Under Sheard's criterion, an exophoric deviation uses which fusional vergence measure at blur/break point?

• A. BI blur/break point
• B. BO blur/break point
• C. BI and BO blur/break points
• D. None of the above

Answer: (B)

Sheard's criterion relates how much fusional vergence a person can muster to their phoria, and for an exophoric deviation the necessary compensating vergence is convergence. Convergence is positive fusional vergence, which is tested with base-out prisms. The relevant measure under this criterion is the positive fusional vergence available at the point of blur or break, i.e., the base-out blur/break point. This is why base-out blur/break point is the best indicator for evaluating an exophoric deviation.