Zoom lens
FIELD OF THE INVENTION 
     The present invention is related to a kind of zoom lens, especially it indicates a kind of zoom lens used by a projective image forming system (e.g. projective television). 
     BACKGROUND OF THE INVENTION 
     Investigating the fact that the zoom lens with zoom rate between 1.3˜2.0, in general, can be classified into two kinds as described in U.S. Pat. No. 4,516,839, the first kind of zoom lens is composed of the first group having Negative Refractive Power and the second group having positive Refractive Power, these kinds of zoom lenses have a serious shortcoming i.e. too long of its back focal length which makes the small type of lens very difficult the second kind of zoom lens is mainly an improved design in light of the shortcoming of the first kind of zoom lens. In order to shorten the focal length, it employs a telephoto type with the first group having Positive Refractive Power and the second group having Negative Refractive Power. The composition of this type of lens has shorter back focal length wherein the distance between the vertex of the first optical plane and the image plane is smaller than 1 comparing with the effective focal length. 
     But lens of projection type, because the size of the formed image of the television is far greater than the size of the negative, if one employs directly the mode of the two groups of the above mentioned two kinds of prior art, the zoom stroke is large, the size of the lens can&#39;t be diminished; besides, the size of the lens will increase. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide a kind of zoom lens used by the projective image forming system (e.g. projection television). The zoom lens of the present invention employs the four-group framework of negative, positive, negative, positive which is composed of four lens groups, starting with object side, in sequence, they are the first lens group with Negative Refractive Power, the second lens group with Positive Refractive Power, the third lens group with Negative Refractive Power and the fourth lens group with Positive Refractive Power. Using the zoom lens framework of the present invention can effectively resolve the problems of large zoom stroke and large-size lenses of the foregoing prior arts, it can also resolve the image difference problem and increase the relative hole diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is the schematic diagram of the lens groups&#39; space layout of the present invention; 
     FIG. 2A is the schematic diagram of each lens&#39;s location of the present invention when in the wide-angle terminal; 
     FIG. 2B is the schematic diagram of each lens&#39;s location of the present invention when in the middle focal length; 
     FIG. 2C is the schematic diagram of each lens&#39;s location of the present invention when in the telephoto terminal. 
    
    
     DESIGNATION IN THE DRAWINGS 
     G1 The first lens group 
     G2 The second lens group 
     G3 The third lens group 
     G4 The fourth lens group 
     1˜10 The lens pieces 
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, that is the schematic diagram of the lens groups&#39; space layout of the present invention; it is known from the figure that the present invention is composed of four lens groups, starting with object side, in sequence, they are the first lens group G1 with Negative Refractive Power, the second lens group G2 with Positive Refractive Power, the third lens group G3 with Negative Refractive Power and the fourth lens group G4 with Positive Refractive Power. 
     The first lens group G1 is further composed of lens pieces 1, 2, 3 of a Positive Refractive Power and a Negative Refractive Power; the second lens group G2 is further composed of lens pieces 4, 5, 6, 7; the third lens group G3 is further composed of lens pieces 7, 8, 9, wherein the first lens closed to the image side is a convex one; the fourth lens group G4 comprise a lens 10, its position is fixed in the course of zooming. 
     The fourth lens group (i.e. lens 10) is composed of Fresnel lens in one of the embodiment example of the present invention. 
     In the course of zoom lens assemblage of the present invention; it is in accordance with the following conditions: ##EQU1## where k1 is the Refractive Power of the first lens group 
     k3 is the Refractive Power of the third lens group 
     During the course of zooming, as it moves from the wide-angle end (as shown in FIG. 2A) toward the telephoto end (as shown in FIG. 2c), the first, second and third lens group G1, G2 and G3 are moving toward the object space simultaneously, in the meantime, the distance between the first lens group G1 and the second lens group G2 is decreasing as the focal length is increasing; the distance between the second lens group G2 the third lens group G3 is slightly increasing and the back focal is increasing but the location of the fourth lens group G4 is fixed. The zooming operation of the present invention satisfies the following conditions: 
     
         0.5fW&lt;|f1|&lt;3fT 
    
     
         0.5fW&lt;|f3|&lt;3fT 
    
     where 
     fw is the focal length of the total system at the wide angle end 
     fT is the focal length of the total system at the telephoto end 
     f1 is the focal length of the first lens group 
     f3 is the focal length of the third lens group 
     Therefore, in the course of zooming, it is accomplished changing through the distances between the first lens group and the second lens group, the second lens group and the third lens group and the fourth lens group. As the zooming process moving from the wide-angle toward the telephoto, the first, second and third lens groups are moving toward the object side but the location of the fourth lens group is fixed. 
     The zoom lens, which is made according to the present invention, can make the total length of zoom shorter and the FNO smaller. 
     DESCRIPTION OF AN EMBODIMENT EXAMPLE 
     
         ______________________________________                                    
SURFACE RADIUS    THICKNESS   N      Abbe No.                             
______________________________________                                    
1       -449.52243                                                        
                  4.000000    1.741060                                    
                                     45                                   
2       64.93259  7.010745                                                
3       791.82499 5.822189    1.742862                                    
                                     44.8                                 
4       209.08832 2.442789                                                
5       89.63345  5.644970    1.718216                                    
                                     29.3                                 
6       672.85307 29.288980(*2)                                           
7       86.66644  6.100000    1.717420                                    
                                     46.9                                 
8       -226.71036                                                        
                  0.296025                                                
9       53.23270  13.499513   1.487701                                    
                                     70.3                                 
10      -114.15475                                                        
                  3.000000    1.740199                                    
                                     29.3                                 
11      133.34274 5.607067                                                
12      infinitive                                                        
                  1.016705(*3)                                            
13      165.47226 3.725753    1.722213                                    
                                     35.5                                 
14      70.27022  22.672875                                               
15      150.32043 9.804426    1.515172                                    
                                     56.8                                 
16      -99.53541 16.422099                                               
17      -36.56901 5.760321    1.744000                                    
                                     447                                  
18      -57.93851 62.625364(*4)                                           
19      infinitive                                                        
                  2.100000    1.492000                                    
                                     58.4                                 
(*1)20  -53.24746 8.100000                                                
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     where 
     SURFACE - - - The curve surface of each lens (a surface of lens) 
     RADIUS - - - Radius of curvature 
     THICKNESS - - - Thickness of lens 
     Abbe No. - - - Glass material 
     N - - - The Index of the Refraction 
     whereto the aspherical surface characteristic of the 19th lens surface can be expressed by the following polynomial formula: ##EQU2## where (*1): Fresnel Surface (threaded curve surface) 
     Z: Convex or concave amount 
     C: Curvature (V=1/R) 
     Y: the radial height of optical surface 
     C1: -2.0839E+01 
     C3: -5.6649E-06 
     C4: 3.4747E-09 
     C5: -1.0216E-12 
     C6: 1.5897E-16 
     C7: -1.2098E-20 
     C8: 3.0000E-01 
     
         ______________________________________                                    
width of pitch (thread width on the threaded curve surface)               
______________________________________                                    
(*2)THI S6 (MM)                                                           
              29.28898   11.92644 1.41862                                 
(*3)THI S12 (MM)                                                          
              1.01671    1.71068  2.16345                                 
(*4)THI S18 (MM)                                                          
              62.62536   80.01553 102.58417                               
EFL (MM)      103.9458   --       163.0009                                
FNO           3.2500     --       4.3300                                  
ANG (DEGREE)  27.8843    --       18.9542                                 
______________________________________                                    
 
    
     where 
     THI is the distance between lenses 
     EFL is the focal length 
     FNO is the aperture 
     ANG is the angle of the field 
     The foregoing is the embodimental description of the present invention, one who is versed in this kind of technique can undertake various modifications and applications thereof, it should be appreciated that any of these changes should not be disengaged from the spirit of this creative work and the scope of the patent defined as follows.