Normal CA/C ratio is which of the following?

• A. 1.00/6 diopters
• B. 0.50/6 diopters
• C. 0.25/6 diopters
• D. 0.75/6 diopters

Answer: (B)

CA/C ratio describes how much accommodation changes in response to a given amount of convergence. When the eyes converge, the lens naturally tends to adjust focus to keep the image clear, so this ratio measures that cross-link between vergence and accommodation. The normal coupling is about 0.50 diopters of accommodation for every 6 prism diopters of convergence. In other words, for a convergence change of 6 Δ, you’d expect roughly half a diopter of additional accommodation. Values can vary slightly between individuals, but 0.50/6 Δ is the standard reference. If the ratio were much larger or smaller, it would indicate atypical cross-link strength between accommodation and convergence.