Zoom lens having magnification factors in the range of 6.5x to 14x for micrographic applications
TECHNICAL FIELD 
     The present application relates to lens systems for use with micrographic projectors and printers and more particularly to such lens systems having variable focal lengths. 
     BACKGROUND ART 
     In order to conserve space, documents are often stored photographically with their size greatly reduced on microfilm, microfiche, or other media. Such processes and media are known generally by the term micrographics. 
     In order to utilize a document which has been stored in micrographic format, a projector or reader is required. Such a reader will produce an enlarged reproduction of the reduced image on a screen for viewing by the operator. Often such readers also include a printer. When a desired document has been located by projection onto the screen the operator may produce a permanent enlarged copy through photographic or plain paper copier techniques. Systems which permit such reading and printing are known as reader/printers. The term reader/printer will be used herein to denote both readers containing only a projector and those including a printer in addition to a projector. 
     A problem which arises in the use of such systems relates to the fact that different reduction factors may be used in recording the micrographic images. As a result different magnifications are required in order to properly reproduce such images. In order to accommodate such variations, lenses having a variety of magnifications are commonly provided with micrographic reader/printers. In some cases as many as fifteen or more lenses are required to accommodate all possible reduction factors which may be used. The frequent changing of lenses can create a great inconvenience to operators of such equipment and can increase the time required for reading and printing documents stored in micrographic form. 
     DISCLOSURE OF INVENTION 
     The present invention provides a zoom lens for use in micrographic applications. The zoom lens of the present invention provides magnification factors in the range of 6.5 to 14. The lens varies from a focal length of 62.9 mm with an aperture of f8.3 to a focal length of 132.9 mm with an aperture of f17.4. The use of such a zoom lens reduces the number of lenses required to view or print micrographic images having different reduction factors. An additional advantage of such a lens arises in reader/printers having a printer which utilizes plain paper copier techniques. In such printers, toner usage is minimized by allowing the operator to optimize image size, thereby eliminating the dark borders often associated with such printers. 
     The lens system of the present invention has the additional benefit of being extremely compact and acting as a telephoto lens. The lens system, with the zoom space fully extended, has a focal length of 132.9 mm and is 104.6 mm long, a telephoto ratio of 0.79. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic representation of the lens system of the invention; 
     FIG. 2 illustrates the decentration defect; and 
     FIG. 3 illustrates the tilt defect. 
    
    
     DETAILED DESCRIPTION 
     The lens system of the present invention provides a variable magnification or zoom lens for use in micrographic reader/printers. The preferred embodiment of the present invention is illustrated in FIG. 1. A film containing micrographic images is placed in film plane 10 and light from a light source, not shown, is projected therethrough. Elements 12, 14, 16, 18, 20, 22, 24 and 26 operate together to enlarge the image used for projection on a screen or for reproduction by a printer. The amount of the enlargement is determined by the spacing between elements 20 and 22. The magnification factor of the lens system of the invention may vary between 6.5 and 14. 
     Detailed information on the lens system of the present invention is shown in the Table below. The values for the radii of curvature, the thickness, the separations, or air space between the elements, the tolerances for element thicknesses and separations and the decentration tolerances are given in millimeters. The values for the tilt tolerances are given in degrees. The Table entries for index of refraction include two values. The first is the actual index of refraction of the element while the second is the dispersion. The tolerances for all indicies of refraction are equal to ±0.0005. In order to read the information in the Table it should be understood that each element of FIG. 1 comprises two surfaces. Thus, element 12 comprises surfaces S1 and S2 having the radii of curvature shown in the Table. This element is 7.60 ±0.50 mm thick on its optical axis. It is made from glass having an index of refraction of 1.69350 with a dispersion of 53.3 ±0.4. Finally it has a maximum decentration of 0.012 mm and a maximum tilt of 0.046°. Element 14 is separated from element 16 by an air space of 16.75 ±0.050 mm on the optical axis of the system. This separation is measured from surface S3 of element 14 to surface S4 of element 16 on the optical axis of the system. Similarly the data regarding the remaining elements and the separations between them may be determined from the information provided in the Table. Those skilled in the art will perceive that deviations from the dispersion values specified in the Table will not necessarily prevent the construction of a functional lens system. Such deviations may, however, introduce chromatic aberation and reduce the resolution of the system. 
     Elements 12 and 14, 16 and 18, and 22 and 24 form cemented doublets and, as such, share common surfaces, S2, S5 and S10, respectively. The decentration and tilt tolerances given for elements 12, 16 and 22 each actually apply to the doublet as a whole. 
     Aperture stop 11 is placed in front of element 12, as shown, rather than in the zoom space, as is more commonly done. Such placement allows the lens system of the invention to be more compact than it would be otherwise. Aperture stop 11 is in the form of an iris-type diaphram which may be adjusted to pass the required amount of light for optimum viewing or printing. 
     The Table indicates that the separation between elements 16 and 17 may vary between 4.075 mm and 17.336 mm. The value of this separation determines the magnification factor. A shorter separation will provide a smaller magnification factor while a longer separation will provide a greater one. 
     Particular note should be given to the decentration and tilt tolerances. These tolerances are commonly given less than their due weight in the design of a lens system. In reality they are quite important if a lens system is to perform adequately. 
     Decentration is defined as the distance between the optical axis and the mechanical axis of the lens at the center of the lens. This is illustrated in FIG. 2. FIG. 2 shows a lens 40 having a concave surface 41 and a planar surface 42. The optical axis of the lens is shown as 43 while the mechanical axis of the lens is 44. These two axes are separated by a distance D which is the decentration of the lens. 
     Tilt is defined as the angle between the optical axis and the mechanical axis of the lens. This is illustrated in FIG. 3. FIG. 3 shows a lens 46 having a convex surface 47 and a planar surface 48. The optical axis of the lens is shown as 49 and the mechanical axis as 50. The angle α between the optical and mechanical axes of the lens is the tilt. When a lens is subject to both decentration and tilt the decentration is measured at the center of the lens. 
     As shown in FIG. 1, the preferred embodiment also includes a prism 28. Prism 28 allows the operator to rotate images to insure proper alignment for viewing or printing. When such a prism is used, image quality will be improved by including one or more collimating lenses. In the preferred embodiment a cemented doublet is utilized. 
     A further aspect of the preferred embodiment relates to the distance between prism 28 and lens element 12 and the distance between lens element 26 and film plane 10. Cams are preferably used to vary these distances as the zoom space between elements 20 and 22 is varied. Thus an image which is in proper focus at one zoom setting will remain in focus as the zoom setting is varied thereby retaining the desired image resolution. 
     
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             RADIUS OF                                                    
                     THICK-                INDEX OF DECEN-                
ELEMENT                                                                   
       SURFACE                                                            
             CURVATURE                                                    
                     NESS  SEPARATION                                     
                                   TOLERANCE                              
                                           REFRACTION                     
                                                    TRATION               
                                                          TILT            
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       S1    19.146                                                       
12                   7.60          0.050   1.69350 -                      
                                                    0.012 0.046           
       S2    -21.208                       53.3 ± 0.4                  
14                   1.50          0.050   1.78590 -                      
       S3    28.652                        43.9 ± 0.3                  
                           16.75   0.050                                  
       S4    -216.043                                                     
16                   6.80          0.100   1.48749 -                      
                                                    0.012 0.086           
       S5    -13.501                       70.4 ± 0.6                  
18                   2.00          0.100   1.83500 -                      
       S6    -33.758                       43.0 ± 0.3                  
                           0.20    0.050                                  
       S7    27.871                                                       
20                   4.90          0.100   1.48749 -                      
                                                    0.025 0.070           
       S8    -354.527                      70.4 ± 0.6                  
                           4.075                                          
                           -17.336                                        
       S9    -65.515                                                      
22                   2.30          0.100   1.77250 -                      
                                                    0.035 0.059           
        S10  28.767                        49.6 ± 0.4                  
24                   4.48          0.100   1.78470                        
                                           26.1 ± 0.2                  
        S11  -312.853                                                     
                           2.30    0.050                                  
        S12  -29.315                                                      
26                   2.50          0.100   1.64000 -                      
                                                    0.025 0.054           
        S13  Plano                         60.2 ± 5                    
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