The difference between measuring the distance and near P.D.'s using a ruler is:

The distinction in measuring distance and near pupillary distance (P.D.) primarily involves the method in which the measurements are taken, particularly how the dispenser aligns with the wearer's eyes during the process. When measuring for distance P.D., the dispenser can generally maintain a direct line of sight with both eyes aligned to the wearer's pupils. This usually involves no change in focus or position, as distance vision typically requires the wearer to look straight ahead.

In contrast, when measuring near P.D., the wearer's gaze is often directed at a closer target, which can involve looking slightly downward or forward. While the dispenser aims to keep the measurement accurate, it may require adjusting their viewpoint or relation to the wearer's eyes, especially if the wearer is instructed to look at a specific point or object. Thus, the correct answer highlights that during the near measurement, the dispenser remains focused on the same eye alignment rather than shifting focus or direction, maintaining sight on the wearer's eyes without changing eyes.

The other options relate to aspects of the measurement process that do not accurately capture this key distinction. For example, moving the head during the measurement can introduce inaccuracies, while the limbus to limbus technique is a concept of refracting rather than a measurement approach in this context