Prism Calculator

Prentice's rule in both directions, plus combined prism vectors and base notation.

Prentice's rule

Solve for
Meridian

Resulting prism

2.00 Δ

Base In · −5.00 D decentred 4 mm

  1. 01Input: F = −5.00 D, c = 4 mm = 0.4 cm
  2. 02Formula: P = c × F
  3. 03P = 0.4 × −5.00
  4. 04P = 2.00 Δ Base In

Combined prism vector

Resolve horizontal and vertical components into a single prism

Resultant prism

5.00 Δ @ 53.1°

H 3.00 Δ (Base In) · V 4.00 Δ (Base Up)

Formula

  1. 01P = c × F (Prentice's rule)
  2. 02P = prism dioptres (Δ)
  3. 03c = decentration in centimetres
  4. 04F = lens power in dioptres
  5. 05c (mm) = 10 × P / F
  6. 06Resultant Δ = √(H² + V²)

Base notation

Convention displayed, never applied silently

Positive decentration is taken as nasal (horizontal) or upward (vertical) displacement of the optical centre. Plus lenses produce base-apex effect in the direction of decentration; minus lenses the opposite. Verify base direction against the patient’s measured deviation.

OptiX Pro provides mathematical calculations, reference information, and fitting assistance. It does not replace professional clinical examination, manufacturer fitting guidelines, or professional judgement. Contact lens selection and prescription must be clinically verified.