When fabricating lenses to fit a frame, which statement best explains why block placement may no longer be centered?

• A. The blocks are always at the lens geometric center
• B. The PD has no effect on lens centering
• C. The frame PD equals the patient PD
• D. The lens optical center is shifted to match the patient's PD, which is usually less than the frame PD

Answer: (D)

When lenses are fabricated to fit a frame, the goal is to place the lens’s optical center under the wearer’s pupil position so that looking straight ahead doesn’t induce unwanted prismatic effect. The frame’s PD—the distance between the centers of the two lenses in the frame—often differs from the patient’s PD. Because of this, the lens block is moved laterally during mounting so the optical center lines up with the patient’s PD within the frame. The result is block placement that isn’t centered on the lens. The statement that best fits this is that the optical center is shifted to match the patient’s PD, which is usually smaller than the frame PD. This decentration ensures the pupil aligns with the lens’s optical center in the frame.