If the trial frame scale at the apex of the cornea reads as the red 5 mm (further), what is the vertex distance?

• A. 10 mm
• B. 15 mm
• C. 20 mm
• D. 5 mm

Answer: (C)

Vertex distance is the distance from the corneal apex to the back surface of the lens in the frame. A common reference vertex distance used in refraction is about 15 mm. If the trial frame scale at the corneal apex shows the red mark 5 mm farther, that means the lens is sitting 5 mm farther from the cornea than this reference. Add the extra distance to the standard 15 mm: 15 mm + 5 mm = 20 mm. So the actual vertex distance is 20 mm. This matters because changing the vertex distance alters the effective lens power, especially for higher prescriptions. Reading 10 mm would imply the lens is closer than standard, while 15 mm would indicate the standard distance with no extra separation, and 5 mm by itself isn’t the scenario described here.