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Vertical imbalance at the reading level
With different powers in each eye, reading below the distance optical centers induces a different vertical prism in each eye. Each eye gets Prentice prism from its vertical meridian power at the reading drop, commonly 10 mm; the imbalance is the difference. A common action level for slab-off is about 1.50 prism diopters.
Vertical imbalance calculator
Enter both distance prescriptions and the reading drop below the distance optical centers. Each eye picks up Prentice prism from its vertical meridian power; the difference is the imbalance the wearer must fuse.
Net vertical imbalance: 3.50 prism diopters
- Vertical (90 degree) meridian power: OD +1.00 D, OS -2.50 D (F = S + C x sin^2(90 - axis)).
- Prentice at 10 mm: OD = 1.0 x 1.00 = 1.00, OS = 1.0 x -2.50 = -2.50 (signed: positive reads base up below a plus lens, negative base down below a minus lens).
- Imbalance = |OD - OS| = |1.00 - -2.50| = 3.50 prism diopters.
At 3.50 prism diopters this meets the common 1.50 prism-diopter level at which slab-off is considered. Conventional slab-off grinds base up on the more minus (or less plus) lens at near; reverse slab adds base down to the more plus lens. Dissimilar segments or separate reading glasses are alternatives. The prescriber chooses the remedy.
Induced prism from an OC height error
A single optical center set too high or too low induces vertical prism the order never asked for: plain Prentice with the vertical meridian power.
Induced prism: 1.20 prism diopters (0.30 cm x 4.00 D in the vertical meridian)
Practice vertical imbalance
Unlimited generated problems with exact answers and worked steps.
Go deeper
The per-eye arithmetic is plain Prentice's rule with the vertical meridian power. The formulas are on the formula sheet.
Frequently asked questions
What is vertical imbalance at the reading level?
With different lens powers in each eye, reading below the distance optical centers induces a different vertical prism in each eye. Each eye gets Prentice prism from its vertical meridian power at the reading drop (commonly 10 mm); the imbalance is the difference between the two.
When is slab-off considered?
A common guideline is a vertical imbalance of about 1.50 prism diopters or more at the reading level. Options include slab-off (bicentric grinding), reverse slab, dissimilar segments, or separate reading glasses. The prescriber decides the remedy.
Which eye gets the slab-off?
Conventional slab-off adds base-up prism to the more minus (or less plus) lens in the reading area. Reverse slab adds base-down to the more plus lens. The calculator reports the imbalance so the remedy can be chosen deliberately.
What about an optical center height error on one eye?
The same Prentice arithmetic applies: an OC set 3 mm too high on a +4.00 D vertical meridian induces 1.2 prism diopters the order never asked for. The height-error tool on this page computes it directly.