When adjusting seg height for a new frame, the difference in A or B measurements is halved. The halved difference is added or subtracted depending on whether the new frame is bigger or smaller.

• A. Compare A measurements to each other or B measurements to each other
• B. Compare A measurements to B measurements
• C. Use the full difference
• D. Do nothing

Answer: (A)

Seg height changes are driven by how much the frame size itself has changed, and you apply that change using the same dimension on both frames. A and B describe the same directional measurements for the frame, so you compare A measurements to A measurements or B measurements to B measurements. Once you find the difference, you take half of it and adjust the seg height in the appropriate direction (add if the new frame is larger, subtract if it’s smaller). This keeps the progressive lens’ corridor aligned with the wearer’s eyes as the frame size changes. For example, if the new frame’s A is larger by 4 mm, you’d add 2 mm to seg height; if it’s smaller by 4 mm, you’d subtract 2 mm.