Which criterion states that the compensatory fusional vergence reserve should be at least twice the demand of the phoria?

• A. Sheard's criterion
• B. Percival's criterion
• C. Both criteria
• D. Neither criterion

Answer: (A)

The main idea here is how much vergence the eyes can muster to counteract a latent misalignment and keep vision comfortable. Sheard's criterion states that the compensatory fusional vergence reserve should be at least twice the demand of the phoria. This creates a healthy margin so everyday tasks, especially sustained near work, don’t push fusion to the breaking point. For example, if a person has a phoria of 6 prism diopters, they should have about 12 prism diopters of compensatory fusional vergence available in the opposite direction to maintain single vision. If the reserve is less than twice the phoria, symptoms like eye strain or intermittent diplopia can occur because the system has little leeway to accommodate fluctuations or stress. Percival's criterion describes a different approach to judging vergence reserves and balance that does not specify this twice-the-demand rule, so the statement aligns with Sheard's criterion rather than Percival's.