what is working distance of loupes, How to measure working distance?

the working distance is one of the important data for the loupes. the working distance is the distance from the loupes user's eyes to the patient's mouth or the objects                   

How do I measure my working distance?
working distance is the distance from your eyes to the objects operated when you working a normal position 
While many things such as height, and personal preference should be considered, the easiest way to figure out 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 the end of the Loupee

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

just need to choose the closer one to your real working distance. because all the loupes have a depth of view. for examples. the loupe's working distance is 420mm. the real working distance is in the range:360mm-460mm.