Final answer:
When measuring monocular PDs, using a ruler alone can result in errors for people with an asymmetrical or symmetrical nose. To overcome this issue, it is recommended to use a frame with adjustable PD markers for precise measurements.
Step-by-step explanation:
Using a ruler alone to measure monocular PDs can lead to errors when a person has an asymmetrical or symmetrical nose. This is because the distance between the two lines on the ruler may not align properly with the center of the person's nose, resulting in inaccurate measurements. To overcome this issue, it is recommended to use a frame with adjustable pupillary distance (PD) markers. The frame allows for precise alignment with the person's nose, ensuring accurate measurements of PDs.