the workig distance is one of the important data for the loupes. the working distace is the distance form the loupes user's eyes to patient mouth.

How do I measure my working distance?

While many things such as height, and personal preference should be considered, the easiest way to figure your working distance is to:

Step 1- measure the distance from between your eyes and the area you want to focus on  (a mouth, an eye, etc), while sitting or standing in the correct ergonomic position, (back straight and arms perpendicular to the floor]. 
Step 2- Subtract 1.5" (the distance from your eye to end of the Loupe

That number is your working distance:
Short       (10"-13")    340mm
Regular    (14"-18")   420mm
Long        (17"-21")    500mm
X Long     (22"-27")   550mm