Slab off is a technique used to correct for vertical imbalance by adding _____ prism to the more minus lens.

• A. BU
• B. BD
• C. BI
• D. BO

Answer: (A)

Vertical imbalance from anisometropia can cause diplopia or fatigue because each eye’s image is effectively at a different vertical position. Slab-off works by placing a small, horizontal slab prism in the more minus lens to shift the image of that eye vertically, helping the two eyes fuse. The prism is oriented base up, so the light entering through the lower part of the lens bends upward, moving that eye’s image higher on the retina relative to the fellow eye. This correction specifically targets vertical disparity without altering horizontal alignment, which is why base-up prism on the more minus lens is used. Base-down prisms or horizontal prisms wouldn’t address the vertical imbalance.