Which of the following is NOT used to correct for vertical imbalance of prism?

• A. Prescribe a separate pair of distance and near spectacles
• B. Wear contact lenses
• C. Dissimilar segs can be used
• D. Use Slab off by adding more BU prism to the more plus lens

Answer: (D)

Vertical imbalance of prism happens when the two eyes experience different vertical prismatic shifts, so the near images don’t line up and diplopia or discomfort can occur. To manage this, several practical approaches are used: you can prescribe separate distance and near spectacles so the near addition doesn’t create imbalance, wear contact lenses to eliminate spectacle-induced prism altogether, or use dissimilar segment powers (different near adds) to balance the vertical image heights between eyes. Slab-off is a specific technique for bifocals that corrects vertical imbalance by creating a partial prism in the lower portion of one lens to equalize the near images between eyes.

What’s described in that option would involve adding more base-up prism to the more plus lens, which isn’t how slab-off is performed. Slab-off relies on a lower-segment prism arranged to offset the near-image height difference, not a uniform base-up prism added to a lens. So that particular method isn’t a standard way to correct vertical imbalance.