Abstract:
A compact large-aperture photographic lens system having six-component seven-element lens configuration and comprising a first, second, third, fourth, fifth and sixth lens components, in which the first and second lens components are positive meniscus lenses, the third lens component is a negative meniscus lens, the fourth lens component is a cemented meniscus doublet, the fifth lens component is a positive meniscus lens and the sixth lens component is a positive lens, the compact large-aperture photographic lens having a short overall length and long back focal length.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a compact large-aperture photographic lens system and, more particularly, to a compact large-aperture photographic lens system for which the back focal length is long, intensity of light in the marginal portion is high and aperture ratio is Fl.2. 
     (B) Description of the Prior Art 
     A Gauss-type photographic lens systems with focal length of 50 mm, various lens systems are known. For most of them, however, the aperture ratio is about Fl.4. To reduce the weight of camera, it is necessary to obtain a compact lens system with short overall length. Moreover, it is generally required to make the back focal length long. 
     However, compact photographic lens systems with large aperture ratio and long back focal length are not yet known so much. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a primary object of the present invention to provide a large-aperture Gauss-type photographic lens system for which the overall length is short, intensity of light in the marginal portion is high, aperture ratio is Fl.2 and back focal length is long. 
     As shown in FIG. 1, the compact large-aperture photographic lens system with long back focal length according to the present invention has six-component seven-element lens configuration and comprises a first, second, third, fourth, fifth and sixth lens components. Out of them, the first lens component is a positive meniscus lens, the second lens component is a positive meniscus lens, the third lens component is a negative meniscus lens, the fourth lens component is a cemented meniscus doublet, the fifth lens component is a positive meniscus lens and the sixth lens component is a positive lens. The stop of the lens system is arranged between the third and fourth lens components. Besides, the compact large-aperture photographic lens system according to the present invention satisfies the conditions given below when reference symbol L represent the overall length of the lens system, reference symbol r 4  represents the radius of curvature of the surface of the image side of the second lens component, reference symbol r 5  represents the radius curvature of the surface on the object side of the third lens component, reference symbol r 8  represents the radius of curvature of the cemented surface of the fourth lens component, reference symbols d 3  and d 5  respectively represent thicknesses of the second and third lens components, reference symbols d 7  and d 8  respectively represent thicknesses of respective lenses constituting the fourth lens component, reference symbol d 4  represents the airspace between the second and third lens components, reference symbols n 1 , n 2 , n 3 , n 4 , n 5 , n 6  and n 7  respectively represent refractive indices of respective lenses, referency symbols ν 4  and ν 5  respectively represent Abbe&#39;s numbers of repective lenses constituting the fourth lens component, reference symbols ν 6  and ν 7  respectively represent Abbe&#39;s numbers of the fifth and sixth lens components, reference symbol f 23  represents the total focal length of the second and third lens components, reference symbol f 25  represents the focal length of the fourth lens component and reference symbol f represents the focal length of the lens system as a whole. 
     
         L/f &lt; 1 (L = Σd )                                    (1) 
    
     
         0.6f &lt; -r.sub.8 &lt; 1.1f                                     (2) 
    
     
         0.1 &lt;n.sub.4 - n.sub.5 &lt; 0.2, 24 &lt; ν.sub.5 - ν.sub.4 &lt; 35 (3) ##EQU1## 
    
     
         1.5 &lt; r5/r4 &lt; 1.87, d.sub.4 &lt; 0.05f                        (5) 
    
     
         0.16f &lt; d.sub.3 + d.sub.4 + d.sub.5 &lt; 0.19f, 0.14f &lt; d.sub.7 + d.sub.8 &lt; 0.17f                                                     (6) 
    
     
         1.7 &lt; n.sub.1, n.sub.2, n.sub.3, n.sub.4 &lt; 1.86            (7) 
    
     
         1.6 &lt; n.sub.5, n.sub.6, n.sub.7 &lt; 1.82                     (8) 
    
     
         40 &lt; ν.sub.5, ν.sub.6, ν.sub.7 &lt; 60               (9) 
    
     To attain an object of the present invention, i.e., to obtain a compact lens system, the overall length of the lens system should be made short and, as a result, the intensity of light in the marginal portion becomes high. For lens systems of Fl.2 class, however, the overall length is liable to become long because the aperture ratio is large. Moreover, to obtain favourable symmetry of coma, the airspace between the lens surfaces just in front and rear of the stop cannot be made very small. Therefore, to obtain a compact large-aperture lens system for which the intensity of light in the marginal portion is high, it is necessary to satisfy the condition (1) L/f &lt; 1. 
     For modified Gauss type lens systems like the lens system according to the present invention, the ratio between the back focal length f B  and f; i.e., f B  /f, is usually about 0.7 at the maximum. To make the ratio still larger up to about 0.75, it is necessary to increase asymmetry of the front lens group arranged in front of the stop and the rear lens group arranged in rear of the stop. As a result, unfavourable influence will be caused, for example, symmetry of coma will become unfavourable, chromatic aberration will be undercorrected and so forth. In the present invention, undercorrection of chromatic aberration is prevented by arranging so that the only cemented surface r 8  has negative power. Besides, to obtain favourable symmetry of coma, the cemented surface r 8  is arranged to be concave toward the object side. The condition (2) is to define the abovementioned cemented surface r 8 . If it becomes 0.6f &lt; -r 8  in the condition (2), chromatic aberration of the lens system will be undercorrected even when materials of respective lenses constituting the fourth lens components are selected to satisfy the condition (3). If it becomes -r 8  &lt; 1.1f, symmetry of coma will become unfavourable. 
     The condition (3) defines the materials of respective lenses constituting the fourth lens component which is a cemented doublet. When the materials of those lenses are selected within the ranges shown by the condition (3), the cemented surface of the fourth lens component serves as a concave surface in cooperation with the condition (2) and, as a result, chromatic aberration is corrected favourably and, moreover, favourable symmetry of coma is obtained. 
     When it is attempted to attain another object of the present invention, i.e., to make f B  long and to increase the ratio f B  /f between f B  and f up to 0.75, symmetry of Gauss type lens system will be lost. Moreover, curvature of field will become large and it becomes impossible to obtain a favourable image over the whole effective field. For Gauss type lens systems, Petzval&#39;s sum generally becomes a large positive value and the image plane becomes concave toward the object side. In the present invention,  the second and third lens components constitute one lens in combination with each other and it is so arranged that the second and third lens components form a negative lens as a whole. Besides, the fourth lens component is also arranged as a negative lens. By arranging so that the component ##EQU2## which contributes to make Petzval&#39;s sum negative, becomes smaller than the upper limit of the condition (4), Petzval&#39;s sum of the lens system as a whole is made small and curvature of field is also made small. If, however, the above-mentioned component becomes smaller than the lower limit of the condition (4), spherical aberration will become unfavourable. 
     In the present invention, the radius of curvature r 4  of the surface on the image side of the second lens component and radius of curvature r 5  of the surface on the object side of the third lens component are selected as defined by the condition (5) so that the air lens (an airspace between lenses which serves like a lens) between the second and third lens components has negative power. Besides, to satisfy the condition (4), it is necessary to arrange so that the combination of the second and third lens components serves as a negative lens of strong negative power. Therefore, it is required to make the value of r 5  /r 4  larger than 1.5 as shown by the condition (5). In Gauss type lens systems, the second and third lens components are generally cemented together to form a cemented doublet and its cemented surface is effective for correction of chromatic aberration. If, therefore, r 5  /r 4  exceeds the upper limit in the condition (5) and the airspace d 4  between the second and third lens components becomes larger than the range defined by the condition (5) as a result of the above, correcting effect of surfaces r 4  and r 5  for chromatic aberration becomes considerably weak and it becomes very difficult to favourably correct chromatic aberration by the other lenses. 
     The condition (6) is required for the purpose of materializing a compact lens system of Fl.2 class. In the lens system, thicknesses d 1 , d 10  and d 12  of the first, fifth and sixth lens components cannot be made small because effective diameters of these lens components are pre-determined. Besides, the airspace d 6  between the third and fourth lens components cannot be made smaller than about (r6 + r7/2 ) because symmetry of coma will otherwise become unfavourable. Therefore, in the present invention, the other d values are made small, i.e., d 3  + d 4  + d 5  &lt; 0.19f and d 7  + d 8  &lt; 0.17f, so that the overall length of the lens system becomes short and the intensity of light in the marginal portion becomes satisfactorily high. If, however, the above values become smaller than the lower limits of the condition (6), i.e., if it becomes 0.16f &lt; d 3  +  d 4  + d 5  or 0.14f &lt; d 7  + d 8 , Petzval&#39;s sum becomes large and curvature of field also increases. 
     The condition (7) is required for the purpose of making the curvature of field small. That is, by arranging so that refractive indices of the first, second and third lens components and of the lens on the object side of the fourth lens component become respectively higher than the lower limit of the condition (7), absolute values of radii of curvature of these lenses are made large so that Petzval&#39;s sum becomes small and curvature of field also becomes small. If, however, any of the above-mentioned refractive indices becomes higher than the upper limit of the condition (7), it is impossible to obtain such material for that lens with which Abbe&#39;s number becomes a reasonable value. 
     Due to the same reason as above, it is prefereable to make refractive indices of the lens on the image side of the fourth lens component and of the fifth and sixth lens components as high as possible. The present invention, however, has an object to make f B  long up to about 0.75f. To attain this object, the rear lens group arranged in rear of the stop should have strong refrative power and, therefore, there is a tendency that chromatic aberration caused in the rear lens group becomes large. To prevent the above, Abbe&#39;s number ν 5  of the lens on the image side of the fourth lens component and Abbe&#39;s numbers ν 6  and ν 7  of the fifth and sixth lens components are made larger than 40 as shown in the condition (9) so that chromatic aberration is minimized. Therefore, materials of these lenses necessarily become such materials for which refractive indices are somewhat low. Due to the above-mentioned reason, refrative indices n 5 , n 6  and n 7  should be higher than 1.6 as shown in the condition (8). If, however, n 5 , n 6  or n 7  becomes higher than 1.82it becomes difficult to obtain a material of adequate Abbe&#39;s number for that lens. In the same way, if Abbe&#39;s number ν 5 , ν 6  or ν 7  in the condition (9) becomes larger than 60, it becomes impossible to obtain such material for that lens for which the refractive index satisfies the condition (8) and at the same time is reasonable for making a lens. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectional view of the compact large-aperture photographic lens system according to the present invention; 
     FIGS. 2A, 2B and 2C respectively show graphs illustrating aberration curves of Embodiment 1; 
     FIGS. 3A, 3B and 3C respectively show graphs illustrating aberration curves of Embodiment 2; 
     FIGS. 4A, 4B and 4C respectively show graphs illustrating aberration curves of Embodiment 3; 
     FIGS. 5A, 5B and 5C respectively show graphs illustrating aberration curves of Embodiment 4; 
     FIGS. 6A, 6B, 6C, 6D and 6E respectively show graphs illustrating aberration curves of Embodiment 5; and 
     FIGS. 7A, 7B, 7C, 7D and 7E respectively show graphs illustrating aberration curves of Embodiment 6. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the compact large-aperture photoraphic lens system with long back focal length according to the present invention are as shown below. 
     
         ______________________________________Embodiment 1:f = 100, F 1.2, 2ω =  46°, f.sub.B = 75.2r1 = 90.851 dl = 10.61    nl = 1.834    ν1 = 37.19r2 = 546.619 d2 = 0.19r3 = 55.369 d3 = 11.25    n2 = 1.8044   ν2 = 39.62r4 = 95.056 d4 = 4r5 = 158.146 d5 = 2.61     n3 = 1.7847   ν3 = 26.22r6 = 36.590 d6 = 34.02r7 = -33.822 d7 = 2.88     n4 = 1.84666  ν4 = 23.9r8 = -82.077 d8 = 13.17    n5 = 1.734    ν5 = 51.52r9 = -48.055 d9 = 0.19r10 = -285.232 d10 = 11.72   n6 = 1.734    ν6 = 51.52r11 = -64.922 d11 = 0.19r12 = 157.079 d12 = 6.73    n7 = 1.734    ν7 = 51.52r13 = -910.494L = Σd = 97.56, f23 = -126.919, f45 = -200.167 ##STR1##r5/r4 = 1.664______________________________________ 
    
     
         ______________________________________Embodiment 2:f = 100, F 1.2, 2ω = 46°, f.sub.B = 74.5r1 = 90.090  dl = 10.6    n1 = 1.834    ν1 = 37.19r2 = 586.320  d2 = 0.2r3 = 54.675  d3 = 11.92   n2 = 1.8044   ν2 = 39.62r4 = 91.779  d4 = 2.79r5 = 156.838  d5 = 3.08    n3 = 1.7847   ν3 = 26.22r6 = 35.379  d6 = 33.99r7 = -33.098  d7 = 2.88    n4 = 1.84666  ν4 = 23.9r8 = -72.065  d8 = 12.88   n5 = 1.734    ν5 = 51.52r9 = -48.356  d9 = 0.2r10 = -244.533  d10 = 11.69  n6 = 1.72916  ν6 = 54.7r11 = -63.601  d11 = 0.2r12 = 157.708  d12 = 6.73   n7 = 1.72916  ν7 = 54.7r13 = -712.917L = Σd = 97.14, f23 = -125.76, f45 = -202.40 ##STR2##r5/r4 = 1.709______________________________________ 
    
     
         ______________________________________Embodiment 3:f = 100, F 1.2, 2ω = 46°, f.sub.B = 74.42r1 = 92.057  dl = 10.6    n1 = 1.834    ν1 = 37.19r2 = 624.402  d2 = 0.2r3 = 54.988  d3 = 11.92   n2 = 1.8044   ν2 = 39.62r4 = 93.467  d4 = 2.79r5 = 159.627  d5 = 3.54    n3 = 1.7847   ν3 = 26.22r6 = 35.479  d6 = 34.00r7 = -33.921  d7 = 2.88    n4 = 1.84666  ν4 = 23.9r8 = -71.957  d8 = 12.88   n5 = 1.72916  ν5 = 54.7r9 = -48.367  d9 = 0.2r10 = -245.942  d10 = 11.69  n6 = 1.72916  ν6 = 54.7r11 = -63.647  d11 = 0.2r12 = 159.177  d12 = 6.73   n7 = 1.72916  ν7 = 54.7r13 = -639.108L = Σd = 97.6, f23 = -127.156, f45 = -200.50r5/r4 = 1.708______________________________________ 
    
     
         ______________________________________Embodiment 4:f = 100, F 1.2, 2ω = 46°, fB = 74.4r1 = 87.472  d1 = 10.6    n1 = 1.834    ν1 = 37.19r2 = 480.599  d2 = 0.2r3 = 55.420  d3 = 11.22   n2 = 1.8061   ν2 = 40.92r4 = 94.348  d4 = 4.01r5 = 152.809  d5 = 2.59    n3 = 1.7847   ν3 = 26.22r6 = 35.214  d6 = 34.05r7 = -33.757  d7 = 2.88    n4 = 1.84666  ν4 = 23.9r8 = -88.400  d8 = 13.08   n5 = 1.734    ν5 = 51.52r9 = -48.424  d9 = 0.2r10 = -257.195  d10 = 11.70  n6 = 1.735    ν6 = 49.82r11 = -63.662  d11 = 0.2r12 =  153.235  d12 = 6.72   n7 = 1.735    ν7 = 49.82r13 = -792.247L = Σd = 97.42, f23 = -125.074, f45 = -189.085 ##STR4##r5/r4 = 1.62______________________________________ 
    
     
         ______________________________________Embodiment 5:  f = 100, F 1.2, f.sub.B = 74.62rl = 82.84  dl = 10.19    nl = 1.83481 ν1 = 42.82r2 = 375.812  d2 = 0.19r3 = 54.248  d3 = 10.71    n2 = 1.834   ν2 = 37.19r4 = 81.369  d4 = 2.88r5 = 122.931  d5 = 3.25     n3 = 1.74    ν3 = 28.29r6 = 34.208  d6 = 34.04r7 = -32.638  d7 = 3.17     n4 = 1.84666 ν4 = 23.9r8 = -101.792  d8 = 11.54    n5 = 1.6968  ν5 = 55.52r9 = -46.762  d9 = 0.19r10 = -183.948  d10 = 11.15   n6 = 1.7725  ν6 = 49.60r11 = -58.952  d11 = 0.19r12 = 169.800   d12 = 5.77   n7 = 1.79952 ν7 = 42.24r13 = -699.104L = Σd = 93.29,  f23 = -134,5, f45 = -157.1 ##STR5##Petzval&#39;s sum = 0.158______________________________________ 
    
     
         ______________________________________Embodiment 6:f = 100, F 1.2, f.sub.B = 74.62r1 = 82.171  dl = 10.22    nl = 1.83481 ν 1 = 42.82r2 = 379.002  d2 = 0.19r3 = 54.883  d3 = 10.78    n2 = 1.834   ν = 37.19r4 = 81.410  d4 = 2.88r5 = 126.319  d5 = 3.29     n3 = 1.72825 ν3 = 28.46r6 = 34.304  d6 = 33.95r7 = -32.819  d7 = 3.11     n4 = 1.84666 ν4 = 23.9r8 = -105.769  d8 = 11.49    n5 = 1.6779  ν5 = 55.33r9 = -47.662  d9 = 0.19r10 = -163.417  d10 = 10.95   n6 = 1.7725  ν6 = 49.6r11 = -57.517  d11 = 0.19r12 = 174.050  d12 = 5.82    n7 = 1.79952 ν7 = 42.24r13 = -435.498Σd = 93.07  f23 = -133.2, f45 = -144.0 ##STR6##Petzval&#39;s sum = 0.159______________________________________ 
    
     In the above-mentioned respective embodiments, reference symbols r1 through r13 respectively represent radii of curvature of respective lens surfaces, reference symbols d1 through d12 respectively represent thicknesses of respective lenses and airspaces between respective lenses, reference symbols n1 through n7 respectively represent refractive indices of respective lenses and reference symbols ν1 through ν7 respectively represent Abbe&#39;s numbers of respective lenses. 
     Besides, Seidel&#39;s coefficients of aberrations for Embodiment 2 are as shown below. 
     
         ______________________________________Spherical  Astig-aberration matism   Coma     Distortion                                Petzval______________________________________1    0.3391    0.0349   0.1087 0.1730  0.50482    0.1966    0.9346   -0.4254                          -1.8829 -0.07763    -0.1005   -0.0377  -0.0615                          0.4759  0.81544    0.2388    1.4771   -0.5939                          -2.4658 -0.48575    -0.6738   -1.9629  1.1500 2.8717  0.28036    -0.8058   -0.1332  -0.3276                          -0.5593 -1.24287    -1.9118   -0.2357  0.6713 0.5576  -1.35218    -0.0015   -0.0235  0.0059 -0.1008 0.04889    0.5060    0.0602   -0.1746                          -0.3227 0.875410   -0.0002   -0.0053  0.0009 1.0496  -0.172411   1.6908    0.0028   -0.0687                          -0.0271 0.663012   -0.0009   -0.3026  0.0167 0.6366  0.267413   0.6271    0.1402   -0.2965                          -0.0942 0.0591Sum  0.1010    -0.0511  0.0055 0.3114  0.1836______________________________________  As explained in the above, the photographic lens system according to the present invention is compact in size and, moreover, its back focal length is long, the intensity of light in the marginal portion is satisfactorily high and aberrations are corrected favourably. This is evident also from numerical values and aberration curves of respective embodiments and also from Seidel&#39;s coefficients of aberrations for Embodiment 2 which are shown for example.