what is Pupil distance？ How to measure it?
upil distance (PD), that is the distance of two pupils. near Pupil distance is the distance when you looking down (looking the near objects when you Do operation or work), Far pupil distance is the pupil distance when you looking straight. TTL loupes need near pupil distance. if you don't know your near pupil distance. How to measure the near pupil distance for TTL loupes?
1. The best way is to go to a nearby optical lab meaure it.
if you want to a 420mm working distance, just need them measure your near pupil distance at 420mm distance. When you measure it, please measure the far pupil distance and near pupil dsitance, choose the pupil dsitance on line as near pupil distance. also can leave if the pupil distace is different between right eye and lest eye.
2. the fast and easy way to measure the Pupil dsitance: meausre BY App.
down load the App "PDcheck Ar" from App store of you smart phone. and then measure the pupil distance as the guardiance.