Refraction
What this domain tests
Refraction questions test objective and subjective technique, prescription transposition, vertex distance, prescribing judgement and the handling of anisometropia and astigmatism. You must show the arithmetic and explain why a prescription is or is not issued.
Core rules and concepts
- Sphero-cylinder form is sphere/cylinder × axis; transposition changes the cylinder sign, changes the axis by 90 degrees, and adds the cylinder power to the sphere.
- Vertex distance matters when power exceeds about ±4.00 D; moving a lens closer or farther changes its effective power.
- Cycloplegic refraction is needed when accommodation is likely to distort the result, especially in children and in hyperopic patients.
- Prescribe for symptoms, binocular balance and visual needs, not merely for the optical finding.
- In anisometropia, consider aniseikonia and amblyopia risk before equalising powers.
Worked example
A retinoscopy result is +2.00/−3.00 × 180. Transpose: change −3.00 to +3.00; change axis 180 to 090; add −3.00 to +2.00 to get −1.00. The result is −1.00/+3.00 × 090. Reasoning: the two forms describe the same optical surface, so the meridian powers must remain equivalent. A common error is changing the axis but not the sign, which creates a different lens.
Traps that cost marks
Forgetting to add the cylinder to the sphere during transposition; rotating the axis by 90 degrees in the wrong direction; ignoring vertex distance in high myopia; prescribing a full cycloplegic result without considering adaptation.
Key points
- Transposition: change cylinder sign, rotate axis 90 degrees, add cylinder to sphere.
- Vertex distance is clinically important above about ±4.00 D.
- Cycloplegia reduces accommodative error in children.
- Prescribe for symptoms and binocular status, not numbers alone.
- Anisometropia carries amblyopia and aniseikonia risk.