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Bi-convex lenses are manufactured with an identical convex surface on both sides of the lens - in other words, there is one concave lens on both faces. Bi-convex lenses have positive focal lengths and form both real and virtual images.
If the lens is biconvex or plano-convex, a collimated beam of light travelling parallel to the lens axis and passing through the lens will be converged to a spot on the axis, at a certain distance behind the lens, known as the focal length.
What happens to the "real" image of an object as it is moved closer to a simple thin bi-convex lens? At points greater than two times the focal length of the lens, the image of an object is real, inverted, and smaller than the object. At two times the focal length, the image is the same size as the object and is real and inverted. At less than two times the focal length, the image is real, inverted, and magnified. When the object approaches closer than the focal length of the bi-convex lens, it appears to be on the same side of the lens as the object. In this instance, the image is magnified and is now a virtual image.
Eschenbach manufactures a 'Large Field' series of Biconvex hand-held magnifiers, which offers the largest fields available in their power range. Lens sizes range from 65 mm to 120 mm, and 80 x 40 mm to 120 x 55 mm. Diopter values range from 3.3 to 7.4 diopters.
The Biconvex PXM® lightweight lenses have anti-static coating and the mount is composed of transparent PXM®. The handle is a shell-type patented design. It is black plastic with a slot for a carrying cord. Black leather cases for each large field biconvex magnifier are also available.