A myope whose spectacle prescription is -8.0 D at a vertex distance of 14 mm undergoes cataract surgery. What contact lens power would provide equivalent correction post-operatively?

• A -8.0 D
• B -8.93 D
• C -9.20 D
• D -7.14 D ✓

Explanation

For a minus lens moved CLOSER to the eye, effective power decreases (less minus needed at the cornea). F_contact = F_spec / (1 − d × F_spec) where d = 0.014 m (14 mm), F_spec = -8.0 D: F_contact = -8.0 / (1 − 0.014 × (-8.0)) = -8.0 / (1 + 0.112) = -8.0 / 1.112 = -7.19 D ≈ -7.14 D. This confirms the optics principle: a minus lens moved closer to the eye has LESS effective power (numerically less minus), so a weaker minus contact lens is required compared to the spectacle correction.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.