Predicting aniseikonia in anisometropia.

In estimating the amount of aniseikonia that may be present in any given case of anisometropia, it is usual to apply the "rule of thumb" approach. The "rule" states that for each diopter of anisometropia, the amount of aniseikonia to be expected is 1 1/2 to 2 percent. The rule is based upon an average value computed for theoretical amounts of aniseikonia in many possible situations that can occur, some of which have little likelihood of being found in everyday clinical practice. Applied to many cases of anisometropia that are common, the rule leads to extreme overstimates and is grossly misleading, often resulting in the mishandling of patients. In this paper, a number of typical cases are dealt with in the refutation of the use of "the rule."