During edging, the optical center is shifted to align with which measurement?

• A. The frame center
• B. The patient's PD, which is typically smaller than the frame PD
• C. The distance between the frame edges
• D. The frame width

Answer: (B)

The important idea is that the optical center of each lens should sit in front of the pupil, so the eye looks through the lens’s center without introducing prism. During edging, you shift the lens so its optical center lines up with the patient’s interpupillary distance. Since the patient’s PD is typically smaller than the frame PD, you move the lens inward (toward the nose) to place the optical center over the pupil position. Aligning with the frame’s overall width or with the frame center would misplace the optical center relative to the eye, causing unwanted prism and distorted vision. The frame measurements themselves don’t determine where the optical center should sit; it’s about matching the lens to the patient’s PD.