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

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

Answer: (B)

Duochrome testing relies on the eye’s different focal points for red and green light due to longitudinal chromatic aberration. When the two color targets are compared, the goal is to have both colors focused on the retina with equal clarity; that balance indicates the spherical power is correct.

If the red image is clearer, the current correction is not bringing both colors to the same focal plane. To shift the focus so that the red and green images coincide on the retina, you adjust the spherical power toward more minus. In practice, adding minus power moves the overall focus in a way that reduces the discrepancy between the colors, leading to equal clarity. Changing axis wouldn’t affect the duochrome balance, and leaving the correction unchanged would maintain the imbalance.