A patient has 2 pd BU for the distance portion of the Rx and 1 pd BU for the near portion of the OD Rx. What is the total prismatic effect of that lens?

• A. 1 pd BU
• B. 3 pd BU
• C. 3 pd BD
• D. 1 pd BD

Answer: (B)

The key idea is that prisms in different zones of a single lens add when they share the same base direction. Here, the distance portion has 2 prism diopters with base up, and the near portion has 1 prism diopter with base up. Since both prisms are oriented the same way (base up), they sum to a total of 3 prism diopters base up. If the prism directions were opposite, you’d subtract instead. So the total prismatic effect of that lens is 3 prism diopters base up.