A vertex-distance calculation with a boundary

A fictional optical exercise supplies a thin spectacle lens of -5.00 diopters located 12 millimeters in front of the corneal plane. The task is to calculate the equivalent power at the cornea using Fc = Fs / (1 - d x Fs), with d expressed in meters and the stated signs retained. The exercise deliberately ignores a tear lens, lens thickness, fit, rotation, accommodation and other clinical variables. One learner substitutes 12 rather than 0.012 into the formula, while another calculates a value near -4.72 diopters but presents it as a complete contact-lens prescription. The instructor separates the valid optical arithmetic from a real fitting and visual assessment.