The fusional demand produced by anisometropic spectacles can be determined from the parameters of the lenses. This study compares several equations designed for this purpose. Two of the equations, recently described in other articles, contain small angle approximations and hence become less accurate for large angles of gaze eccentricity. Another two equations, introduced in this article, contain no approximations and can be used for accurate results at large gaze eccentricities. Results for all four equations are compared through a range of eccentricities of gaze and for various spectacle magnifications and typical examples of spectacle corrections. The conventional method of using Prentice's rule for finding interocular prismatic differences is also included in the comparisons. For eccentricities corresponding to the near visual point, the equations produce similar results, except Prentice's rule, which produces very large errors. Beyond the reading point eccentricity, results for the equations containing small angle approximations deviate significantly from each other and from the results obtained with the equations containing no approximations. It is recommended that the latter equations be used for large eccentricities.
Arnulf Remole (Mon,) studied this question.
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