Songzi Optics 2.5X Binocular Dental Surgical Loupes Anti-fog Titanium Sport Frame (Working Distance:R(420mm)) Magnification:2.5X ,Color: Black,Frame: Anti-fog Sport Frame,Field Of View: 100mm, Viewing Angle: 20-150,Depth of field: 80mm ,Weight:30g. Certificate: ISO9001:2008,CE 3 optional kinds of working distance : R(420mm), L(460mm),XL(500mm). How To Choose Your Loupes.Determine your working distance, which will also automatically determine your field of view. While working on a friend or family member have someone measure the distance from your eyes to your patient's : this is you working distance. Choose the distance that is closest to the selection we offer.IF you are a dentist , your height is in the range from 160cm to 180CM,we recommend regular (420mm) working distance.IF you're a surgeon, you are sitting while working, your height is from 160cm to180cm, we recommend working distance R (420mm ). IF you are standing while working, your height is from 170cm to 180cm, we recommend L (460mm) or XL(500mm) working distance for you. Frame can be weared separately ,also can be weared over glasses as the picture shows.