A patient's refraction is: right eye +3.00 DS / +2.00 DC × 90°. Converting to minus cylinder notation, the equivalent prescription is:
- A +3.00 DS / +2.00 DC × 180°
- B +5.00 DS / -2.00 DC × 90°
- C +5.00 DS / -2.00 DC × 180° ✓
- D +1.00 DS / +2.00 DC × 90°
Correct answer: C. +5.00 DS / -2.00 DC × 180°
Explanation
To transpose from plus to minus cylinder: (1) add sphere and cylinder algebraically (+3.00 + 2.00 = +5.00 DS); (2) change sign of cylinder (+2.00 becomes -2.00 DC); (3) rotate axis by 90° (90° + 90° = 180°). The transposed prescription is +5.00 DS / -2.00 DC × 180°. This represents the same optical power as the original, just expressed in the other cylinder convention. The ability to transpose accurately is essential when dispensing lenses and when ordering from labs that use a different convention.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.