Abstract:
A telephoto lens system which can focus down to unity magnification comprising two lens groups that are displaced during focusing. The front lens group of positive refractive power has five or six elements, namely, a positive lens, a positive meniscus lens, a negative lens, a lens element with a slightly negative refractive power and a positive lens. The rear lens group of negative refractive power has three elements, namely, a positive, a negative and a positive lens element. Additionally, there are four numerical conditions on focal lengths and radii of curvature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a telephoto lens system capable of closeup shooting that features an aperture ratio of about 1:2.8 and a viewing angle of about 24.4° which ensures high performance over the full shooting distance ranging from the imaging of an infinitely distanct object to a life-size shot. 
     2. Background of the Invention 
     Micro or macro lenses have been chiefly used for shooting objects at a near distance. Since most of these lenses are designed to provide best focusing in near-distance shooting, they are not capable of achieving as good aberrational correction when shooting an object at infinity as normal imaging lenses. In response to this situation, a certain type of lens has been proposed that employs a floating mechanism which is capable of correcting the aberrational changes that occur as a result of variation in the shooting distance. Some of the lenses that employ such a floating mechanism have aperture ratios of about 1:2.8 but their focal length is comparatively short (e,g., about 50 mm on a 35 mm camera). In addition, the magnification that can be attained by this lens alone in shooting at the closest focusing distance is approximately 0.5, so that an adapter such as a closeup ring is necessary when shooting the object at a magnification of unity (life size) with this lens. A lens that is capable of near-distance shooting without employing a floating mechanism is also known but because of limitations on its performance this lens can not be made brighter than an aperture ratio of about 1:3.5 to 1:4. In addition, most of the lenses of this type feature magnifications of 0.25-0.5 at the closest focusing distance. 
     A macro lens that is capable of closeup shooting and which features a focal length of approximately 100 mm in terms of a lens on a 35 mm camera is also known. However, the long focal length of this lens requires a longer lens extension and it is very difficult to produce a system that is capable of shooting at a magnification of unity with this lens alone. Therefore, in order to effect proper focusing when shooting over a very wide range of distances from infinity to a life-size shot, an accessory such as a closeup ring must be used as an aid to permit the lens to be displaced by a distance equal to its focal length. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished in order to solve the aforementioned problems of the prior art. Accordingly, an object of the invention, is to provide a compact and high-performance lens system that has a telephoto focal length of the order of 100 mm in terms of a lens on a 35 mm camera, which has a relatively bright aperture ratio of about 1:2.8, and which yet has a capability of imaging from an infinite distance to a life-size shot with the lens alone. 
     The invention can be summarized as a telephoto lens system which can focus down to unity magnification comprising two lens groups that are displaced during focusing. The front lens group of positive refractive power has five or six elements, namely, a positive lens, a positive meniscus lens, a negative lens, a lens element with a slightly negative refractive power and a positive lens. The rear lens group of negative refractive power has three elements, namely, a positive, a negative and a positive lens element. Additionally, there are four numerical conditions on focal lengths and radii of curvature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1, 5, 9, 13, 17, 21 and 25 are simplified cross-sectional views of lens systems according to Examples 1 to 7 when they are focused for shooting an object at an infinite distance; 
     FIGS. 2, 6, 10, 14, 18, 22 and 26 are graphs plotting the aberration curves obtained in Examples 1 to 7 when shooting an object at an infinite distance; 
     FIGS. 3, 7, 11, 15, 19, 23 and 27 are simplified cross-sectional views of lens systems according to Examples 1 to 7 when they are adjusted for a life-size shot, or imaging at a magnification of unity; and 
     FIGS. 4, 8, 12, 16, 20, 24 and 28 are graphs plotting the aberration curves obtained in Examples 1 to 7 when imaging at a magnification of unity. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to attain the aforementioned object, the present invention provides a lens system that comprises, in order from the object side, a first lens group L 1  having a positive refractive power and a second lens group L II  having a negative refractive power. Embodiments are illustrated respectively in FIGS. 1, 5, 9, 13, 17, 21 and 25 at infinite focus and in FIGS. 3, 7, 11, 15, 19, 23 and 27 for unity magnification. The lens system effects focusing from an infinite distance to the closest distance by increasing the aerial distance of the first lens group L 1  relative to the second lens group L II . The lens system of the invention is characterized in that the first lens group L I  has either a five-unit, six-element or five-unit, five-element configuration consisting of a first lens element 11 which is a positive lens, a second lens element 12 which is a positive meniscus lens, a third lens element 13 which is a negative lens, a cemented or a single lens element 14 having a slightly negative value of refractive power (if it is a cemented lens element 14, it is composed of a negative lens 15 and a positive lens 16), and a positive lens element 17. The second lens group L II  has a three-unit, three-element configuration consisting of a positive lens element 21, a negative lens element 22 and a positive lens element 23. 
     The lens system satisfies the following conditions (1) to (4): 
     
         0.5&lt;f.sub.I /f&lt;0.7                                         (1) 
    
     
         1/2&lt;f.sub.1,2,3 /f&lt;1.7                                     (2) 
    
     
         -0.4&lt;f.sub.r6 /f&lt;-0.2                                      (3) 
    
     
         -0.6&lt;f.sub.IIn /f&lt;-0.3.                                    (4) 
    
     In these equations, f r6  =r 6  /(1-n 3 ) and f IIn  =r IIn  /(1-n IIn ), wherein f I  is the focal length of the first lens group L I , f is the focal length of the overall system for shooting an infinitely distanct object and f 1 ,2,3 is the composite focal length of the first, second and third lens elements 11, 12 and 13. The previously defined f r6  is the focal length of the surface of the third lens element 13 on the image side where n 3  is the refractive index at the d-line of the third lens element 13 and r 6  is the radius of curvature of the surface of the third lens element 13 on the image side. Similarly, f IIn  is the focal length of the surface on the image side of the negative lens element 23 in the second lens group L II  where n IIn  is the refractive index at the d-line of the negative lens 23 in the second lens group L II  and r IIn  is the radius of curvature of the surface on the image side of this negative lens element 23. 
     According to the lens configuration used in the present invention, the amount of displacement of the first lens group L I  relative to the second lens group L II  is reduced to less than half the amount required in the conventional system which achieves focusing by displacing the overall lens system. This small displacement permits the use of a relatively small and lightweight lens barrel for achieving a great improvement in system operability. 
     If the focal length of the first lens group is written as f I , the focal length of the overall system for shooting an infinitely distant object as f, and the magnification by the overall system as m (m&gt;0), then the amount of displacement of the first lens group, ΔD, can be expressed as ΔD=m·f I .spsb.2 /f. 
     The four conditions that should be satisfied by the lens system of the present invention are described below. 
     Condition (1) sets forth the required value for the focal length of the first lens group L I  having a positive refractive power which is positioned on the object side. Such a focal length reduces the amount of displacement of that first lens group L I  to less than half the amount required for achieving proper focusing in the conventional system. If the lower limit of this condition is not reached, the amount of displacement of the first lens group L I  becomes very small but, on the other hand, the negative refractive power of the second lens group L II  becomes so strong that the Petzval sum will decrease until it assumes a negative value. Furthermore, compensation of aberrations such as curvature of field and astigmatism becomes too difficult to attain a bright aperture ratio of about 1:2.8 or satisfactory imaging performance. If the upper limit of condition (1) is exceeded, the negative refractive power of the second lens group L II  becomes weak to provide for easy compensation of aberrations and an aperture ratio brighter than 1:2.8 but, on the other hand, the amount of displacement of the first lens group L I   rapidly increases to a level that is no smaller than when the overall system is displaced in order to attain proper focusing. In either case, the object of the present invention is not attainable. 
     The second lens group L II  of the lens system of the present invention has a negative refractive power so that in order to ensure that the overall system has an aperture ratio of about 1:2.8, the aperture ratio of the first lens group must be brighter than 1:2.8. Therefore, in the present invention, the first lens group L I  has an aperture ratio within the range of 1:1.95 to 1:1.57 in order to satisfy condition (1). The requirement that should be met to provide the first lens group L I  with an aperture ratio brighter than 1:2.8 is condition (2). This condition (2)sets forth the range of the composite focal length f 1 ,2,3 of the first, second and third lens elements 11, 12 and 13 in the first lens group over which aberrational compensation can be achieved with the aperture ratio of the overall system being at about 1:2.8. If the lower limit of condition (2) is not reached, compensation of aberrations such as spherical and coma aberrations becomes too difficult to ensure a bright aperture ratio of about 1:2.8. If, on the other hand, the upper limit of condition (2) is exceeded, the increase in the composite refractive power of the other lens elements in the first lens group is inevitable and it becomes particularly difficult to compensate for spherical aberration. In order to achieve effective compensation of aberrations when the upper limit of condition (2) is exceeded, the upper limit of condition (1) is unavoidably exceeded and the amount of displacement of the first lens group L I  necessary to effect proper focusing is increased to an extent at which the object of the present invention is by no means attainable. 
     Condition (3) sets forth the required focal length f r6  of the surface r 6  on the image side of the third lens element 3 in the first lens group L I . A focal length f r6  in such a range achieves effective compensation of spherical aberration, astigmatism and coma aberration. If this conditon is met, particularly effective compensation of the spherical aberration and astigmatism that occur in shooting at a magnification close to unity can be achieved. If the lower limit of condition (3) is not reached, it becomes difficult to compensate for an excessive amount of spherical aberration, as well as the outward coma that is caused by a downward ray of light and the rear curvature of field that occurs with respect to the best central image plane. If, on the other hand, the upper limit of condition (3) is exceeded, an inward coma is produced by the downward ray of light and the frontal curvature of field that occurs is too great to be effectively compensated. These aberrational variations will progressively increase as the magnification approaches unity. Therefore, if one makes an attempt to effect aberrational compensation outside the range specified by condition (3), he will find that merely displacing the first lens groupe L I  is not sufficient to provide satisfactory imaging performance over the full shooting distance ranging from an infinitely distant object to a life-size shot. As a result, it becomes necessary to employ a floating mechanism for displacing the distance between certain lenses but then this leads to the use of a lens barrel of complicated construction, which is contrary to the purpose of providing a small and lightweight system. 
     Condition (4) sets forth the required focal length f IIn  of the surface r IIn  on the image side of the negative lens element 22 in the second lens group L II . Such a value for the focal length f IIn  achieves effective compensation of astigmatism and provides an appropriate value of the Petzval sum without causing any substantial change in spherical aberration when the shooting distance is varied from infinity to a life-size shot. 
     There are two cases where the lower limit of condition (4) is not reached. In the first case, that is, if the radius of curvature r IIn  of the surface r IIn  on the image side of the negative lens element 22 becomes excessive, it becomes difficult to compensate for the rear curvature of field and the Petzval sum becomes too small to provide satisfactory imaging performance on the periphery of the field. In the second case, that is, if the refractive index n IIn  of this particular negative lens element 22 becomes unduly small, the Petzval sum which is excessive cannot be reduced to a sufficiently small value to minimize the curvature of field. In addition, astigmatism has a tendency to increase as the magnification approaches unity. 
     There are also two cases where the upper limit of condition (4) is exceeded. In the first case, that is, if the radius of curvature r IIn  of the surface at issue becomes unduly small, it becomes difficult to compensate for the frontal curvature of field. In addition, the astigmatism cannot be reduced and the Petzal sum becomes excessive, to thereby make it difficult to compensate for the astigmatism occurring in the middle portion of the optical field when shooting a distant object. The second case is where the refractive index n IIn  of the negative lens element 22 becomes excessively high but this is also undesirable for the purpose of the present invention since the optical material that can be used is limited to one having a dispersion index that is not suitable for achieving achromatism. 
     Data sheets for seven examples of the present invention are presented below. In these data sheets: f, F NO , ω and f B  denote the focal length, aperture ratio, half viewing angle and the back focus, respectively, of the overall system when it is focused for an object at an infinite distance. In these data sheets r is the radius of curvature of an individual lens surface, d is the thickness or aerial distance of an individual lens element n d  is the refractive index of an individual lens element at the d-line, and ν d  is the Abbe number of an individual lens element at the d-line. Also, L I  signifies the first lens group and L II  denotes the second lens group. 
     EXAMPLE 1 
     
         ______________________________________f = 100.18 F.sub.NO = 1:2.8 2ω = 24.4° F.sub.B = 45.51surface  No.    r         d        n.sub.d                                    ν.sub.d______________________________________L.sub.I      1      64.764  4.300    1.78590                                      44.2        2      473.491 4.773        3      31.180  4.800    1.80610                                      40.9        4      75.635  1.800        5      152.788 1.500    1.80518                                      25.4        6      25.817  11.674        7      -24.386 1.800    1.74000                                      28.3        8      -83.740 4.500    1.80610                                      40.9        9      -32.411 0.150        10     250.715 4.602    1.72000                                      42.0        11     -54.589 2.500-48.587L.sub.II     12     -906.318                       3.701    1.80518                                      25.4        13     -67.189 5.356        14     -51.897 1.500    1.80610                                      40.9        15     47.120  10.398        16     50.044  5.428    1.51633                                      64.1        17     -466.238______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________f = 100.16 F.sub.NO = 1:2.8 2ω = 24.4° F.sub.B = 46.93surface  No.    r          d        n.sub.d                                     ν.sub.d______________________________________L.sub.I      1      64.000   4.500    1.78590                                       44.2        2      750.150  3.050        3      31.747   5.410    1.80610                                       40.9        4      66.120   1.900        5      128.547  1.500    1.80518                                       25.4        6      26.047   10.640        7      -25.160  1.800    1.74000                                       28.3        8      -120.000 5.000    1.80610                                       40.9        9      -34.000  0.150        10     244.668  4.440    1.72000                                       42.0        11     -57.000  2.500-48.587L.sub.II     12     -1351.297                        3.340    1.80518                                       25.4        13     -68.597  4.640        14     -55.555  1.500    1.80610                                       40.9        15     46.035   11.590        16     51.387   5.510    1.51633                                       64.1        17     -370.000______________________________________ 
    
     EXAMPLE 3 
     
         ______________________________________f = 100.46 F.sub.NO = 1:2.8 2ω = 24.4° F.sub.B = 40.32surface  No.    r         d        n.sub.d                                    ν.sub.d______________________________________L.sub.I      1      61.428  4.003    1.80400                                      46.6        2      -828.516                       0.150        3      34.419  4.000    1.83481                                      42.7        4      84.536  4.046        5      1726.841                       1.500    1.78472                                      25.7        6      27.243  7.784        7      -27.071 1.800    1.72825                                      28.5        8      -92.450 3.632    1.83400                                      37.2        9      -34.908 0.150        10     149.627 3.777    1.77250                                      49.7        11     -60.691 2.000-32.000L.sub.II     12     124.956 3.182    1.80518                                      25.4        13     -65.902 0.485        14     -66.592 1.500    1.88300                                      40.8        15     34.039  23.902        16     42.722  3.984    1.51633                                      64.1        17     106.221______________________________________ 
    
     EXAMPLE 4 
     
         ______________________________________f = 100.03 F.sub.NO = 1:2.8 2ω = 24.5° F.sub.B = 42.65surface  No.    r          d        n.sub.d                                     ν.sub.d______________________________________L.sub.I      1      58.324   4.286    1.80400                                       46.6        2      -3268.671                        0.126        3      27.176   5.044    1.80400                                       46.6        4      46.785   3.038        5      96.990   1.500    1.80518                                       25.4        6      21.790   12.883        7      -22.910  3.000    1.80400                                       46.6        8      -29.198  0.200        9      164.912  4.609    1.61272                                       58.8        10     -38.915  1.000-41.000L.sub.II     11     -272.659 3.091    1.80518                                       25.4        12     -65.698  4.446        13     -54.137  1.500    1.88300                                       40.8        14     44.177   12.451        15     57.202   5.315    1.72916                                       54.7        16     -493.517______________________________________ 
    
     EXAMPLE 5 
     
         ______________________________________f = 100.12 F.sub.NO = 1:2.8 2ω = 24.5° F.sub.B = 47.16surface  No.    r          d        n.sub.d                                     ν.sub.d______________________________________L.sub.I      1      64.000   4.450    1.78590                                       44.2        2      727.416  3.290        3      31.747   5.110    1.80610                                       40.9        4      66.762   2.000        5      130.320  1.500    1.80518                                       25.4        6      26.047   10.830        7      -24.940  1.800    1.68893                                       31.1        8      -403.219 5.260    1.74400                                       44.7        9      -34.018  0.150        10     244.668  4.150    1.72000                                       42.0        11     -57.000  2.500-48.087L.sub.II     12     -1351.297                        3.310    1.80518                                       25.4        13     -68.597  4.650        14     -55.555  1.500    1.80610                                       40.9        15     46.035   11.410        16     51.387   5.520    1.51633                                       64.1        17     -370.000______________________________________ 
    
     EXAMPLE 6 
     
         ______________________________________f = 100.48 F.sub.NO = 1:2.8 2ω = 24.4° F.sub.B = 53.77surface  No.    r          d        n.sub.d                                     ν.sub.d______________________________________L.sub.I      1      62.504   4.021    1.80400                                       46.6        2      -9446.370                        0.150        3      34.697   3.307    1.83481                                       42.7        4      61.586   4.849        5      166.095  1.500    1.76182                                       26.6        6      28.381   9.331        7      -27.285  1.800    1.74077                                       27.8        8      -161.506 4.939    1.83400                                       37.2        9      -35.702  0.150        10     206.844  3.700    1.80400                                       46.6        11     -75.061  0.806-40.806L.sub.II     12     -392.189 3.000    1.84666                                       23.9        13     -80.499  3.717        14     -79.786  1.500    1.88300                                       40.8        15     44.076   13.382        16     61.272   4.799    1.71300                                       53.8        17     -576.557______________________________________ 
    
     EXAMPLE 7 
     
         ______________________________________f = 100.45 F.sub.NO = 1:2.8 2ω = 24.4° F.sub.B = 45.64surface  No.    r          d        n.sub.d                                     ν.sub.d______________________________________L.sub.I      1      63.251   4.058    1.80400                                       46.6        2      -605.257 0.150        3      33.672   3.600    1.83481                                       42.7        4      61.511   4.154        5      225.046  1.500    1.76182                                       26.6        6      27.280   8.902        7      -27.217  1.800    1.74077                                       27.8        8      -189.691 4.738    1.83400                                       37.2        9      -36.014  0.150        10     196.875  3.700    1.80400                                       46.6        11     -68.648  2.000-36.000L.sub.II     12     318.268  3.021    1.80518                                       25.4        13     -75.777  1.548        14     -74.102  1.500    1.88300                                       40.8        15     38.840   19.669        16     51.202   4.853    1.56883                                       56.3        17     460.483______________________________________ 
    
     The numerical values that satisfy conditions (1) to (4) in each of Examples 1 to 7 are listed below: 
     
         ______________________________________    Conditional formula    (1)  (2)         (3)     (4)    f.sub.I /f         f.sub.1,2,3 /f                     f.sub.rs /f                             f.sub.IIn /f______________________________________Example 1  0.678  1.679       -0.320                               -0.583Example 2  0.678  1.571       -0.323                               -0.570Example 3  0.546  1.305       -0.346                               -0.384Example 4  0.632  1.372       -0.271                               -0.500Example 5  0.678  1.607       -0.323                               -0.570Example 6  0.631  1.398       -0.371                               -0.497Example 7  0.582  1.231       -0.357                               -0.438______________________________________ 
    
     In Examples 1 to 7, the first lens group L I  is displaced by the following amounts when the system focused for an object at infinity is adjusted to life-size shot (magnification of unity): 46.09 in Examples 1, 2 and 5; 30.00 in Example 3; 40.00 in Examples 4 and 6; and 34.00 in Example 7. 
     The seven examples are described also in the accompanying drawings. The structures of the seven examples as focused for an infinite shooting distance are respectively shown in FIGS. 1, 5, 9, 13, 17, 21 and 25. The corresponding structures as focused for life-size shooting (magnification of unity) are shown in FIGS. 3, 7, 11, 15, 19, 23 and 27. 
     Associated with each of the infinite-focus structures is a drawing for aberration curves at that focus. These curves are contained in FIGS. 2, 6, 10, 14, 18, 22 and 26. Each drawing contains four graphs. The first graph plots spherical aberration (SA) and sine condition (SC) as a function of aperture. The second graph plots chromatic aberration as a function of aperture for the d-line, g-line and C-line. The third graph plots astigmatism as a function of viewing angle for the sagittal (S) direction and the meridional (M) direction. The fourth graph plots distortion as a function of viewing angle. 
     There are also provided aberration curves for the seven examples as adjusted for life-size shooting. These curves are displayed in FIGS. 4, 8, 12, 16, 20, 24 and 28. The format of these unity-magnification curves is the same as for the infinite-focus curves except that the aperture F e  is the appropriate aperture at unity focus and that image height Y is substituted for viewing angle.