Slab-Off Prism and Vertical Imbalance: When and How It’s Calculated

When the two lenses have different vertical powers, reading below the optical centers creates different vertical prism in each eye. The difference is the vertical imbalance, found with Prentice’s rule in the 90° meridian. Slab-off, made by bicentric grinding, adds base-up prism low on the more minus or less plus lens to offset it.

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Anisometropia and reading level

When the eyes have unequal prescriptions, looking down through both lenses below their optical centers produces different vertical prism in each eye. In multifocals the reading area is low in the lens, so this matters at near.

Use the vertical meridian power

Vertical prism comes from the power in the 90° meridian. For axis 180 that is sphere + cylinder; for axis 090 it is the sphere. For oblique axes see oblique meridian power.

Calculate

Find each eye’s prism with Prentice’s rule at the reading level, note base up or down, then combine. Same base direction: subtract. Opposite directions: add. Check each eye in the Prentice’s rule calculator.

Example: OD −1.00 D, OS −3.50 D sphere, reading 9 mm below the OCs. Minus lenses give base-down prism below the OC. OD 0.9 × 1.00 = 0.9Δ BD; OS 0.9 × 3.50 = 3.15Δ BD. Imbalance 3.15 − 0.9 = 2.25Δ, with the OS more minus.

What slab-off does

Bicentric grinding gives the lens two optical centers, one for distance and one below the slab line for near. Conventional slab-off adds base-up prism low on whichever lens is more minus (or less plus). A reverse slab adds base-down prism to the other lens instead. How much to correct is the prescriber’s and fitter’s decision.

Practice set

Original problems.

1. OD +2.00 −1.00 × 180, OS +2.00 sphere, reading 10 mm below the OCs. Vertical imbalance?

   Show answer

   90° meridian: OD +1.00, OS +2.00. Plus gives base up below the OC: OD 1.0Δ BU, OS 2.0Δ BU. Imbalance 1.0Δ.

2. In that example, which lens would take conventional slab-off?

   Show answer

   OD, the less plus lens in the vertical meridian.

3. OD +1.50, OS −1.50 sphere, reading 8 mm down. Imbalance?

   Show answer

   OD 1.2Δ BU, OS 1.2Δ BD; opposite directions add: 2.4Δ.

4. OD −2.00 −2.00 × 180, OS −2.00 sphere, reading 10 mm down. Imbalance?

   Show answer

   90° meridian: OD −4.00, OS −2.00. OD 4.0Δ BD, OS 2.0Δ BD: 2.0Δ, OD more minus.

5. A lab uses reverse slab for that last example. Which lens gets it?

   Show answer

   OS, the less minus lens, with base-down prism.

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Standard text used for this page: Brooks CW, Borish IM. System for Ophthalmic Dispensing, 3rd ed. Butterworth-Heinemann, ch. 21 (vertical imbalance, slab-off and reverse slab, reading level). The Iowa optics review and NAO prism course are linked under sources.

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