Some wearers with unequal right and left powers see well at distance but feel strain when looking down to read. As both lines of sight pass below the optical centers, the two lenses can induce different amounts of vertical prism.
01 · DOWNGAZE
Slab-off targets differential vertical prism during near downgaze
Prentice’s rule predicts prism when viewing away from the optical center. If the relevant powers differ between the two lenses, equal downgaze can create unequal vertical prism.
Slab-off adds a compensating vertical-prism region to reduce that binocular difference.
02 · 90 MERIDIAN
With astigmatism, use the 90-degree meridian power rather than sphere alone
Because the near line of sight is displaced mainly vertically, power in the vertical meridian is the relevant value for the induced vertical prism. Comparing only SPH can miss an important cylinder contribution.
Evaluate the 90-degree meridian powers together with the actual vertical distance to the reading level.
03 · CONVENTIONAL
Conventional slab-off generally creates a Base Up effect on the more-minus or less-plus lens
Traditional slab-off adds a base-up compensation region on one lens. The correct eye is determined from the relative 90-degree meridian powers and direction of the predicted imbalance.
A vague rule such as “put it on the stronger eye” can fail when plus/minus signs or astigmatism are involved.
04 · REVERSE
Reverse slab-off creates the complementary Base Down effect on the opposite type of lens
Reverse slab-off generally places base-down compensation on the more-plus or less-minus lens to achieve the same binocular goal. Choice can depend on manufacturing method, appearance and lens-design availability.
05 · NOT AUTOMATIC
Anisometropia does not automatically require slab-off
Consider the magnitude of vertical imbalance, actual near gaze displacement, symptoms, previous adaptation and lens design. Practical thresholds exist, but one universal cutoff should not be treated as mandatory for every wearer.
Prescribed vertical prism and slab-off compensation for near imbalance should also be recorded and verified as distinct purposes.
SOLUTION GUIDE
A practical check order
Compare the vertical-meridian powers in the same notation convention.
Estimate the actual vertical displacement to the reading level.
Calculate which eye needs a BU or BD compensating effect.
Check the slab line and intended reference regions using the manufacturer’s layout.
GLANOS CHECK
Slab-off does not eliminate anisometropia; it compensates differential vertical prism in downgaze.
Need depends on 90-degree meridian powers, gaze displacement, symptoms and adaptation.
CAUTION
Do not prescribe slab-off automatically just because anisometropia is present.
Confirm actual vertical imbalance and near symptoms, then consider the feasible lens design.
GLANOS EYEWEAR
There is usually a reason behind eyewear discomfort.
slab-off · anisometropia · vertical prism · downgaze · Prentice rule · progressive lenses