If the trial frame scale at the apex of the cornea reads 0 mm, what is the vertex distance?

If the trial frame scale at the apex of the cornea reads 0 mm, what is the vertex distance?

• A. 15 mm
• B. 10 mm
• C. 20 mm
• D. 0 mm

Answer: (A)

Vertex distance is the gap between the back surface of the spectacle lens and the corneal apex along the optical axis. When the trial frame is adjusted so that the scale at the apex of the cornea reads zero, it is using the standard reference position for spectacles, which is about 15 mm from the cornea. So the vertex distance is 15 mm. Values like 0, 10, or 20 mm would reflect nonstandard or unusually short/long distances, not the baseline used when the apex reading is zero.

Vertex distance is the gap between the back surface of the spectacle lens and the corneal apex along the optical axis. When the trial frame is adjusted so that the scale at the apex of the cornea reads zero, it is using the standard reference position for spectacles, which is about 15 mm from the cornea. So the vertex distance is 15 mm. Values like 0, 10, or 20 mm would reflect nonstandard or unusually short/long distances, not the baseline used when the apex reading is zero.