Javal's rule

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Javal's rule is a mathematical formula used to estimate astigmatism based on keratometry readings. [1] 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.

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LASIK Corrective opthalmological surgery

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Presbyopia

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Eyeglass prescription Order written by an eyewear prescriber

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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. The term comes from the Greek α- (a-) meaning "without" and στίγμα (stigma), "a mark, spot, puncture".

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References

  1. Schwartz, S. Geometric and Visual Optics. pg 219-221. McGraw-Hill. 2002.