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

Working Distance (WD) refers to the vertical distance from the user’s eyes to the operating site when the surgeon maintains a natural upright working posture during clinical operations.

Note: For ERGO loupes, measure with eyes looking straight horizontally forward.

This parameter can be physically measured with a tape measure, then select the loupe whose rated working distance best matches your measured value. Each specified working distance of surgical loupes covers a valid range of distances for practical use.  
         

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.