For determining vertical prismatic imbalance, if both eyes have BU prism, do you add or subtract this prism?

For determining vertical prismatic imbalance, if both eyes have BU prism, do you add or subtract this prism?

• A. Add
• B. Subtract
• C. Multiply
• D. Divide

Answer: (B)

Vertical prismatic imbalance is determined by comparing how much each eye is shifted by prism. When both eyes have prism in the same direction (for example, both bases up), the shared effect cancels to the extent of the smaller prism, and the remaining difference between the two eyes is what creates imbalance. That’s why you subtract the two vertical prism values to get the imbalance. A simple way to think about it: if one eye has BU 3 diopters and the other BU 1 diopter, the vertical imbalance is 2 diopters, toward the eye with the larger BU prism. If prisms were in opposite directions (one BU, one BD), you would add them to get the total imbalance. Multiplication or division aren’t appropriate for combining prisms in this context.

Vertical prismatic imbalance is determined by comparing how much each eye is shifted by prism. When both eyes have prism in the same direction (for example, both bases up), the shared effect cancels to the extent of the smaller prism, and the remaining difference between the two eyes is what creates imbalance. That’s why you subtract the two vertical prism values to get the imbalance. A simple way to think about it: if one eye has BU 3 diopters and the other BU 1 diopter, the vertical imbalance is 2 diopters, toward the eye with the larger BU prism. If prisms were in opposite directions (one BU, one BD), you would add them to get the total imbalance. Multiplication or division aren’t appropriate for combining prisms in this context.