In an automated lensometer, keeping the lens flat against the lens stop helps ensure accurate readings.

• A. True
• B. False
• C. Sometimes
• D. Not applicable

Answer: (A)

Keeping the lens flat against the lens stop is essential because the stop provides a reference plane for the lens surface. In an automated lensometer, the reading depends on the light path interacting with the lens surfaces along a fixed optical axis. If the lens isn’t pressed flat, it can tilt or decenter, changing the orientation of the curved surfaces relative to that axis. This tilt alters the apparent curvature and prism effects, leading to inaccurate vertex power (and axis for cylinders) readings. Seating the lens flush against the stop ensures the surface is properly aligned with the instrument’s optical axis, minimizing tilt and decentration errors and giving a more reliable, repeatable measurement.