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Prism Calculator
Prentice's Rule
Calculate induced prism from decentration or find the decentration needed for prescribed prism.
Use sphere power or power in the meridian of interest
Distance from optical center to pupil position
Understanding Prentice's Rule
Prism (Δ) = Decentration (cm) × Power (D)
or: Δ = d × F (where d is in cm and F is in diopters)
Key Concepts:
- •Induced prism occurs when the optical center doesn't align with the pupil
- •Base direction: For minus lenses, base is toward the thicker edge (toward decentration). For plus lenses, base is toward the thinner edge (away from decentration)
- •Vertical imbalance: When eyes look through different prism amounts, usually >1.5Δ difference is problematic
Common Applications:
- •Checking vertical imbalance in progressive lenses
- •Calculating slab-off requirements
- •Grinding prism by decentration to save cost
- •Troubleshooting patient complaints about new glasses