AC/A Ratio

Accommodative Convergence to Accommodation Ratio

Ratio = Change in deviation / change in accommodation
Fixation distance is fixed.
Measure deviation with and without a modifying lens
The lens is changing the amount of accommodation
Stimulate Accommodation
- Measure alignment fixating at 6 meters * Remeasure with a -1.00 lens before both eyes
- Calculate the difference between the two alignment measures to get the AC/A ratio
Relax Accommodation
- Measure alignment fixating at 0.33 meters
- Remeasure with a +3.00 lens over both eyes
- Divide the difference in the two alignment measures by 3 to get the AC/A ratio
Results
- Average: 3.7
- Range: 0.9 - 9.8
- Low: 0 - 2.0
- Normal: 2.5 - 5.0
- High: > 5.0

Assumes that convergence is wholly due to accommodation and ignores tonic convergence or proximal convergence (fusion) mechanisms
- AC/A = Pupil Distance + (Dn - D0/ D)
  - Dn = alignment measurement at near
  - D0 = alignment measurement at distance
  - D = diopters of accommodation
Results
- Average: 4.4
- Range 2.7 - 7.7

Measure the alignment at a set distance with the following lenses: +3.00, +1.00, -1.00, -3.00
Graph values and determine slope

Gradient Method