A 30-year-old myope with -4.00 DS spectacles wants contact lenses. If the vertex distance is 14 mm, what contact lens power is required? (Use vertex formula; approximate answer acceptable.)
- A -4.00 DS
- B -3.50 DS ✓
- C -3.75 DS
- D -4.25 DS
Correct answer: B. -3.50 DS
Explanation
Vertex correction for minus lenses: F_contact = F_spectacle / (1 + d × F_spectacle), where d = 0.014 m. F_contact = -4.00 / (1 + 0.014 × -4.00) = -4.00 / 0.944 = -4.24 D. However, rounding to nearest 0.25 D gives -4.25 D. Wait, recalculating: the standard formula gives F_c = F_s / (1 - dF_s) where d is positive distance moved toward eye. At 14 mm, F_c = -4 / (1 - 0.014 × -4) = -4 / 1.056 = -3.79 D, rounded to -3.75 D. The closest standard answer is -3.75 D.
Reference: AAO BCSC Section 3: Clinical Optics, 2023-2024 ed.
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