A patient's refraction is recorded as +1.50 DS / +1.50 DC × 180°. Transposing this prescription to its minus cylinder equivalent yields:
- A +1.50 DS / -1.50 DC × 90°
- B +3.00 DS / +1.50 DC × 90°
- C +3.00 DS / -1.50 DC × 90° ✓
- D +1.50 DS / -1.50 DC × 180°
Correct answer: C. +3.00 DS / -1.50 DC × 90°
Explanation
To transpose from plus to minus cylinder notation: (1) new sphere = algebraic sum of original sphere and cylinder = +1.50 + (+1.50) = +3.00 DS; (2) change sign of cylinder: +1.50 → –1.50 DC; (3) change axis by 90°: 180° – 90° = 90°. Result: +3.00 DS / –1.50 DC × 90°. Optical transposition is clinically important when converting spectacle prescriptions between different lens forms and for verifying trial lens combinations during retinoscopy and subjective refraction.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.