A young adult can see distant objects clearly only when they are brought to within 40 cm of his eye. What spectacle lens power will allow him to see clearly at infinity?
- A -1.50 DS
- B -2.50 DS ✓
- C -2.00 DS
- D -4.00 DS
Correct answer: B. -2.50 DS
Explanation
The far point of a myopic eye lies in front of the eye, and a diverging lens must place parallel rays at that far point. The required lens power equals the reciprocal of the far point distance in metres: 1/0.40 = 2.50 dioptres of minus sphere. A -1.50 D lens corresponds to a far point of 67 cm, and -4.00 D would correspond to 25 cm, so both understate or overshoot this patient's measured far point.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.