A hypermetropic patient wears a +4.0 D spectacle lens at a vertex distance of 12 mm. If a contact lens is fitted on the eye (vertex distance = 0), the required contact lens power is:
- A +4.20 D ✓
- B +4.00 D
- C +3.82 D
- D +4.50 D
Correct answer: A. +4.20 D
Explanation
When a plus lens is moved CLOSER to the eye (reducing vertex distance), effective power at the eye INCREASES and MORE plus power is needed. Using the effective power formula: F_contact = F_spectacle / (1 − d × F_spectacle) where d = vertex distance in metres = 0.012 m, F = +4.0 D: F_contact = 4.0 / (1 − 0.012 × 4.0) = 4.0 / (1 − 0.048) = 4.0 / 0.952 = +4.20 D. A plus lens moved closer to the eye loses effective vergence advantage, requiring more plus power in the contact lens than in the spectacle.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.