What is the trial frame scale?

• A. Shows us vertex distance, ranging from 0 to 15 mm
• B. Measures ocular surface curvature
• C. Indicates pupillary distance
• D. Displays accommodation amplitude

Answer: (A)

The trial frame scale is a built‑in ruler on the trial frame that shows the vertex distance—the distance from the back surface of the spectacle lens to the eye. This distance matters because the effective power of a lens changes as you move it closer to or farther from the eye, especially with higher prescriptions. The scale usually runs from about 0 to 15 millimeters, covering the typical range used in refraction and fitting. Setting and recording this distance ensures the prescription you determine matches what the patient actually experiences when wearing the glasses, preventing under- or over-correction.

The other options don’t fit because ocular surface curvature is measured with a keratometer or similar instrument, pupillary distance is obtained with a PD ruler or pupillometer, and accommodation amplitude is assessed with near-vision/acuity tests—not with the trial frame scale.