What is the A 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: (A)

The A value is the horizontal width of one lens—the distance across the lens from edge to edge at its widest point. This measurement, often seen in frame sizing like 50-20-135, tells you how wide the lens itself is so the frame can accommodate the wearer's eyes and sit properly on the face. It is different from the vertical height of the lens, which is measured separately, and from any center-to-center distances or other dimensions. Knowing the horizontal lens width helps ensure the frame looks balanced and fits comfortably.