Abstract:
A zoom camera lens which is particularly compact, has a small number of elements and is capable of good performance at relatively wide aperture has a front negative lens group, a middle positive lens group and a rear negative lens group. The rear negative lens group has a focal length not more than 1.25 times the focal length of the middle positive lens group. Preferably, the lens has seven or eight elements and is designed without the use of aspheres.

Description:
This invention relates to zoom camera lenses having at least three moving groups. 
     U.S Pat. No. 5,153,776, issued Oct. 6, 1992 to Nozawa, is typical of a large number of complex multi-element wide angle zoom camera lenses having a field angle of 70° or more in the wide angle position and a zoom ratio of 3 or more. These prior lenses have a front group of negative power, a middle group of positive power and a rear group of negative power with all three groups moving independently to vary focal length. The middle positive group includes a number of air-spaced elements with a stop placed either immediately in front of or immediately to the rear of the group. Such lenses commonly have 10 to 15 elements, and may include some aspheric surfaces. They are relatively long from the front surface to the film plane. Negative power in the lens is somewhat evenly divided between the front and rear negative groups while the middle positive group is more powerful than either of the negative groups. 
     U.S. Pat. No. 5,175,648 to Mori, Dec. 29, 1992, shows a similarly constructed lens in which power of the rear negative group is increased to make the lens more compact. This lens also has ten or more elements with some aspheric surfaces. 
     U.S. Pat. No. 4,955,700, issued Sep. 11, 1990 to Yamaguchi, shows a ten element zoom lens of generally the same type in which the focal length of the rear group is less than the focal length of the front group to provide compactness to the system. The front group contains a negative and positive element which are meniscus to the front. The lens has a four or five element middle positive group and a four element rear negative group. 
     Japanese Laid-Open Patent Applications 63-15412 and 64-21027, laid open Aug. 1, 1989 and Aug. 9, 1990, respectively, show eight element designs in which the front and rear groups each have two elements with a four element middle group. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a zoom camera lens having at least three moving lens groups and substantial zoom range with a minimum number of lens elements. 
     This and other objects are accomplished by a zoom camera lens having at least three movable lens groups, arranged, from the object side, a front negative group, a middle positive group and a rear negative group. The front and rear negative groups each have two elements. The focal length of the rear group is not more than 1.25 times the focal length of the middle group. A stop for the lens is positioned adjacent to but slightly in front of the middle group and is movable with the middle positive group. 
     According to a preferred embodiment, the two elements in the front negative group are a negative element and a positive element which are movable with respect to each other to better control distortion in the lens. 
     According to another preferred embodiment, the middle positive group consists of two components, a positive singlet and a positive doublet, providing a well corrected compact 3X zoom lens having only seven elements which provide high quality images without the use of aspheres. 
     According to another preferred embodiment, both elements of both the front and rear groups have indexes of refraction of 1.7 or greater. 
     In general, these lenses are effective over a 3 to 1 range (3X), a wide angle f/number of 4.6 or less, and are well corrected despite a small number of elements and without a need for aspheric surfaces. 
     Like some of the prior art cited above, these lenses are longer from the front of the first element to the film plane in the telephoto range than they are in the wide angle position, but the elements are closer together in the telephoto position and more widely separated in the wide angle position. According to a preferred embodiment of the invention, the lenses have a position in which the groups are more compacted even than the telephoto position and are positioned closer to the film plane than the wide angle position. This is not a position for taking pictures but is extremely useful as a stored position allowing the closing of covers and general compactness to the camera when not in use. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-4 are cross-section schematics of zoom lenses. 
     FIGS. 5-7 are cross-section schematics illustrating an arrangement of lens groups of the FIG. 1 lens in its wide angle, telephoto, and stored conditions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Further advantages will be apparent from the Examples below. 
     Note that Examples 1, 2, 3 and 5 contain just seven elements and that Examples 1-5 have the stop immediately in front of the middle group. This location of the stop has minimized the diameter of the lens barrel encasement. 
     The wide angle field coverage, especially of Examples 1-5, is extremely large, for example, a semifield angle of 40.03° in a lens constructed according to Example 1. 
     In Examples 1-5 the indices of refraction in both the first and last negative groups contain elements which are all in excess of 1.7. In both groupings, the positive element has a lower abbe V number than does the negative element. 
     Although some aspects of these lenses can be used in lenses having aspheres, a significant advantage of these designs is that they operate well across the range in apertures of which they are designed with a minimum number of elements without aspheres. 
     In the following Examples distances are in millimeters. The index is for the e line of the spectrum and V is the abbe number. 
     
         __________________________________________________________________________Example I (FIG. 1)__________________________________________________________________________ CLEAR  AXIALSURFACE APERTURE        BEAM RADIUS  THICKNESS                             INDEX                                  V__________________________________________________________________________1     26.64  5.60 23.8205 1.300   1.713                                  53.82     21.16  5.46 11.9916 7.8003     19.44  6.73 34.9306 3.400   1.755                                  27.64     17.98  6.77 90.1422 A 7.71   7.71 DIAPHRAGM                     1.6415     9.49   7.85 79.3527 5.735   1.487                                  70.46     12.74  7.94 -20.3517                     .2507     13.74  7.83 253.018 5.682   1.487                                  70.48     14.82  7.21 -10.1265                     1.000   1.755                                  27.69     16.33  7.23 -17.6583                     B10    21.33  3.37 -23.6888                     4.666   1.755                                  27.611    22.81  2.87 -15.5713                     2.95712    21.81  1.87 13.8594 1.300   1.713                                  53.813    27.50  1.69 102.021__________________________________________________________________________FOCAL                  ZOOM SPACINGSLENGTH BACK FOCUS         FRONT FOCUS                  A    B     LENS LENGTH__________________________________________________________________________25.75 7.97    12.06    12.647                       12.503                             60.88143.31 30.20   27.56    10.403                       5.318 51.45272.81 66.05   53.97    7.598                       1.080 44.408__________________________________________________________________________ 
    
     Example 1 is a 3X zoom lens which has a semi-field which varies from 40.03° to 16.55° and an f/number which varies from 4.6 to 7.0 as focal length increases. It has an unusually large angular field in this wide angle position for a lens with only seven elements and no aspheric surfaces. The diaphragm location has minimized the diameter of the lens barrel encasement. The front negative group (surfaces 1-4), middle positive group (surfaces 5-9) and rear negative group (surfaces 10-13) have focal lengths of -81.08, 22.19 and -25.44. 
     
         __________________________________________________________________________Example 2__________________________________________________________________________ CLEAR  AXIALSURFACE APERTURE        BEAM RADIUS  THICKNESS                             INDEX                                  V__________________________________________________________________________1     29.53  10.56             26.0035 1.500   1.713                                  53.82     23.31  10.22             13.0257 8.0233     21.65  12.78             23.8960 4.000   1.755                                  27.64     19.64  12.55             34.2671 A 13.80  13.80             DIAPHRAGM                     3.0455     15.94  14.43             53.1290 4.039   1.487                                  70.46     16.50  14.50             -24.6319                     .2807     16.62  14.12             373.842 7.000   1.487                                  70.48     16.99  13.20             -11.3452                     1.200   1.755                                  27.69     18.73  12.53             -20.1044                     B10    23.43  12.35             -27.4405                     4.899   1.755                                  27.611    24.82  9.55 -17.3869                     3.42812    23.52  9.31 -15.0024                     1.500   1.713                                  53.813    29.34  2.60 132.817__________________________________________________________________________FOCAL                  ZOOM SPACINGSLENGTH BACK FOCUS         FRONT FOCUS                  A    B     LENS LENGTH__________________________________________________________________________30.63 10.90   15.39    14.109                       13.408                             66.43151.50 36.10   35.20    10.929                       5.876 55.71986.60 76.37   68.58    7.596                       1.402 47.911__________________________________________________________________________ 
    
     Example 2 is a 3X zoom lens which has a semi-field from 38.84° to 15.15° and an f/number from 4.1 to 8.2. The front negative group (surfaces 1-4), middle positive group (surfaces 5-9) and rear negative group (surfaces 10-13) have focal lengths of -70.97, 24.37 and -29.07. It has the same advantages as Example 1. 
     
         __________________________________________________________________________Example 3 (FIG. 1)__________________________________________________________________________ CLEAR  AXIALSURFACE APERTURE        BEAM RADIUS  THICKNESS                             INDEX                                  V__________________________________________________________________________1     20.25  8.31 29.2550 1.000   1.713                                  53.82     15.98  8.12 9.54200 3.2773     15.69  9.71 16.3100 3.205   1.575                                  41.54     15.00  9.89 -2039.98                     A 10.57  10.57             DIAPHRAGM                     2.4135     10.92  10.79             -31.1150                     2.092   1.487                                  70.46     11.29  11.05             -13.0600                     .2007     11.78  10.73             32.7600 4.766   1.487                                  70.48     11.85  10.17             -7.99000                     1.000   1.755                                  27.69     12.76  10.31             -13.7050                     B10    16.89  9.61 -17.2500                     2.782   1.805                                  25.411    17.75  9.58 -12.2500                     2.18212    17.26  7.44 -11.2600                     1.000   1.713                                  53.813    21.78  7.21 64.0100__________________________________________________________________________FOCAL                  ZOOM SPACINGSLENGTH BACK FOCUS         FRONT FOCUS                  A    B     LENS LENGTH__________________________________________________________________________20.60 5.69    13.17    9.417                       9.978 43.31234.63 22.65   25.89    7.975                       4.345 36.23758.17 50.54   47.23    6.079                       1.000 30.997__________________________________________________________________________ 
    
     Example 3 is a 3X zoom lens having a semi-field varying from 39.24° to 16.24° and an f/number varying from 4.6 to 7. It has similar advantages to Examples 1 and 2. The front negative group (surfaces 1-4), middle positive group (surfaces 5-9) and rear negative group (surfaces 10-13) have focal lengths of -118.06, 17.61 and -19.04, respectively. 
     
         __________________________________________________________________________Example 4 (FIG. 2)__________________________________________________________________________ CLEAR  AXIALSURFACE APERTURE        BEAM RADIUS  THICKNESS                             INDEX                                  V__________________________________________________________________________1     29.50  10.55             51.5789 1.500   1.713                                  53.82     23.86  10.37             15.3923 8.3383     22.11  13.44             41.6405 4.000   1.755                                  27.64     20.99  13.67             -321.578                     A 14.32  14.32             DIAPHRAGM                     1.0005     14.69  14.43             107.407 6.454   1.583                                  46.56     15.39  14.47             -15.3498                     0.3647     15.30  14.28             -14.2795                     1.200   1.750                                  35.08     16.80  14.90             24.8353 6.037   1.517                                  64.29     17.76  15.69             -16.4388                     0.83310    21.21  15.20             45.1331 5.019   1.487                                  70.411    22.05  14.42             -45.1330                     B12    25.78  13.85             -33.2522                     4.796   1.755                                  27.613    26.97  13.51             -20.4160                     3.10214    25.82  10.90             -18.1258                     1.500   1.713                                  53.815    30.73  10.57             74.5288__________________________________________________________________________FOCAL                  ZOOM SPACINGSLENGTH BACK FOCUS         FRONT FOCUS                  A    B     LENS LENGTH__________________________________________________________________________30.62 11.67   14.21    12.500                       14.805                             71.44851.48 39.93   31.25    9.901                       6.375 60.41986.53 86.70   59.32    7.120                       1.328 52.591__________________________________________________________________________ 
    
     Example 4 is a 3X zoom lens with a semi-field varying from 38.84° to 15.55° and an f/number varying from 4.1 to 8.2. The front negative group (surfaces 1-4), middle positive group (surfaces 5-11) and the rear negative group (surfaces 12-15) have focal lengths of -164.09, 27.29 and -30.21, respectively. 
     
         __________________________________________________________________________Example 5 (FIG. 3)__________________________________________________________________________ CLEAR  AXIALSURFACE APERTURE        BEAM RADIUS  THICKNESS                             INDEX                                  V__________________________________________________________________________1     29.24  10.39             28.2283 1.300   1.713                                  53.82     23.55  10.12             13.7289 A3     17.45  12.77             22.5046 2.884   1.575                                  41.54     16.48  12.72             127.293 B 12.83  12.83             DIAPHRAGM                     4.5485     13.72  12.89             -39.3727                     2.342   1.487                                  70.46     14.25  13.10             -20.0532                     .2007     14.85  12.80             37.8954 5.721   1.487                                  70.48     15.21  12.07             -10.5998                     1.000   1.755                                  27.69     16.78  12.14             -20.0815                     C10    21.73  11.56             -22.7888                     4.513   1.805                                  25.411    23.40  11.57             -16.1794                     4.07912    22.55  8.39 -14.4274                     1.300   1.713                                  53.813    28.65  8.17 115.109__________________________________________________________________________FOCAL BACK    FRONT    ZOOM SPACINGS                              LENSLENGTH FOCUS   FOCUS    A   B   C   LENGTH__________________________________________________________________________25.75 7.04    11.32    14.774                      9.393                          11.347                              63.40043.27 25.22   30.83    10.511                      9.550                          5.519                              53.46772.76 57.12   61.04    8.606                      7.473                          1.099                              45.064__________________________________________________________________________ 
    
     Example 5 is a 3X zoom lens constructed similarly to Examples 1-4 except that there are four moving groups rather than three. The first two elements which are moved as a group in Examples 1-4, are now moved differentially. This differential movement improves control of distortion. Focal lengths of first (surfaces 1 and 2), second (surfaces 3 and 4), third (surfaces 5-9) and fourth (surfaces 10-13) groups are -38.77, 46.80, 27.21 and -26.59. Example 5 has a semi-field which varies from 39.77° to 16.45° and an f/number which varies from 4.6 to 7.0 as overall focal length increases. 
     
         __________________________________________________________________________Example 6 (FIG. 4)__________________________________________________________________________ CLEAR  AXIALSURFACE APERTURE        BEAM RADIUS  THICKNESS                             INDEX                                  V__________________________________________________________________________1     83.55  7.47 -460.060                     3.000   1.517                                  64.22     67.42  7.49 45.0930 A3     56.61  13.22             50.8870 14.420  1.713                                  53.84     53.99  12.47             -206.036                     4.4775     30.90  11.73             19.6276 8.159   1.517                                  64.26     26.58  9.48 55.9770 4.2947     19.74  8.03 -48.5177                     2.400   1.755                                  27.68     15.77  7.64 20.3558 1.9239     14.79  7.74 29.1091 3.596   1.620                                  60.310    13.57  7.52 -29.1091                     2.551 6.74   6.74 DIAPHRAGM                     B11    19.08  3.91 -48.0070                     3.982   1.755                                  27.612    20.80  3.42 -19.1574                     1.60513    21.18  2.84 -15.7039                     3.000   1.713                                  53.814    27.42  2.45 -67.5502__________________________________________________________________________FOCAL                  ZOOM SPACINGSLENGTH BACK FOCUS         FRONT FOCUS                  A    B     LENS LENGTH__________________________________________________________________________30.63 10.16   -16.58   60.013                       10.170                             123.59051.49 30.24   19.52    26.042                       3.982 81.78286.59 50.20   85.88    1.439                       1.110 55.956__________________________________________________________________________ 
    
     Example 6 is a 3X zoom lens having a semi-field which varies from 38.61° to 15.22° and an f/number which varies from 4.10 to 8.67 as focal length increases. The front (surfaces 1-2), middle (surfaces 3-10) and rear (surfaces 11-14) groups have focal lengths of -79.01, 38.07 and -109.0, respectively. This lens does not have the relatively equal focal length relationship of the middle and rear groups of Examples 1-5 and, therefore, does not have the remarkable compactness of those lenses. By arranging the four elements of the second (middle) group and the two elements of the third (rear) group in configuration of separated air-spaced elements with a stop in close proximity to the fourth element of the middle group, spherical aberration, corona and chromatic aberrations are easily controlled.