When lenses are cut to the shape of the frame, why aren't the blocks at the center of the lens anymore?

• A. The lens optical center has shifted to match the patient's PD, which is usually less than the frame PD
• B. The blocks are repositioned to the center of the frame
• C. The frame PD has no bearing on lens centering
• D. The blocks stay at the center due to frame PD being larger

Answer: (C)

The key idea is that lens centering is driven by the wearer’s pupil alignment, not by the frame’s geometry. When you edge a lens to fit the frame, you still want the optical center of the lens to line up with the wearer’s line of sight. If the frame’s PD doesn’t match the patient’s PD, the finished lens must be decentered relative to its geometric center so the pupil taps the correct part of the lens. That means the blocks won’t sit at the literal center of the lens because centering is about the optical axis over the pupil, not about the frame’s center. The frame PD itself doesn’t dictate where the lens’ center sits, so centering is not controlled by the frame PD.