A lens is defined in the dictionary of optics as " ... a body limited mostly by two spherical surfaces, made of transparent material with refractive effect". Lenses used in magnifiers and magnifying readers are convex lenses. i.e. at least one surface corresponds to the surface curvature of a sphere. These lenses are also described as spherical lenses ( sphere ... Greek for ball ). Convex lenses curve outward and have a light-gathering effect and are, therefore, known as convergent lenses. Lenses that curve inward are known as divergent lenses and have a reducing effect ( reducing glasses ).

Due to the demand for greater magnification, more and more aspherical lenses have been used in the last few years( the name "aspheric" simply means non- spherical ). With aspherical lenses, magnifications of between x2 and x 12.5 ( DIN ), can be achived. However, when these are used, it should be noted that an aspherical lens is calculated for a particular interocular distance, in order to achieve the widest field of view with a distortion-free image.

For some years now, the computed interocular distance has also been stated in mm. on aspherical Eschenbach magnifying readers in addition to the magnification, e.g. 4/180. Optimum lens performance is only achieved if the computed interocular distance is observed.