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

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

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

Answer: (B)

Vertex distance is the distance from the back surface of the lens to the corneal apex. In a trial frame, the apex scale shows how the lens sits relative to the eye compared with a standard reference distance (about 15 mm). If the scale indicates the lens is 5 mm closer than that reference, you subtract 5 from 15, giving a vertex distance of 10 mm. So the vertex distance is 10 mm.

Vertex distance is the distance from the back surface of the lens to the corneal apex. In a trial frame, the apex scale shows how the lens sits relative to the eye compared with a standard reference distance (about 15 mm). If the scale indicates the lens is 5 mm closer than that reference, you subtract 5 from 15, giving a vertex distance of 10 mm. So the vertex distance is 10 mm.