Donegan ELS 2.5 Watch and Clock Makers Single Eye Loupe, 4x Magnification, 5 Diopers, 2-1/2" Focal Length, 24mm Lens Diameter Single-eye loupe magnifier provides hands-free magnification and viewing for a variety of applications such as jewelry-making, watch-making, clock-making, needlework, electronic inspection, book-reading, print inspection, and stamp-collecting Available in multiple lenses that provide a specific magnification power (X), depending on the distance from the object A dioptric number and a magnification number are not the same thing. Diopter strength represents a specific magnification power (X). For example, a 5-diopter provides 2.5X magnification (at a distance of 8) Lens attaches to prescription glasses, aligns in front of the eyeglass for magnification, and can pivot to the side when not in use Optical glass lenses are ground and polished into prismatic lenses for quality and clarity