Javal's rule
Encyclopedia
Javal's rule is a mathematical formula used to estimate astigmatism
based on keratometry readings. The estimate is useful for high cylinder measures, generally over -2.00 diopters.
The estimate is found by multiplying the difference in power between the two meridians by 1.25 and factoring in the average lenticular astigmatism, which is -0.50 x 090.
Astigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. 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...
based on keratometry readings. The estimate is useful for high cylinder measures, generally over -2.00 diopters.
The estimate is found by multiplying the difference in power between the two meridians by 1.25 and factoring in the average lenticular astigmatism, which is -0.50 x 090.