Prisms in Practice: Prescribing, Splitting, Decentration and Fresnel Prisms
Last updated: November 2026
RPC Rounds for Optometrists · B.Optom study guide
A prism displaces the image towards its apex without changing focus. In practice prisms are prescribed to relieve double vision or symptomatic phorias, appear unintentionally when lenses are decentred, and are used diagnostically in cover tests and fusional reserves. Getting the base direction right is the whole skill.
Basics
- A prism of 1 prism dioptre (1Δ) deviates light by 1 cm at 1 m. 1° is about 1.75Δ for small angles.
- Light bends towards the base; the image is displaced towards the apex. To correct a deviation, the apex points in the direction the eye has deviated: base out for eso, base in for exo, base down before the higher (hyper) eye.
- Base directions are written per eye: base in (BI), base out (BO), base up (BU), base down (BD); oblique prisms can be resolved into horizontal and vertical parts.
When to prescribe prism
| Situation | Approach |
|---|---|
| Small vertical deviation with diplopia (for example decompensated fourth nerve palsy) | Prescribe the smallest vertical prism that gives single vision in primary gaze and downgaze reading; split between the eyes |
| Acquired horizontal diplopia (sixth nerve palsy, decompensated phoria) | Fresnel prism as a temporary measure while waiting for recovery |
| Convergence insufficiency with symptoms | Exercises first; base-in prism for near (reading glasses) if exercises fail, especially in presbyopes |
| Divergence insufficiency in older adults | Base-out prism for distance |
| Decompensated phoria | Sheard's criterion: prism = (2 × phoria − opposing reserve) ÷ 3 |
| Nystagmus with a null point | Yoked prisms to shift the null point to primary gaze |
| Homonymous hemianopia | Field-expanding prisms (Peli) in specialist low vision practice |
Splitting prism and decentration
- Split prism equally between the eyes to balance weight and thickness: 4Δ BO needed becomes 2Δ BO each eye; 2Δ vertical becomes 1Δ BD right and 1Δ BU left (for a right hyper).
- Prism can be produced by decentring the lens (Prentice: c = P ÷ F) if the power is high enough; otherwise it is ground into the lens.
- Unwanted prism from a wrong PD: a −5.00 D pair with optical centres 4 mm too wide (2 mm each) gives 0.2 × 5 = 1Δ base in per eye, 2Δ in total.
Fresnel press-on prisms
- Thin flexible plastic sheets of tiny parallel prisms, cut and stuck on the inside of the spectacle lens (wet application).
- Available up to about 30Δ; useful for temporary or changing deviations and for trial before ordering ground prism.
- Drawbacks: reduced acuity and contrast (more with higher powers), visible lines, chromatic fringes.
Tolerances
Unwanted horizontal prism of more than about 0.5–1Δ (base out tolerated better than base in) and vertical prism of more than about 0.25–0.5Δ in total can cause symptoms. Vertical tolerances are tighter because vertical fusional reserves are small (about 2–4Δ).
Practical pearls
- Before prescribing prism for diplopia, measure the deviation in the nine gaze positions; comitant deviations suit prism, highly incomitant ones may not.
- Try a Fresnel prism in the trial frame and let the patient walk around before ordering ground prism.
- Patients with recent-onset diplopia need a cause found (cranial nerve palsy, myasthenia, thyroid eye disease) before prism is given.
Where students slip
- Putting the base towards the deviation (it should point away: apex towards the deviation).
- Forgetting that vertical reserves are small, so vertical prism errors matter most.
- Prescribing the full measured prism for a phoria instead of the minimum needed for comfort.
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Related: Prentice's Rule: Induced Prism With Worked Examples, Orthoptic Tests Made Easy: Cover Test, Hess Chart, Synoptophore, Worth 4-Dot and Stereopsis, Strabismus Measurement and Surgical Dose Tables: Quick Reference, Approach to Diplopia: Monocular vs Binocular and the Three-Step Test, Convergence Insufficiency and Convergence Excess: Diagnosis, Treatment and Exercises. Back to the optometry curriculum.
Key sources
- Jalie M. Ophthalmic Lenses and Dispensing. 3rd ed. Butterworth-Heinemann; 2008
- Evans BJW. Pickwell's Binocular Vision Anomalies. 5th ed. Butterworth-Heinemann; 2007
- Rowe FJ. Clinical Orthoptics. 3rd ed. Wiley-Blackwell; 2012
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