accommodative convergence/accommodation (AC/A) ratio
Also called AC/A, AC/A ratio
The accommodative convergence/accommodation (AC/A) ratio is the amount of convergence that occurs reflexly with a change of accommodation of 1 diopter.1 The ratio is expressed as accommodative convergence in prism diopters divided by the accommodative response in diopters.2 The expected value is 4:1, with a standard deviation of 2.3
The AC/A ratio sets how the near phoria differs from the distance phoria. A high ratio produces more eso or less exo at near, and a low ratio produces less eso or more exo at near.3 Near esophoria with a high AC/A ratio generally responds well to plus lenses. The same esophoria with a normal or low ratio calls for prism, vision therapy, or both.3
Clinical tests control the stimulus to accommodation, but the accommodative response is generally about 10% less than the stimulus. A gradient ratio calculated from the stimulus therefore tends to underestimate the AC/A ratio.3
More detail
The calculated method compares the distance and near phorias, using , with esophoria entered as plus and exophoria as minus.3 The gradient method measures the near phoria again through added lenses and takes the change in phoria per diopter of added power.1,3 The calculated ratio is usually the larger of the two, because proximal convergence raises the near phoria, while the gradient method holds proximal cues constant.1,3 The gradient ratio better predicts the effect of a refractive correction on the deviation at a given distance.1
Sources (3)
- Pickwell's Binocular Vision Anomalies, ch. 2
- Brooks and Borish, System for Ophthalmic Dispensing, ch. 19
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 1
Reviewed October 2026
Related terms
Nearby in the glossary
- absorptive lens
- accommodation
- accommodative convergence
- accommodative convergence/accommodation (AC/A) ratio
- accommodative esotropia
- accommodative insufficiency
- adduction