AC/A ratio describes which relationship?

• A. Convergence per diopter of accommodation
• B. Accommodation per diopter of convergence
• C. Vergence per diopter of accommodation
• D. Prism diopter per degree of rotation

Answer: (A)

The AC/A ratio measures how much the eyes converge for each diopter of accommodation. It expresses the amount of vergence (specifically convergence) that accompanies a given change in focusing power, usually written as prism diopters of vergence per diopter of accommodation. So when you increase accommodation by one diopter to focus on a nearer object, the eyes typically converge by a certain amount, and that relationship is what the AC/A ratio quantifies. This concept helps explain why near tasks can shift eye alignment and underpins issues like accommodative esotropia and how lenses influence vergence. The other phrasing would either invert the relationship (accommodation per diopter of convergence) or use unrelated units, which isn’t how this ratio is defined.