The measurement from the geometric center to the furthest edge doubled is called which term?

• A. Effective Diameter
• B. Frame PD
• C. A Measurement
• D. Geometric Center

Answer: (A)

The main idea is that a distance from the center of a circle to its edge is the radius, and doubling that radius gives the full width across the circle, its diameter. In ophthalmic optics, the term used for that diameter—derived from a center-to-edge measurement—is the effective diameter. It’s described as “effective” because it reflects a diameter obtained from a central measurement rather than a direct edge-to-edge measurement, and it’s a standard term used in lens and frame sizing. The other options don’t describe a width across the circle: the geometric center is just the center point, a vague “A Measurement” isn’t specific, and frame PD refers to the distance between the centers of the pupils in a frame, not the circle’s diameter.