Rule proposed by Sheard to establish whether a patient is going to experience discomfort in binocular vision. It states that the amount of heterophoria should be less than half the opposing fusional convergence in reserve. If the criterion is not met, appropriate prisms, spherical lenses or visual training can be used. If a patient has 10 Χof exophoria, the positive fusional vergence should be at least 20 Χto satisfy this criterion. See relative convergence; binocular vision single zone of clear.