These prescriptions take special time, care, and attention! The ability to accurately prescribe prism also differentiates an optical assistant from a true optician. We recently saw a patient who had glasses remade several times at a local optical shop, with no success!
from Review of Optometry:
Prism can be very useful when treating patients with symptomatic binocular vision disorders, but opinions vary widely about how to best determine the amount to prescribe. Indeed, common methods for determining prismatic prescriptions can result in different magnitudes of recommended prism for the same patient. This lack of consensus, along with rough guidelines and apparently contradictory rules of thumb, may make some clinicians uncomfortable prescribing prism and may even deter them for doing so.