What measurement is crucial for fitting monocular lenses?

Monocular pupillary distance is a critical measurement for fitting monocular lenses because it indicates the distance from the center of each pupil to the bridge of the nose. This measurement is essential for ensuring that the optical center of each lens is aligned with the wearer's pupils, which is vital for comfort, clear vision, and effective lens performance.

Using the monocular pupillary distance helps in creating well-fitted lenses that reduce the risk of visual distortion and strain. Proper alignment can enhance the overall experience for individuals requiring monocular lenses, making it an indispensable measurement in the fitting process.

Other options, like inter-pupillary distance, while useful in fitting bifocal or progressive lenses, do not provide the specific measurement needed for monocular lenses. Ocular pressure and visual acuity, while important metrics in eye care, do not directly influence the physical fitting of lenses. Thus, monocular pupillary distance remains the key measurement for this context.