How should the final prescription be written when back vertex power differs from the trial-frame powers?

• A. from the individual lens powers
• B. from front vertex power
• C. from resultant back vertex power
• D. from subjective refinement

Answer: (C)

The main idea is that the distance between the eyes and the spectacle lenses changes the effective power of the correction. The final prescription must reflect the back vertex power—the power at the back surface of the lenses as it sits in front of the eye—because that is the power the retina actually experiences when the glasses are worn. When the back vertex power differs from what the trial frame shows, you convert to the back vertex power so the lenses placed in front of the eyes deliver the correct refraction.

This conversion accounts for the vertex distance (the gap between the cornea and the lens). The trial-frame powers are recorded at the frame’s vertex, but the final glasses sit at a specific back vertex distance. By determining the resultant back vertex power, you ensure the patient gets the intended correction through the lenses. For example, a trial-frame power of a certain diopter value at a given vertex distance may translate to a slightly different power at the back of the lens; the prescription would then be written to match that back vertex value.

Using the individual lens powers without adjusting for vertex effects, or selecting the front surface power, would not reflect what reaches the eye. Relying on subjective refinement alone also misses the objective correction needed at the retina. The appropriate approach is to base the final prescription on the resultant back vertex power after accounting for vertex distance.