A patient has a prescription of -3.00 / -1.00 × 180 in the right eye. In cross-cylinder notation, the two principal meridians and their powers are:

• A 180°: -4.00 D; 90°: -3.00 D
• B 180°: -3.00 D; 90°: -2.00 D
• C Both meridians: -3.50 D (spherical equivalent)
• D 180°: -3.00 D; 90°: -4.00 D ✓

Explanation

For the prescription -3.00 / -1.00 × 180, the cylinder axis lies at 180°, so the cylinder exerts its power in the perpendicular (90°) meridian. At the axis meridian (180°) only the sphere acts: power = -3.00 D. At the 90° meridian the power = sphere + cylinder = -3.00 + (-1.00) = -4.00 D. With both principal meridians myopic (-3.00 D and -4.00 D), this is compound myopic astigmatism, the vertical (90°) meridian carrying the greater minus power.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.