What is the total prism in the following Rx: OD: +0.50 -1.00 x 180 2 diopters BO OS: +0.25 -1.50 x 180 2 diopters BO?

• A. 4 diopters base out
• B. 2 diopters base out
• C. 4 diopters base in
• D. 4 diopters base up

Answer: (A)

Prism diopters show how much light is shifted and in what direction the base (the thick edge) is oriented. Here, each eye has 2 diopters of prism with the base pointing outward (horizontal BO). The sphere and cylinder values are about refractive correction and astigmatism, not prism, and don’t change the amount of prism prescribed. When both eyes require the same horizontal prism in the same direction, you simply add the prisms: 2 + 2 = 4 diopters, base outward. The other options would represent different directions (base in) or a vertical prism (base up), which aren’t indicated here. So the total prism is 4 diopters base out.