A prescription written as -3.00DS / -2.00DC × 180 (negative cylinder form) is transposed to its positive cylinder equivalent. What is the correct positive cylinder form?
- A +2.00DS / -2.00DC × 90
- B -5.00DS / +2.00DC × 90 ✓
- C -1.00DS / +2.00DC × 90
- D +1.00DS / +2.00DC × 180
Correct answer: B. -5.00DS / +2.00DC × 90
Explanation
Transposition rule: new sphere = algebraic sum of old sphere and old cylinder (-3.00 + (-2.00) = -5.00DS); new cylinder = same magnitude, sign reversed (+2.00DC); new axis = old axis ± 90° (180° - 90° = 90°). Result: -5.00DS / +2.00DC × 90. This represents the identical optical power expressed with a plus cylinder. This transposition skill is important for converting prescriptions between the forms used by different practitioners and for spectacle lens verification with a lensometer.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.