If the interocular power difference is 2.0 D, approximately how much aniseikonia is expected according to the 1% per diopter rule?

• A. 0%
• B. 1%
• C. 2%
• D. 3%

Answer: (C)

The central idea is that when the two eyes don’t have the same refractive power, the images each eye receives are magnified differently, creating aniseikonia. A handy rule of thumb used in practice is that the perceived image size difference is about 1% per diopter of interocular refractive power difference. So with a 2.0 diopter difference, the approximate aniseikonia would be 2% (2 D × 1% per D). This is an estimate, and actual perceptual differences can vary with distance, lens design, and fitting, but 2% is the best quick approximation here.