Which vertex distance should not be used for refraction?

• A. 12-15 mm
• B. 10 mm
• C. 20 mm
• D. 8 mm

Answer: (C)

Vertex distance is the gap between the back of the spectacle lens and the eye during refraction. The refractive error you determine is tied to this distance, so refractions are done at a standard vertex distance to ensure the result matches what the patient will see with glasses.

A distance of 20 mm sits farther from the eye than is typical. Moving the lens farther away changes the effective power at the eye, so the measured prescription would not translate correctly into a wearing prescription. That’s why 20 mm should not be used for refraction. The other distances fall within commonly used ranges, depending on technique and patient needs.