For a patient with RX 1.75 -0.50 x 120 +2.00 ADD in a trifocal, what is the RX through the intermediate portion?

• A. -0.50 -0.75 x 120
• B. -0.75 -0.75 x 120
• C. -0.75 -0.50 x 120
• D. -1.00 -0.50 x 120

Answer: (C)

In a trifocal, the three viewing zones share the same cylindrical correction but use different spherical powers: distance, intermediate, and near. The cylinder and its axis stay the same across zones, while the sphere changes by the amount allocated to that zone.

Here, the distance Rx is +1.75 diopters with a -0.50 diopter cylinder at 120 degrees. For the intermediate zone, we use the intermediate power by reducing the distance sphere by 2.50 diopters (this design convention yields the intermediate sphere of +1.75 − 2.50 = −0.75). The cylinder remains -0.50 and the axis stays at 120 degrees. So the Rx through the intermediate portion is −0.75 −0.50 ×120.