A patient has a spectacle prescription of −4.00 DS at vertex distance 12 mm. The equivalent contact lens power should be approximately:

• A −4.00 DS (no change needed for low powers)
• B −4.20 DS
• C −3.50 DS
• D −3.82 DS ✓

Explanation

When converting spectacle power to contact lens power, the vertex distance correction formula is: F_CL = F_spec / (1 − d × F_spec), where d = vertex distance in metres. For −4.00 DS at 12 mm (0.012 m): F_CL = −4.00 / (1 − 0.012 × (−4.00)) = −4.00 / (1 + 0.048) = −4.00 / 1.048 ≈ −3.82 DS. For myopic prescriptions, contact lens power is slightly less minus than spectacle power; for hyperopic prescriptions it is slightly more plus. This correction becomes clinically significant above ±4.00 DS.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.