When measuring the binocular interpupillary distance with a ruler, you may measure from:

The correct method for measuring binocular interpupillary distance (PD) when using a ruler involves measuring from the inner limbus to the outer limbus of the eye. This approach is based on the anatomical landmarks of the eye, where the limbus is the border between the cornea and the sclera. The inner limbus refers to the nasal side of the eye where the cornea meets the sclera, while the outer limbus is the temporal side. Measuring from these specific points helps ensure that the glasses or other optical devices are properly aligned with how a person views through their pupils.

Using the inner limbus to outer limbus measurement accounts for the whole width of the face and provides a more accurate representation of the distance between the two pupils, which is critical for effective optical correction and comfort.

In contrast, measuring from inner limbus to inner limbus or from inner canthus to inner canthus does not take the full width of each eye into account and could lead to inaccuracies in fitting. Measuring from outer canthus to inner canthus would also be misleading since it involves the outer edges of the eyes instead of a precise line of sight through the pupils. Thus, the inner limbus to outer limbus provides the most relevant and accurate