An important factor in measuring lenses with an automated lensometer is keeping the lens flat against the lens stop.

• A. curved
• B. flat
• C. angled
• D. rotated

Answer: (B)

In an automated lensometer, measurements rely on the lens being positioned on a stable, level reference plane. When the lens sits flat against the stops, the optical surfaces align with the instrument’s measurement plane, so the light path enters and exits as the device expects. This gives true, repeatable readings of sphere, cylinder, and axis without extra distortions.

If the lens isn’t flat, it can tilt or rock. That tilt changes the direction of the light entering the lens, introducing unintended prism and altering the apparent powers and axis. A curved lens or one that isn’t resting evenly can cause uneven contact, while an angled or rotated lens can misalign the optic relative to the instrument’s reference, all of which degrade accuracy. Keeping the lens flat ensures the measurement reflects the lens’s true optical properties, making flat the best condition for measurement.