During lens edging, the optical center alignment is influenced by the relationship between patient PD and frame PD; what is the typical scenario?

• A. It remains at the geometric center of the lens
• B. It is aligned with the frame center
• C. It is shifted to match the patient's PD, which is usually smaller than the frame PD
• D. It is determined solely by the bridge width

Answer: (C)

The main idea is to place each lens’s optical center where the wearer’s pupil will look through it when the glasses are worn. This means matching the lens centers to the patient’s pupillary distance (PD), not just sitting at the lens’s geometric center or the frame’s center.

In practice, most frames have a frame PD that is larger than the patient’s PD. To align the optical centers with the eyes, each lens is decentered inward toward the nose by roughly half the difference between frame PD and patient PD. This positions the optical center at the same horizontal plane as the patient’s pupil, minimizing prismatic effects and ensuring proper, comfortable vision through the lenses.

So, the typical scenario is that the optical center is shifted to match the patient’s PD, which is usually smaller than the frame PD.