During subjective refraction, the patient views the duochrome chart. He reports that the letters on the red half appear sharper than those on the green half. The appropriate refinement is to:
- A Add plus 0.50 D cylinder
- B Remove minus 0.25 D sphere
- C Add minus 0.25 D sphere ✓
- D Stop refinement, the endpoint is reached
Correct answer: C. Add minus 0.25 D sphere
Explanation
The duochrome test exploits longitudinal chromatic aberration: green light focuses about 0.25 D in front of red light. When the eye is focused on the midpoint, both halves look equally clear. If red letters are clearer, the focal point lies behind the retina, meaning residual undercorrection, so additional minus 0.25 D is added until the two halves are balanced. Green clarity indicates overminus and calls for removing 0.25 D.
Reference: Clinical Procedures for Ocular Examination by Carlson and Kurtz, 4th ed.
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