Check the listing for details. Quality Optics® Professional Series Glass Headband Magnifier Jewelers Visor 3.5x. Condition: New. Listed at 16.99 USD. Quality Optics® Professional Series Glass Headband Magnifier Jewelers Visor 3.5x • Dual 29mm x 38mm / Optical Grade Precision Ground GLASS Lenses• Interchangeable with other brands, Mount holes are spaced 3 ¾" on center• Knob adjustable Felt Padded headband lined w Hypo-Allergenic Leatherette• Visor swings up when not in use, fits all head sizes, Durable lightweight classic design• Approx. working distance (Lens): 3.5X 4” QO-USA Since 2008--------------------------------------------------------------------Precision binocular headband magnifier, provides hands-free magnification, allows three-dimensional vision. Can be worn directly over eyeglasses.Lens exchange/installation directions:Tab type: using the flat side or butt of a screwdriver, gently push out tabs from the inside of the magnifier, align new lens and reinsert tabs.Screw type: first verify screw holes are spaced 3 ¾" on center. Remove screws and lens. Insert new lens and fasten screws do not over tighten. Posted working distance of lenses are approximate and can vary slightly with the user's vision, head size/shape.