If the PD is smaller than the frame PD, what effect does this have on lens centering after shaping?

• A. The optical center remains at the lens geometric center
• B. The blocks move to the center of the frame
• C. The frame PD and lens PD become equal
• D. The optical center shifts to match the patient's PD

Answer: (D)

When lenses are centered for a wearer, the optical centers should sit directly over each pupil when the frame is worn. If the frame PD is larger than the patient’s PD, the pupils are closer together than the centers of the two lenses in the frame. To make the optical centers align with the actual pupil positions, each lens is decentered toward the bridge (nasal direction). In other words, the optical centers move inward to match the patient’s PD. The result is that the centers are no longer at the lenses’ geometric centers, and the frame PD and the lens PD do not become equal by shaping alone. This adjustment ensures proper alignment and minimizes unwanted prismatic effects.