The present invention broadly relates to a magnifier lens system and, more particularly, pertains to a new and improved construction of a magnifier lens for use in optoelectronic devices. 
Magnifier lenses of this type are known in the art for observing the image on the anode surface of an electronic tube.  These magnifiers must have both a very good image quality and a short overall length.  This is because the electronic tubes are predominantly employed in portable optoelectronic devices, e.g. observation telescopes or gun sights for low light level or heat image applications. 
A magnifier lens is known from German Patent Publication No.  2,708,691, published Sept. 8, 1977, which consists of only five lenses and has an exit pupil with a distance greater than 0.58f from the last lens vertex, f being the system focal length.  These magnifiers suffer from an exceptionally great overall length of 2.53f and a sum of the thicknesses of the lens elements of more than 2.2f.  Moreover, the glasses employed have specific gravities of 3.78 g/cm.sup.3 or more.  Apart from the resulting considerable weight and poor handling properties, such magnifiers are also rather costly to fabricate. 
A magnifier lens is known from the European Patent application No.  29,005, published May 20, 1981, which has a shorter overall length of 1.5f and a sum of the lens thicknesses of 1.3f.  However, this magnifier system consists of six lens elements.  Its main disadvantage is a rather short distance of the exit pupil from the last lens vertex of only 0.57f.  This is the reason why the magnifier cannot be made with a short focal length and why it cannot be used for devices mounted on a firearm.  Moreover, this magnifier system has a rather substantial distortion of 10.8 percent at a field angle of 24.6.degree..
From the above examples of the state of the art it may be seen that a short overall length entails a short distance of the exit pupil from the last lens vertex.  A magnifier of simple construction having a short overall length, a great distance of the exit pupil and a good correction of distortion is not known in the art. 
If one tries to modify known magnifier systems in order to shorten the overall length, the distance of the exit pupil is reduced at the same time and the distortion increases.  This fact is demonstrated by the above-mentioned known examples:
______________________________________ Overall Distance Distortion Example length of Exit Pupil at 20.degree. ______________________________________  German Patent 2.53 f 1.33 f approx.  5% Publication No.  2,708,691 European Patent 1.5 f 0.57 f more than 8% Application No.  29,005 ______________________________________
As already mentioned, the reduction in overall length of the system according to the aforementioned European Patent Application No.  29,005 not only brings about a shortened distance of the exit pupil from the last lens vertex and an increased distortion, but the system also has one additional lens element.