A spectacle lens prescription reads −3.00 DS / −1.50 DC × 90. When converted to the cross-cylinder notation, what is the power in the 90-degree meridian?

• A −4.50 D ✓
• B −3.00 D
• C −1.50 D
• D −1.50 D combined with +1.50 D in the 180° meridian

Explanation

In the prescription −3.00 DS / −1.50 DC × 90, the cylindrical axis is 90°, meaning the cylinder power acts in the meridian perpendicular to the axis — i.e., at 180°. The 90° meridian has only the spherical power: −3.00 D. The 180° meridian has sphere plus cylinder: −3.00 + (−1.50) = −4.50 D. Therefore the power in the 90° meridian is −3.00 D and in the 180° meridian is −4.50 D.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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