When measuring DBL on lenses that are in a frame, what should you do before recording the measurement?

• A. Subtract 0.5 mm from both sides
• B. Add 0.5 mm to both sides
• C. Subtract 1 mm from one side
• D. Record as is

Answer: (A)

DBL is the horizontal distance between the inner edges of the two lenses. When you measure with lenses mounted in a frame, you need to compensate for the lens edge thickness (bevel) so the value reflects the true space available for the finished lenses. Subtracting 0.5 mm from each side does exactly that—it accounts for that edge thickness on both sides and yields the DBL you’ll use for lens edging and mounting. Recording the measurement as-is would overestimate the space, and changes like adding to one side or subtracting a larger amount would misbalance the measurement.