What is the B value on a spectacle frame?

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

Answer: (B)

The B value is the vertical height of the lens—the distance from the top edge to the bottom edge of each lens opening. This measurement shows how tall the lens is and helps ensure the frame can physically fit the lens and place the pupil appropriately, especially with lenses that require more vertical room like progressives. The other descriptions refer to horizontal width, an uncommon diagonal measure, or the distance between the centers of the lenses, none of which define the vertical height.