Determining exact prismatic deviations in spectacle corrections

For nearly 100 years, Prentice's rule has been used to determine the prismatic effect and prismatic differences at peripheral points in the binocular spectacle field. However, calculations show that the current application Prentice's rule is valid only for a single lens and only if that lens is considered as a thin lens approximation. When center thickness and base curves are taken into account, Prentice's rule is no longer valid. In addition, when the prismatic difference in an off-axis region of the lenses is determined with the assumption that both eyes look through the lenses at identical distances from their optical centers, the errors are further compounded. If the eyes are to maintain fusion, this assumption is optically impossible and will produce incorrect results. It is also shown that Prentice's rule cannot be applied to iseikonic corrections to find differential prismatic effects. The article demonstrates how exact prismatic effects can be determined from the spectacle-generated images and the angles they subtend at the ocular rotation centers.