Once the manual lensometer is turned on, what is the next step?

After turning on the manual lensometer, the most logical next step is to focus the eyepiece. This adjustment ensures that the operator can accurately read the measurements displayed through the eyepiece. Proper focus is critical because it allows the eye to see the scale and the markings clearly, which directly affects the readings obtained from the lens being measured. A well-focused eyepiece enhances precision and reduces the possibility of errors in the determination of the lens power.

While the other options are certainly important steps in the overall lens measurement process, focusing the eyepiece is foundational and must be done first to gather accurate data. Placing the lens in the lens holder, calibrating the device, and turning on the light source are all crucial steps that follow after ensuring the eyepiece is correctly focused, providing a clear view of the readings necessary for detailed lens analysis.