Which statement best describes when horizontal prism is considered unacceptable?

• A. It is >0.67 PD and PRP distance from IPD exceeds 2.5 mm
• B. It is >0.5 PD
• C. It is >1.0 PD and PRP within 1 mm
• D. It is >0.67 PD but PRP equals IPD

Answer: (A)

Horizontal prism from decentration occurs when a lens is shifted horizontally away from the pupil centers, creating a prismatic effect. The amount of induced prism depends on how far the lens is decentered and the lens power, and there are practical limits to keep wear comfortable.

The best description is that horizontal prism is unacceptable when the induced prism exceeds about 0.67 prism diopters and the prism reference point is more than 2.5 mm away from the interpupillary distance. If both conditions are met, the prismatic effect is typically noticeable and uncomfortable for most wearers, making the frame unsuitable. If either condition isn’t met—prism not that large or the PRP not that far from IPD—the frame may still be acceptable or adjustable.