What is the formula to determine decentration per lens from frame PD and patient PD?

• A. (Frame PD - Patient PD)/2
• B. (Patient PD - Frame PD)/2
• C. Frame PD + Patient PD
• D. Frame PD - Patient PD

Answer: (A)

Determining how much each lens must shift relies on comparing the frame PD to the patient’s PD. The frame PD is the distance between the frame’s lens centers, while the patient PD is the distance between the centers of the pupils. The difference between these two distances represents how much wider the frame is than the patient’s eyes, and this extra width must be split equally between the two lenses. So the amount each lens must decenter is half of that difference: (Frame PD − Patient PD) / 2. If the frame PD is larger, the lenses move outward; if smaller, the movement is inward. For example, frame PD 66 mm and patient PD 62 mm yields 2 mm per lens. The other formulas either omit dividing by 2 or use the wrong order, which doesn’t reflect the per-lens decentration.