How is the intermediate add power of a trifocal lens determined?

How is the intermediate add power of a trifocal lens determined?

• A. 1/2 the ADD power
• B. Full ADD power
• C. 1/3 the ADD power
• D. ADD power minus 1.00 D

Answer: (A)

In a trifocal, the three segments are designed to cover distance, intermediate, and near vision. The distance portion has no added power, the near portion uses the full ADD to bring close objects into focus, and the intermediate portion is given roughly half of that ADD to provide mid-range vision without compromising either distance or near. This half-ADD level balances clarity at computer/arm’s-length distances (about 60–80 cm) with sharp distance and near vision. For example, if the near ADD is +2.50 D, the intermediate segment is about +1.25 D above distance. Using the full ADD for intermediate would blur distance and mid-range, while using too little would make mid-range vision fuzzy.

In a trifocal, the three segments are designed to cover distance, intermediate, and near vision. The distance portion has no added power, the near portion uses the full ADD to bring close objects into focus, and the intermediate portion is given roughly half of that ADD to provide mid-range vision without compromising either distance or near. This half-ADD level balances clarity at computer/arm’s-length distances (about 60–80 cm) with sharp distance and near vision. For example, if the near ADD is +2.50 D, the intermediate segment is about +1.25 D above distance. Using the full ADD for intermediate would blur distance and mid-range, while using too little would make mid-range vision fuzzy.