Lenses in Trial Frame: Where does the largest sphere lens go?

• A. Front surface
• B. Top surface
• C. Back surface
• D. Bottom surface

Answer: (C)

Placing the strongest spherical lens closest to the eye minimizes unwanted prism and keeps the optical axis more stable as the trial frame moves or tilts. Any decentration of a lens relative to the pupil creates prism proportional to the lens power, so a high-power sphere would produce a larger prismatic effect if it were farther from the eye. By keeping the largest sphere on the back surface (near the eye) and the smaller powers on the front, the frame maintains better alignment and reduces errors during refraction.