What is the ED value on a spectacle frame?

• A. Longest horizontal portion of a lens
• B. Longest vertical portion of a lens
• C. Longest diameter of a lens
• D. The distance from geometric center to geometric center

Answer: (C)

The ED value represents the maximum size of the lens, essentially its longest diameter. It is the greatest distance between two points on the lens, typically measured through the center. This matters because it helps determine if a lens will fit within a frame and how much of the lens will be exposed or edge-thick, especially for larger or irregularly shaped lenses. For a circular lens, ED equals the diameter; for other shapes, it's simply the maximum distance across the lens. That’s why the ED is best described as the longest diameter of a lens.