During subjective refraction on a duochrome (red-green) chart, the patient reports that the letters on the green half are distinctly sharper than those on the red half. This finding indicates:

• A Undercorrection, so additional minus power should be added
• B Accommodative spasm, so cycloplegia is indicated
• C Uncorrected astigmatism, so cylinder refinement is needed
• D Overcorrection, so minus power should be reduced ✓

Explanation

Because of chromatic aberration the myopic eye focuses green light in front of the retina and red light behind it. Sharper green letters mean the retina sits in front of the focal point, that is, the patient has been overminused or is slightly myopic for the current correction. The remedy is to add plus or remove minus until the two sides appear equally clear or red is slightly preferred, the endpoint deliberately biased to avoid overminus. The duochrome test is insensitive to small cylinders, so astigmatism is not what it detects.

Reference: Elkington AR, Clinical Optics, 3rd ed.

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Written and medically reviewed by the StethoPrep medical team.