When the red image is clearer during a duochrome test, which lens adjustment should you make?

• A. Add plus power
• B. Add minus power
• C. No change
• D. Change cylinder power

Answer: (B)

Duochrome testing uses the eye’s chromatic aberration: red and green light don’t focus at the same plane. In a typical eye, red light comes to focus a bit behind green light. When you see the red image clearer, it means the current focus is biased toward the red plane and you need to move the overall focus forward so red and green can be equally sharp. Adding minus power does exactly that by reducing convergence, shifting the focal point forward and bringing red into sharper focus. If instead the green image were clearer, you’d add plus power to move the focus backward until both colors balance.