To measure lens thickness, where should the calipers be placed?

• A. At or near the optical center.
• B. At the edge of the lens.
• C. Near the frame rim.
• D. At the temple.

Answer: (A)

Measuring lens thickness is done at the optical center because thickness varies across a lens and the center is the reference point through which the wearer looks. The optical center gives the standard, representative thickness used in manufacturing and for weight and edge-thickness considerations. If you measure at the edge, frame rim, or temple, you’ll capture edge or bevel thickness, which doesn’t reflect what the wearer sees through the pupil. So placing calipers at or near the optical center provides the most consistent, meaningful measurement of lens thickness.