A child wearing +4.00 D spectacles at the standard vertex distance of 12 mm. If contact lenses are prescribed, the required contact lens power will be:

• A Greater than +4.00 D (more plus power needed) ✓
• B +4.00 D (no change)
• C Less than +4.00 D (less plus power needed)
• D The same as the spectacle lens power converted to prism

Explanation

Moving a plus (convex) lens closer to the eye — from the spectacle plane to the corneal/contact-lens plane, a 12 mm reduction in vertex distance — requires MORE plus power to give the same correction. Using the vertex-distance formula F_CL = F_spec / (1 − d × F_spec) with d = 0.012 m: F_CL = 4.00 / (1 − 0.012 × 4.00) = 4.00 / 0.952 ≈ +4.20 D. So the contact lens needs slightly more plus than the +4.00 D spectacle. Clinical rule: hyperopes require more plus power in contact lenses than in glasses, whereas myopes require less minus power.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.