In transposition of astigmatic prescriptions, a cylinder of -2.00 DS / -1.50 DC × 90° transposed to positive cylinder form gives:

• A -0.50 DS / +1.50 DC × 180°
• B -2.00 DS / +1.50 DC × 180°
• C -3.50 DS / +1.50 DC × 180° ✓
• D -3.50 DS / +1.50 DC × 90°

Explanation

To transpose from negative to positive cylinder: (1) Add the sphere and cylinder algebraically to get new sphere: -2.00 + (-1.50) = -3.50 D. (2) Change the sign of the cylinder: -1.50 becomes +1.50 DC. (3) Rotate the axis by 90°: 90° becomes 180°. Result: -3.50 DS / +1.50 DC × 180°. This is a fundamental optical calculation tested in refractive practice. The sphere power in negative and positive cylinder forms differs because the first principal meridian represented changes — in the negative form, the sphere represents the least minus (or most plus) meridian power, while in positive form it represents the most minus (least plus) meridian.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.