After shaping lenses to the frame, which factor most directly explains why the center of the lens may not coincide with the optical reference center?

• A. The lens thickness
• B. The material of the lens
• C. The optical center shifts to match the patient's PD
• D. The frame width

Answer: (C)

The key idea is that the optical center must line up with the patient’s line of sight, which is set by interpupillary distance. When lenses are fitted into a frame, we decenter the lens so that the optical center sits where the pupil looks straight through. This decentration shifts the optical center from the lens’s geometric or original reference point, so the center of the lens no longer coincides with the optical reference center. Lens thickness or material don’t drive this shift, and frame width only influences how much decentration is needed rather than causing the alignment itself. The practical effect is that the optical center moves to match the patient’s PD to minimize unwanted prismatic effects.