AC/A Ratio
Optics and Optometry
Both the heterophoria and the gradient method, and why they give different answers for the same patient.
How it is calculated
- The AC/A ratio is how much convergence, in prism dioptres, each dioptre of accommodation brings with it.
- Heterophoria method: AC/A = interpupillary distance in centimetres + (near phoria − distance phoria) ÷ the accommodative stimulus at the near test distance.
- Gradient method: AC/A = (phoria without the lens − phoria with it) ÷ the lens power, measured at one fixed distance.
- Esophoria counts as positive and exophoria as negative throughout. Getting that convention backwards is the commonest source of a wrong answer.
- The heterophoria method reads higher for the same patient, because moving the target closer also recruits proximal convergence. The gradient method holds the distance fixed and so excludes it.
- The usual range is about 3 to 5 prism dioptres per dioptre. A high ratio underlies convergence excess and divergence excess; a low one underlies convergence insufficiency and divergence insufficiency.
References
- Standard binocular vision texts — see Scheiman M, Wick B, Clinical Management of Binocular Vision, for both methods and their normative values.
About these tools
OcuLearning Clinical Tools is part of an educational platform. These calculators are provided for learning and reference, and are not medical advice, a diagnosis, or a treatment recommendation.