigmatism:- * Veical meridian is more curved than the horizontal meridian * Eg : Normal eyes (0.25 D), pterygium * 90deg axis >>>> 180deg axis (+ lens at 90deg/- lens at 180deg) Against the Rule Astigmatism:- * When the horizontal meridian is more curved than the veical meridian * 180deg>>>>90deg axis (- lens at 90deg /+ lens at 180deg) * Eg: Post op astigmatism -suture related Oblique Astigmatism :- * The two principal meridias are at right angles to each other ; but not the veical and horizontal meridian like 45deg and 135deg Bioblique Astigmatism:- * The two principal meridias are not at right angles and also not the veical and horizontal meridia.eg : 30deg & 100deg Reference :- Parson's Diseases of the eye 22nd edition; page:- 53,76
- 2.00 x 90deg
- 1.50 x 180deg
+ 2. 00 x 90deg
+ 1.50 x 180deg
Optics and refraction