When focusing the eyepiece of a lensometer, why is it important to stop turning the eyepiece when a sharp focus is first obtained?

• A. Continuing to turn could cause accommodation.
• B. It will damage the lensometer.
• C. It will cause the image to invert.
• D. It will not affect the reading.

Answer: (A)

When using a lensometer, the reading should reflect the lens under test with the eye as relaxed as possible. Stopping as soon as a sharp image is first obtained prevents the eye from starting to accommodate to the target. If you continue turning the eyepiece, the eye may increase its accommodation, which changes how clearly the target is perceived and can shift the reading, leading to an inaccurate measurement. The main point is to keep the eye relaxed to avoid introducing a measurement error. The other options aren’t the reason this step matters; turning further won’t typically damage the instrument, it doesn’t invert the image in ordinary use, and the important effect here is accommodation altering the reading.