Which criterion generally requires a larger compensatory vergence reserve relative to the phoria demand?

• A. Sheard's criterion
• B. Percival's criterion
• C. Both yield the same requirement
• D. Neither criterion

Answer: (A)

When we think about keeping both eyes aligned, the amount of fusional (compensatory) vergence a person can use relative to how much misalignment they have matters. Sheard's criterion says that for exophoria, the convergence reserve needed should be at least twice the phoria magnitude. That set-up demands a relatively large reserve to keep vision comfortable during typical tasks. Percival's criterion, by contrast, essentially requires the smaller of the available fusional vergence ranges to exceed the phoria, which is a smaller requirement than doubling the phoria. So, in general, Sheard's criterion calls for a larger compensatory vergence reserve relative to the phoria demand. For example, if exophoria is 6 prism diopters, Sheard's would favor about 12 prism diopters of convergence reserve, whereas Percival's would be satisfied with a reserve around the phoria level.