In the AC/A formula, what is subtracted inside the parentheses?

• A. Near phoria - Distance phoria
• B. Distance phoria - Near phoria
• C. Near phoria + Distance phoria
• D. Distance phoria + Near phoria

Answer: (A)

AC/A is about how much vergence (accommodative convergence) you get for each diopter of accommodation. Clinically, you look at how a person’s phoria changes from distance to near. The part inside the parentheses is the difference between those phorias, specifically near phoria minus distance phoria. This difference represents the extra convergence the eyes must produce when shifting from looking at distance to looking at near, driven by the accommodation needed for near vision. If near phoria is more exo or more eso than distance phoria, the result reflects the direction and magnitude of that vergence change, and dividing by the change in accommodation demand (near minus distance) gives the AC/A ratio. Subtracting in the opposite order or adding would not correctly represent the vergence change tied to the accommodation shift.