what is working distance of dental surgical loupes? and How to measure it?
 

Dental surgical loupes are magnifying glasses that are used by dentists to improve their visual acuity during dental procedures. The working distance is the distance between the front of the loupe lens and the patient's teeth or other objects being viewed. It is an important consideration when selecting loupes as it determines the range of motion that the dentist has during the procedure.
                                  

How do I measure my working distance?
(the distance form your eyes to the objects when you working )
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

just need chose the closer one to your real working distance. becaue all the loupes has depth of view. for examples. the loupes working disatnce is 420mm. the real working distance is the range is :380mm-460mm. .