For vertical prismatic imbalance when BU prism is present in both eyes, what operation is used to combine prisms for calculation?

For vertical prismatic imbalance when BU prism is present in both eyes, what operation is used to combine prisms for calculation?

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

Answer: (B)

Vertical prismatic imbalance hinges on how prisms in the two eyes combine to shift the relative image positions. When both prisms have the same orientation—such as both bases up—their effects oppose each other only to the extent of their magnitude difference. So you use subtraction to find the net imbalance. For example, if one eye has 2 prism diopters BU and the other eye has 1 prism diopter BU, the net vertical imbalance is 1 prism diopter BU. If they’re equal, there’s no vertical imbalance. This is distinct from when prisms are in opposite directions (one BU, one BD), where you would add them.

Vertical prismatic imbalance hinges on how prisms in the two eyes combine to shift the relative image positions. When both prisms have the same orientation—such as both bases up—their effects oppose each other only to the extent of their magnitude difference. So you use subtraction to find the net imbalance. For example, if one eye has 2 prism diopters BU and the other eye has 1 prism diopter BU, the net vertical imbalance is 1 prism diopter BU. If they’re equal, there’s no vertical imbalance. This is distinct from when prisms are in opposite directions (one BU, one BD), where you would add them.