Which statement accurately describes why the optical center may shift when lenses are cut to frame shape?

• A. The frame PD is always equal to the patient PD
• B. The block positions remain centered regardless of PD
• C. The optical center is kept at the center by the frame manufacturer
• D. The optical center is shifted to match the patient's PD, which is usually smaller than the frame PD

Answer: (D)

Aligning the optical center with the wearer's pupil is essential to avoid unwanted prism and ensure comfortable vision through every part of the lens. When a lens is cut to fit a frame, the frame’s interpupillary distance (frame PD) is typically larger than the patient’s PD. To place the lens’s optical center where the eye actually looks through the lens, the lens must be decentered toward the nose. This shifts the optical center to match the patient’s PD, which is usually smaller than the frame PD. If you kept the lens centered in the frame, the pupil wouldn’t align with the lens’s optical center, causing unwanted prism and distorted vision.