A myopic spectacle lens of -10.00 DS is moved from a vertex distance of 12 mm to 15 mm. The effective power of the lens at the new position is approximately:
- A -10.30 DS
- B -10.00 DS (no change)
- C -9.70 DS ✓
- D -9.50 DS
Correct answer: C. -9.70 DS
Explanation
For a minus lens, moving it further from the eye (increasing vertex distance) makes it less effective (less negative). Using the vertex power formula: F_new = F / (1 - dF), where d = 0.003 m and F = -10. F_new = -10 / (1 - 0.003 × -10) = -10 / 1.03 = -9.70 D. A common trap is adding the correction instead of subtracting it.
Reference: AAO BCSC Section 3: Clinical Optics, 2023-2024 ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.