# Prescribing Refractive Errors > Prescribing refractive errors in Optometry: when to give hyperopia, myopia and astigmatism corrections, transposition, vertex distance and SRK biometry basics. - Canonical URL: https://prepelephant.com/topics/allied/optometry/prescribing-refractive-errors - Exam / course: Allied Health · Subject: Optometry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Prescribing Refractive Errors", PrepElephant, https://prepelephant.com/topics/allied/optometry/prescribing-refractive-errors ## Direct answer A prescription is a clinical decision, not a printout: the same +1.00 D of hyperopia is ignored in an asymptomatic 22-year-old, prescribed uncorrected trial in an asthenopic reader, and given in full to a child with accommodative esotropia. Myopia is prescribed whenever distance blur is symptomatic — in children essentially always, and in full, since under-correction has been shown to accelerate rather than slow progression. Astigmatism of 0.75 D or more, or any cylinder that explains headaches and squinting, deserves correction, with axis fidelity respected meticulously in long-standing adult wearers. Anisometropia beyond about 1.00 D in a child is an amblyopia risk demanding correction, and high prescriptions beyond roughly ±4.00 D need vertex distance arithmetic before contact lens conversion or intraocular lens power calculation. ## What you must remember - **Myopia:** prescribe full correction for children; deliberate under-correction worsens progression in controlled studies — an exam-standard fact against old shop practice. - **Hyperopia by context:** full cycloplegic plus for esotropia; partial or observed correction for the school-age child with +3.00 D and no symptoms; trial-frame relief for the asthenopic young adult. - **Astigmatism:** 0.75 D is the usual symptomatic threshold; in adults changing a long-worn cylinder axis, rotate gradually toward the new axis rather than jumping, or induce adaptation failure. - **Anisometropia:** correct children from about 1.00 D of interocular difference (amblyopia risk); contact lenses solve the aniseikonia of spectacle corrections beyond about 2.50-3.00 D difference. - **Transposition:** sphere + cylinder sign change — +2.00 DS/+1.00 DC × 90 becomes +3.00 DS/−1.00 DC × 180: add cylinder to sphere, flip cylinder sign, rotate axis 90 degrees. - **Vertex distance:** effective power changes beyond about ±4.00 D; a −8.00 D spectacle at 12 mm vertex is roughly −7.25 D as a contact lens, while a +10.00 D spectacle converts to about +11.25 D. - **IOL arithmetic (SRK):** implant power P = A − 2.5 × axial length − 0.9 × average keratometry; a typical emmetropic implant is about +19 to +21 D, with A-constants near 118. - **Presbyopic addition:** demand minus half the amplitude of accommodation, cross-checked against the age table and the patient's real working distance. ## A clinic morning of prescription decisions First chair: an eight-year-old with intermittent inward turning and cycloplegic +4.50 DS each eye. Prescribe the full +4.50 immediately — accommodative esotropia is treated optically before any surgical thought, and the deviation often straightens within weeks of wear. Second: a 26-year-old engineer, 6/6 unaided, +0.75 DS latent hyperopia, frontal headaches after screen work. Trial-frame +0.50 for four weeks before the review; if symptoms persist, prescribe, because adult asthenopia with small hyperopia is a trial-and-verdict exercise, not an automatic prescription. Third: a 19-year-old with −1.50/−1.00 × 175 and a worn pair at −1.25/−0.75 × 165. Update the sphere fully; nudge the axis toward 175 in steps — the visual cortex forgives small steps, not leaps. Fourth: a high myope wanting contact lenses. Spectacles read −9.00 DS; at 12 mm vertex, effective power at the corneal plane computes to about −8.00 D — that, not −9.00, is the contact lens starting point. Fifth: a cataract camp biometry slip — axial length 22.6 mm, keratometry 44.00 D, A-constant 118.2. SRK gives 118.2 − 56.5 − 39.6 = +22.1 D, a plausible implant for a shortish eye, refined later by the surgeon's formula choice (Hoffer Q for short eyes, SRK/T for long). ## How exams frame it Papers test decision thresholds, not arithmetic alone: whether hyperopia in a child with esotropia gets full plus (yes, always), whether a young myope may be under-corrected to "slow" myopia (no — evidence says it accelerates), and whether a −6.00 D anisometrope gets spectacles or contact lenses (lenses, for aniseikonia). The transposition and vertex-distance sums appear as short calculations where sign errors and the 90-degree axis rotation are the planted traps; in the SRK question the units do the filtering — axial length in millimetres, keratometry in dioptres, forgetting the 0.9 factor loses the mark. Viva examiners add one classic: prescribe for the patient's life — a mechanic's addition follows his 50 cm working distance, not the textbook 40 cm. ## Frequently asked questions ### When must hyperopia be prescribed in a child? When esotropia, amblyopia, or anisometropia coexists, or when symptoms demand it — and in accommodative esotropia the full cycloplegic correction is given, not a token plus. ### Why should progressing myopia not be under-corrected? Controlled studies show under-correction speeds axial progression; full correction plus formal myopia-control measures is the evidence-based stance. ### How is +2.00/+1.00 × 90 transposed to minus cylinder? Add cylinder to sphere (+3.00), reverse the cylinder sign (−1.00), and rotate the axis 90 degrees (180), giving +3.00 DS/−1.00 DC × 180. ### What is the contact lens power for a −8.00 D spectacle worn at 12 mm? About −7.25 D, applying the vertex correction F/(1 − dF); minus lenses lose minus power moving to the corneal plane. ### What does the SRK formula calculate? Intraocular lens power: P = A − 2.5 × axial length − 0.9 × keratometry, yielding a typical emmetropic implant of roughly +19 to +21 D.