Optical Calculations

Prentice's Rule

Prism = (Power × Decentration) / 10
Calculates prism induced when a lens's optical centre is decentered from the pupil.
Lens Power (D)
Decentration (mm)

Decentration for Prism

Decentration = (Prism / Power) × 10
Calculates how much decentration is needed to induce a given prism.
Prism (Δ)
Lens Power (D)

Vertical Prism Imbalance

Imbalance = |(Right × h) - (Left × h)| / 10
Calculates vertical imbalance at reading level for bifocal or multifocal wearers.
Right Eye Power (D)
Left Eye Power (D)
Reading Level (mm)

Combine Horizontal and Vertical Prism

Total Prism = √(H² + V²)
Enter horizontal and vertical prisms (with direction) to calculate total prism and axis.
Horizontal Prism (Δ)
Vertical Prism (Δ)

Split Total Prism and Axis into Components

H = Prism × cos(Angle), V = Prism × sin(Angle)
Enter total prism and axis to find the horizontal and vertical components.
Total Prism (Δ)
Axis (°)