Effect of asymmetry on calculations of convergence in prism diopters

Calculations of convergence in units of prism diopters are based on an assumption that convergence is symmetrical, that is, the visual axes intersect on the median plane. The effect of departure from symmetrical convergence is shown to produce systematic errors in calculations of convergence distance, if the convergence angle is constrained to be constant. On the other hand, if the convergence distance is constrained to be constant, departure from symmetry results in a systematic reduction in the convergence angle. Because these errors are disregarded in the specification of convergence in prism diopters, care should be taken in using these units for larger amounts of asymmetrical convergence.