The Donegan replacement lens plate for the OptiVISOR hands-free, headband binocular magnifier, has an optical-grade, acrylic lens ground and polished into a prismatic lens, and is available in multiple dioptric strengths which provide specific magnification power (X) at specified focal lengths, or distance from object. This magnifier enhances the size and resolution of an object or image, and is commonly used for a variety of applications such as jewelry-making, watch-making, knitting, artwork, electronic inspection, book-reading, print inspection, and stamp-collecting.Determining MagnificationDiopterMagnificationFocal Length (Distance from Object)Sample View20/20 vision20" (50.8cm)2-diopter lens1.50X20" (50.8cm)3-diopter lens1.75X14" (35.56cm)4-diopter lens2.00X10" (25.4cm)5-diopter lens2.50X8" (20.32cm)7-diopter lens2.75X6" (15.24cm)10-diopter lens3.50X4" (10.16cm)The OptiVISOR replacement lens plate is made of a durable plastic material. It is constructed to interchange in and out of the OptiVISOR body with ease. The replacement lens plate comes housed in a protective carrying case, and has a one-year limited warranty.Diopter (D) is a technical term for the measurement of the light curvature and thickness of a lens. 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?). The higher the diopter, the more magnification a lens can provide. As magnification increases, distance to the object (focal length), and field of view (diameter, or narrowness of view) decrease. Magnifiers and prescription eyeglasses have a diopter value, such as 1.5 or 2. The dioptric strength of an accessory lens, such as prescription eyeglasses or an eye loupe, must be added to the dioptric strength of the magnifier to calculate total magnification.Magnifiers are devices used to enlarge the visual appearance of an object or image. Magnifiers come in a variety of styles such as hand-held, headband, standing, clip-on, eye loupe, and those that hang in a necklace form, or fit in a trouser pocket. Magnifiers are commonly made of plastic or glass. They can have one or more lenses with varying magnification abilities, and can have a binocular configuration with a single lens, or two separate lenses. They are sometimes used with LED or fluorescent light sources to help control viewing capabilities. Magnifier ability is often expressed as magnification at a specific length. For example, 1.75X at 14? means that when a magnifier is held at 14? from a viewer?s eyes, the object will be magnified 1.75 times (X) its actual size. As magnification increases, viewing areas and focal length decrease. Magnifiers sometimes have a diopter number, expressed as + or - D, which is a measurement of strength (or power) of the lens. Some magnifiers are marked with a dioptric number on the lens. A high diopter number has a higher magnification than a low diopter number. Aspheric magnifiers, unlike spheric magnifiers, produce a sharp image to the edge of the lens for less distortion. Magnifiers are widely used in jewelry inspection and fabrication, watch-making, electronic inspection, needlework, artwork, reading, print inspection, and stamp-collecting.Donegan Optical manufactures and distributes precision visual devices for hobbyists and professionals in industry, home, office and crafts applications. The company was founded in 1952 and is headquartered in Lenexa, Kansas.What?s in the Box?Replacement lens plate for OptiVISOR magnifierFitted carrying caseInstruction manual