By convention, which prism value do we choose as the actual amount of prism when two values are measured?

• A. The lower amount, or if both equal: either value
• B. The higher amount
• C. The average
• D. The maximum

Answer: (A)

When two prism magnitudes are measured, there is often some variability in the readings due to small changes in fixation, accommodation, or instrument alignment. The safest, most conservative approach is to use the smaller prism value. This minimizes the risk of overcorrecting the eye alignment, which can push the eyes beyond the patient’s fusional reserve and lead to diplopia or discomfort. If both measurements come out the same, either value is reasonable because they reflect the same amount of correction. Choosing a larger prism could overdo the correction and create new visual problems, so the smaller value is the preferred convention.