When lenses are cut to the shape of the frame, the optical center is typically aligned with which reference?

• A. The geometric center of the lens
• B. The frame PD
• C. The patient's PD
• D. The bridge width

Answer: (C)

The key idea is that the optical center should sit where the eye’s line of sight passes through the lens without prismatic deviation. When lenses are cut to the frame, you place the optical centers to correspond with the patient’s PD, so each eye’s visual axis aligns with the lens’ optical center in primary gaze. This minimizes unwanted prism and distortion as the wearer looks straight ahead or moves their head.

Aligning to the frame PD can be misleading because the frame’s geometry—wrap, pantoscopic tilt, and how the frame sits on the nose—can shift where the pupil actually lines up with the lens. The patient’s PD is the true reference for where the eyes want to look straight through the lens, whereas the frame PD is a measurement of the frame itself, not of the eyes. The geometric center of a lens is not guaranteed to coincide with the pupil, especially in non-symmetric or progressive lenses, and the bridge width is just a frame dimension, not a determinant of optical alignment.