Another way to measure frame PD is:

• A. PD = A + B
• B. FPD=A+DBL
• C. PD = A - DBL
• D. FPD = B + DBL

Answer: (B)

Frame PD is the distance between the centers of the two lenses in the frame. A practical way to get this is by using the frame’s own measurements: the width of one lens (A) and the distance between the lenses (DBL, the bridge). Since each lens center sits at the middle of its lens, the center-to-center distance is half a lens width from the left center to the inner edge, plus the bridge, plus half a lens width from the inner edge to the right center. That adds up to A/2 + DBL + A/2, which simplifies to A + DBL. So the frame PD equals A plus DBL. This works best when the pupils are centered in the lenses; otherwise it’s an approximation.