CA/C ratio measurement requires adding how many prism diopters of demand?

• A. 3 prism diopters of demand must be added.
• B. 12 prism diopters of demand must be added.
• C. 1 prism diopter of demand must be added.
• D. 6 prism diopters of demand must be added.

Answer: (D)

CA/C ratio measures how much eyes converge for a given change in accommodation. To measure it reliably, you establish a fixed vergence demand with prisms so the eyes must converge by a known amount, then you change the accommodative demand with lenses and observe how much additional convergence occurs. Six prism diopters is the standard amount used for this fixed demand because it provides a clear, moderate stimulus: it’s large enough to yield a measurable vergence response but not so large that it complicates the measurement with excessive fusional adjustments. Using this fixed 6 pd demand lets you compute the CA/C ratio as the change in convergence (in prism diopters) per diopter of accommodation change.