What observable effect indicates that a keratometer is out of focus on the cornea?

When a keratometer is out of focus on the cornea, one of the observable effects is that the lower right mire appears doubled. This doubling effect occurs because the keratometer is not accurately aligning with the curvature of the cornea, leading to a distortion in the reflection of the mires.

In a properly focused keratometer, the mires are clear and distinct, giving precise measurements of the corneal curvature. If the device is out of focus, however, the misalignment causes the reflected mires to overlap, which results in the doubling effect. This doubling can indicate a need for adjustment to ensure accurate measurements for fitting contact lenses or diagnosing corneal conditions.

Observing aspects such as the clarity of individual mires or the stability of readings can also suggest problems, but the presence of doubled mires is a direct and clear indicator of focus issues with the keratometer.