In the duochrome test, green images are located which relative to red images?

• A. More anteriorly
• B. More posteriorly
• C. At the same depth
• D. Not applicable

Answer: (A)

Longitudinal chromatic aberration of the eye means different wavelengths focus at different depths along the visual axis. Shorter wavelengths (green) focus closer to the eye’s front, while longer wavelengths (red) focus further back, behind the retina. In the duochrome setup this makes green images fall at a more anterior plane than red images. When the eye is properly focused on the retina, both colors should appear equally sharp; if one color looks clearer, it indicates which way to tweak the spherical correction to move the focus onto the retina. Thus green images are located more anteriorly than red images.