AC/A ratio (accommodative convergence/accommodation ratio) can be measured using either the gradient method or the heterophoria method.

A patient has a 12^D esophoria with a -2.00D lenses R and L and 4^D exophoria with +2.00D lenses R and L. 

a. Which method can be used to calculate the AC/A ratio?

b. What is the AC/A ratio in this patient?

 

Answer to aa. Gradient method. 
For the heterophoria, one needs to know the interpupillary distance.

 

Answer to b
b. For the gradient method, the following formula is used:
AC/A = 
(heterophoria with minus lenses - heterophoria with plus lenses)/ Difference in power of lenses used
= 12-(-4)/2-(-2)
= 16 / 4
= 4:1