Prentice's Rule
Optics and Optometry
Prism from decentration, solved in any direction — plus the vertical imbalance behind a slab-off.
How it is calculated
- Prism in dioptres = decentration in centimetres × lens power in dioptres. Enter any two and the third follows.
- Note the units: the rule is written in centimetres, so a decentration measured in millimetres must be divided by ten first. This is the classic factor-of-ten error.
- In anisometropia the two eyes induce different amounts of vertical prism when the patient looks down to read, and it is the difference between them — not either amount on its own — that is symptomatic.
- The vertical meridian is what matters there, so a cylinder has to be resolved rather than ignored. A −2.00 cylinder at axis 180 contributes its full power vertically; the same cylinder at axis 090 contributes nothing.
- Roughly 1.0 to 1.5 prism dioptres of vertical imbalance is where a slab-off, separate reading glasses, or a change of lens design is usually considered. Below that most patients simply fuse through it.
References
- Prentice CF. Ophthalmic Lenses. Philadelphia, 1900 — the original statement of the rule.
About these tools
OcuLearning Clinical Tools is part of an educational platform. These calculators are provided for learning and reference, and are not medical advice, a diagnosis, or a treatment recommendation.