Prism is required when the line of sight must change direction. A prescription or lens formula with prism specifies the amount and the direction of the base (thickest part) of the prism. The amount of prism is usually, but not always, the same in both eyes.
In prescriptions without prescribed prism, the goal is to align the optical center of the lens horizontally with the patient’s line of sight as measured by interpupillary distance (PD). The refraction part of the eye exam determines the binocular and prescription needs of the patient i.e., the centering, prism and prescription values from which lenses are created. In terms of their effects on vision, these represent the 1st (binocularity) and 2nd order effects (defocus in each of the principal meridians). This should result in a properly centered Rx with the correct sphere, cylinder and axis values as well as any prescribed prism required for the patient. Patients do not easily tolerate vertical prism differences, Prism imbalance tolerances of 2/3∆horizontally and 1/3∆vertically are generally acceptable. For patients with greater vertical prism imbalance who must look down to read, slab-off design or reading lenses should be considered. Individual tolerance varies, but a difference between the right and left lens of one prism diopter (1) or more of vertical prism can cause asthenopia, (eye strain) adaptation problems, reading difficulties and even diplopia (double vision). Patients with single-vision lenses and vertical prism imbalance usually learn to turn their heads to read through the lenses at a point closer to the optical centers.