How is total decentration calculated?

• A. Frame PD minus patient PD
• B. Patient PD minus frame PD
• C. ED plus decentration
• D. 2 mm added to ED

Answer: (A)

When designing or fitting spectacles, decentration describes how far the lens centers must shift from the pupil centers so that the optical centers line up with the visual axes. The frame PD is the distance between the centers of the two lenses in the frame, while the patient’s PD is the distance between the centers of the pupils. To align both eyes, each lens is shifted by half the difference between these distances, so the total amount of decentration across both lenses equals the difference between frame PD and patient PD. In other words, total decentration = frame PD − patient PD. If the frame PD is larger, the lenses are decentered inward by half the difference for each eye (sum total equals the full difference); if the patient’s PD is larger, decentration would be in the opposite direction. This differs from measures like edge distance or other adjustments, which aren’t used to determine how far the optical centers must move to match the pupils.