When the green image is clearer during a duochrome test, which lens adjustment is appropriate?

When the green image is clearer during a duochrome test, which lens adjustment is appropriate?

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

Answer: (A)

The duochrome test uses longitudinal chromatic aberration: the eye focuses green light at a slightly different plane than red light. When the green letters are clearer, it means the current spherical correction is tipping the balance so that the green image is in focus before the red image. To bring the red image into equal focus with the green, you shift the overall refraction toward more plus power. Adding plus power increases the converging strength of the eye, moving the focal balance so that both colors appear equally sharp. Cylinder power isn’t involved in this refinement, which is why the appropriate adjustment is to add plus power.

The duochrome test uses longitudinal chromatic aberration: the eye focuses green light at a slightly different plane than red light. When the green letters are clearer, it means the current spherical correction is tipping the balance so that the green image is in focus before the red image. To bring the red image into equal focus with the green, you shift the overall refraction toward more plus power. Adding plus power increases the converging strength of the eye, moving the focal balance so that both colors appear equally sharp. Cylinder power isn’t involved in this refinement, which is why the appropriate adjustment is to add plus power.