Bifocals are typically measured at which location related to the patient’s face?

Bifocals are measured at the location of the bottom lid, specifically where eyeliner would typically be applied. This particular measurement is crucial because it ensures that the bifocal segment aligns correctly within the visual plane for the wearer.

When fitting bifocals, the optical centers of both segments (distance and near vision) must be accurately positioned to provide clear vision at varying distances. The bottom lid is a reliable landmark for determining the correct height for the segment of the lens used for near vision. Positioning the bifocal segment too high or too low can cause discomfort and visual distortion, affecting the patient's ability to see clearly.

Using the bottom lid as a reference point takes into account the natural positioning of the eyes and the common viewing habits of individuals. For instance, when looking down, the eyes naturally fall, and this relationship is crucial for proper lens function. Other landmarks such as the bottom lashes, top lashes, or the center of the pupil do not provide as consistent or functional a measurement for bifocal placement compared to the bottom lid, leading to potential inaccuracies in fitting.