During subjective refraction, a patient views the duochrome (red-green) chart and reports that the letters on the red side appear sharper than those on the green side. The correct adjustment of the trial lens is:

• A Add cylinder power at 180 degrees
• B Add plus sphere until both sides are equal
• C No change, red clearer indicates the ideal endpoint
• D Add minus sphere until both sides are equal ✓

Explanation

Chromatic aberration focuses green light slightly in front of and red light behind the retina in an emmetropic eye. If red letters look sharper, the eye is relatively myopic for the chart and more minus sphere is needed to bring the red focus forward onto the retina. The endpoint is equality or slight red superiority, since a final endpoint of green clarity signals over-minusing and accommodative strain. The duochrome test relies on chromatic aberration, not astigmatism, so cylinder change is irrelevant here.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.