When viewing the lens in a manual lensometer, if the target lines run diagonal to their intended course, what does this represent?

When observing the target lines in a manual lensometer, if they appear diagonal to their intended course, it indicates that the lens is misaligned or is not aligned properly with the optical axis. This misalignment is referred to as being "off-axis."

In a properly aligned lens, the target lines should be perfectly horizontal or vertical, depending on the specific configuration of the lensometer being used. However, when they tilt at an angle, it suggests that the optical center of the lens is not aligned with the optical axis of the lensometer. This misalignment can affect the accuracy of the lens power measurement and may lead to incorrect prescriptions if uncorrected.

Understanding this concept is vital for ensuring accurate lens measurements and ultimately contributing to better patient outcomes in vision correction.