Why should the keratometer eyepiece not be moved back and forth when focusing the cross?

• A. It can scratch the eyepiece
• B. It can cause glare
• C. It can stimulate the accommodative system, resulting in an incorrect setting
• D. It has no effect

Answer: (C)

When measuring corneal curvature with a keratometer, you want the eye to be in a relaxed state, without forcing accommodation. Moving the eyepiece back and forth changes the optical focus the eye must use to see the cross clearly. That shifts the momentary focusing effort into the eye’s accommodative system, causing the lens to change shape and produce an artificial change in the refractive state. The result is an inaccurate reading of the corneal curvature. Keep the eyepiece focus fixed and use appropriate instrument controls or patient fixation to minimize accommodation. The other concerns aren’t the main issue here, and saying there’s no effect would ignore the impact of accommodation on the measurement.