What is the correct method to adjust a lensometer for prism measurement?

To accurately measure prism with a lensometer, rotating the lens holder is the correct method. This action aligns the optical axis of the prism in the correct position relative to the measurement system. When the lens holder is rotated, it allows the user to adjust the orientation of the prism being tested so that the light beam passes through the prism at the appropriate angle. This is crucial for obtaining precise readings of the prism's power and orientation, as any misalignment could result in incorrect measurements.

In contrast, adjusting the eyepiece primarily focuses on the clarity of the sight image but does not affect the prism measurement itself. Calibrating the scale ensures the device is functioning correctly and that readings are accurate but does not involve adjusting the lens for prism measurement. Changing the light filter might alter the visibility or contrast of the measurement, but it doesn't aid in the precise alignment of the prism during the measurement process. Therefore, rotating the lens holder is fundamental for achieving reliable and valid prism measurements.