Centration and the unit conversion
A laboratory exercise supplies a frame boxing width of 54 mm and bridge distance of 18 mm. The right monocular pupillary distance is 32 mm. Assume a symmetric frame reference for this exercise and a single-vision lens whose relevant meridional power is+5.00 D. The learner must first find the frame center distance, then the displacement between the right geometric center and the pupil, and finally the magnitude of prism associated with a 4 mm offset under Prentice’s rule. A colleague inserts 4 directly into the formula even though it requires centimeters. The exercise is intended to test geometry and unit handling, not establish a universal fitting instruction. Actual design, face position and monocular measurements still need practical verification before a clinical order is accepted.
1. Compute the frame relationship.
Read the worked answer
The frame center distance is 54+18=72 mm. Half is 36 mm, so aligning the right center to a 32 mm monocular measure requires 4 mm nasal decentration in this symmetric exercise.
2. Compute the prism magnitude under the supplied rule.
Read the worked answer
Four millimeters equals 0.4 cm. Multiplying 0.4 by 5.00 gives 2.0 prism diopters in magnitude. Base direction requires the actual lens sign, viewing geometry and stated convention; do not substitute millimeters directly.