Abstract:
A zoom lens is disclosed comprising, from front to rear, a first lens unit of positive power, a second lens unit of negative power, a third lens unit of negative power and a fourth lens unit of positive power, zooming being performed by moving the second and third lens units axially in differential relation, whereby the focal length F2 of the second lens unit lies in the following range: 
     
       0.09&lt;|F2/FT|&lt;0.14 
     
     where FT is the longest focal length of the entire lens system. With this, though the optical total length is shorter than ever, the zoom lens gets a higher range of variation of the focal length.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to zoom lenses and, more particularly, to zoom lenses having an F-number of 2.0, an increased zoom ratio to about 8, and a good optical performance over the entire extended zooming range, with a relatively few number of constituent lenses constructed in simple form to a small size and a light weight, suited to photographic cameras, video cameras, etc. 
     2. Description of the Related Art 
     In the photographic camera, video camera, etc., the demand for zoom lenses of large relative aperture, high range and a high optical performance has been growing. 
     Of these, for example, a zoom lens for home video camera, which in view of an increase of the number of resolving cells of the image pickup element such as a CCD and an improvement of the recording technique such as S-VHS, high-band 8 mm, etc., is required to heighten its resolving power over the entire area of the image frame to, for example, 50 lines/mm in the spatial frequency. 
     Among the zoom lenses, there is a so-called 4-unit zoom lens comprising, from front to rear, a first lens unit of positive refractive power for focusing, a second lens unit of negative refractive power for varying the image magnification, a third lens unit of positive or negative refractive power for compensating for the shift of an image plane resulting from the variation of the magnification, and a fourth lens unit of positive refractive power for forming an image. Since this type allows the zoom ratio and the aperture ratio to be increased with relative ease, it has been employed in various kinds of cameras. 
     A proposal for increasing the range of the 4-unit zoom lens to about 6 has been made in, for example, Japanese Laid-Open Patent Applications Nos. Hei 1-120521 and Hei 1-120522. In addition there are U.S. Pat. Nos. 4,832,471, 4,846,563 and 4,934,796, and U.S. patent application Ser. No. 475,749 filed on Feb. 6, 1990. 
     In these publications, for every lens unit, proper rules of design are set forth to obtain a relatively good optical performance over the entire zooming range. But, because the refractive power of the first lens unit or the second lens unit is somewhat weak, the total length of the complete lens tends to become long. 
     With the use of the 4-unit type in the zoom lens design, to allow a minimization of the size of the entire lens system, the refractive power of every lens unit and the construction and arrangement of the members of the first lens unit and the zooming lens unit must be optimized. Otherwise, the variation of aberrations would be caused to increase, making it difficult to obtain a good optical performance throughout the entire zooming range. 
     SUMMARY OF THE INVENTION 
     A first object of the invention is to provide a zoom lens of short optical total length and shortened diameter in the first lens unit. 
     A second object is to provide a zoom lens which, though getting as high a zoom ratio as about 8, maintains good stability of optical performance throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section view of a numerical example 1 of a zoom lens of the invention. 
     FIGS. 2(A), 2(B) and 2(C), FIGS. 3(A), 3(B) and 3(C), FIGS. 4(A), 4(B) and 4(C) and FIGS. 5(A), 5(B) and 5(C) are graphic representations of the aberrations of numerical examples 1 to 4 of zoom lenses of the invention respectively. Of the aberration graphs, the ones whose figure numbers are suffixed (A) are in the wide-angle end, the ones whose figure numbers are suffixed (B) in the intermediate position, and the ones whose figure numbers are suffixed (C) in the telephoto end. 
    
    
     In the drawings, I, II, III and IV denote respectively the first, second, third and fourth lens units. ΔM represents the meridional image surface, and ΔS represents the sagittal image surface. d stands for the d-line, g for the g-line, and SP for the stop. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 in lens block diagram shows an embodiment of the zoom lens according to the invention comprising, from front to rear, a first lens unit I of positive refractive power for focusing, a second lens unit II of negative refractive power which, when zooming from the wide-angle end to the telephoto end, moves monotonously rearward, a third lens unit III of negative refractive power which, when zooming from the wide-angle end to the telephoto end, moves axially on a locus convex toward the front to keep constant the position of an image plane, and a fourth lens unit IV of positive refractive power which remains stationary during zooming and performs the image forming function, with a fixed stop SP positioned within the fourth lens unit IV. 
     Further, the present invention sets forth the following condition: 
     
         0.01&lt;(F1.sup.2 ·F.sub.NOT ·tan ωT)/FT.sup.2 &lt;0.049 (1) 
    
     where, F1 is the focal length of the first lens unit, and FT, F NOT  and ωT are respectively the focal length of the entire lens system, the F-number at full open aperture and the half angle of view, in the telephoto end. 
     In the present embodiment, since the first, second and third lens units are arranged as described above, and the refractive power of the first lens unit lies in the range given by the inequalities of condition (1), the zoom lens has a relatively small total number of lens elements, an F-number of about 2 and the range of variation of the focal length increased to about 8 while well correcting the variation of aberrations with zooming. Thus, a 4-unit zoom lens having good stability of high optical performance throughout the entire zooming range is achieved. 
     Further, the first and second lens units are provided with cemented lens surfaces to correct chromatic aberrations. 
     Next, the technical significance of the above-described condition is explained. 
     The inequalities of condition (1) give a proper range for the refractive power of the first lens unit and have an aim to minimize the bulk and size of the entire lens system while still permitting good correction of all aberrations to be performed. 
     When the refractive power of the first lens unit is weak as exceeding the upper limit of the condition (1), aberration correction becomes easy to perform, but the interval between the first lens unit and the stop increases largely. To receive the off-axial light beam, the diameter of the first lens unit must be increased. When the refractive power of the first lens unit is strong as exceeding the lower limit, the total length of the complete lens becomes short, but its distance from the second lens unit shortens to increase the possibility of occurrence of physical interference therebetween. Further, the variation with focusing of the aberrations comes to increase. So, that is not good. 
     To achieve a further improvement of the stability of aberration correction, it is recommended that the first lens unit is constructed from a cemented lens of positive refractive power having a cemented surface convex toward the front and a meniscus-shaped positive lens convex toward the front, the second lens unit is constructed from a negative lens whose rear surface is of strong curvature and a cemented lens of negative refractive power having a cemented surface convex toward the front, and the third lens unit is constructed from a negative lens whose front surface is concave toward the front. 
     In the zoom lens according to the invention, under the various conditions described above, the following conditions for the focal lengths F2 and F3 of the second and third lens units respectively are satisfied: 
     
         0.09&lt;|F2/FT|&lt;0.14                        (2) 
    
     
         0.45&lt;|F3/FT}&lt;0.65                                 (3) 
    
     These are preferable in reducing the variation of aberrations with zooming when the good stability of optical performance over the entire zooming range is obtained. 
     The inequalities of condition (2) give a proper range for the negative refractive power of the second lens unit and aim to get a predetermined increase of the zoom ratio and to achieve a shortening of the total length of the entire lens, particularly at the zooming section, with the limitation of the variation of aberration with zooming to a minimum. 
     When the refractive power of the second lens unit is weak as exceeding the upper limit, the required total movement of the second lens unit for securing the prescribed zoom ratio increases largely, which in turn causes the physical length of the zooming section to increase and the interval between the first lens unit and the stop to increase. To receive the off-axial light beam, the diameter of the first lens unit then increases objectionably. When the refractive power of the second lens unit is too strong and beyond the lower limit, the total movement of the second lens unit becomes short when the prescribed zoom ratio is secured. So, the total length of the complete lens gets short. However, the variation with zooming of aberrations is caused to increase objectionably. 
     The inequalities of condition (3) are concerned with the negative refractive power of the third lens unit and have an aim that as the third lens unit comprises only one lens of negative refractive power whose front surface is concave toward the front, the varying aberrations are corrected in good balance, and another aim to prevent the diameter of the front member of the first lens unit from increasing in the wide-angle end. 
     When the negative refractive power of the third lens unit is too weak as exceeding the upper limit, the total zooming movement of the third lens unit is caused to increase largely, thereby elongating the total length of the entire lens system. At the same time, the diameter of the front member of the first lens unit in the wide-angle end is increased largely. 
     When the negative refractive power of the third lens unit is too strong as exceeding the lower limit, the Petzval sum increases largely in the negative direction. So, the astigmatism comes to increase objectionably. 
     All the conditions given above suffice for accomplishing the objects of the invention. To further improve such a zoom lens by correcting the varying aberrations in good balance throughout the entire zooming range, it is preferred to satisfy the following condition: 
     
         1.1&lt;|(R.sub.III2 +R.sub.III1)/ (R.sub.III2 -R.sub.III1)|&lt;1.6                                (4) 
    
     where R IIIi  is the radius of curvature of the i-th lens surface, when counted from the front, in the aforesaid third lens unit. 
     The inequalities of condition (4) are concerned with figuration of the single lens of negative refractive power constituting the third lens unit. When the lower limit is exceeded, the higher-order spherical aberrations increase in the positive direction, and large outward coma is produced. This should be avoided. 
     When the upper limit is exceeded, the reverse results are effected. That is, the higher-order spherical aberrations increase in the negative direction, and large inward coma is produced. This makes it difficult to obtain high optical performance. 
     Besides this, the invention is to reduce the amount of image aberrations at any station in the zooming range, thus affording a high optical performance to attain. For this purpose, the fourth lens unit is constructed as follows: 
     The fourth lens unit is divided into two parts with the widest air spacing being the boundary, namely, in the order from the object side, a front lens sub-unit and a rear lens sub-unit. The front lens sub-unit is constructed from four lenses, that is, a positive first lens having a rear refracting surface of strong convex curvature toward the image side, a positive second lens having a front refracting surface of convex curvature toward the object side, a negative third lens having a front refracting surface of strong convex curvature toward the object side, and a positive fourth lens having a front refracting surface of strong convex curvature toward the object side. The rear lens sub-unit is constructed from three lenses, that is, a negative fifth lens having a rear refracting surface of strong concave curvature toward the image side, a positive sixth lens of bi-convex form, and a positive seventh lens. And, letting the focal length of the fourth lens unit be denoted by F4, the focal lengths of the front lens sub-unit and the rear lens sub-unit by F4-1 and F4-2 respectively, and the focal length of the i-th lens, when counted from the front, in the fourth lens unit by F4,i, the following conditions are satisfied: 
     
         0.62&lt;F4-1/F4&lt;0.83                                          (5) 
    
     
         0.27&lt;|F4,5/F4-2|&lt;0.36                    (6) 
    
     The inequalities of condition (5) are concerned with the refractive power of the front lens sub-unit and chiefly aim to correct spherical aberration. When the positive refractive power is too strong and exceeds the lower limit, the zonal spherical aberration on the wide-angle side comes to increase. When the positive refractive power is too weak as exceeding the upper limit, over-correction of spherical aberration results and, further, the outer diameter of the rear lens sub-unit increases objectionably. 
     The inequalities of condition (6) are concerned with the ratio of the negative refractive power of the fifth lens to the positive refractive power of the rear lens sub-unit and have an aim to correct chiefly off-axial aberrations in good balance. 
     When the negative refractive power of the fifth lens is too strong as exceeding the lower limit, large higher order astigmatisms are produced. When the negative refractive power of the fifth lens is too weak as exceeding the upper limit, negative distortion increases. This becomes difficult to correct well. 
     Incidentally, the term &#34;front refracting surface of strong curvature&#34; used above means that its refractive power is stronger than that of the other lens surface, that is, the rear lens surface. The same applies to the rear refracting surface of strong curvature. 
     Numerical examples 1 to 4 of zoom lenses of the invention are shown below. In the numerical examples 1 to 4, Ri denotes the radius of curvature of the i-th lens surface or air separation, when counted from the front, and Ni and νi are respectively the refractive index and Abbe number of the glass of the i-th lens element, when counted from the front. Incidentally, R28 and R29 define a glass block such as face plate or filter. 
     The values of the factors in the above-described conditions (1) to (6) for the numerical examples 1 to 4 are listed in Table-1. 
     
         ______________________________________Numerical Example 1 (FIGS. 1, 2(A), 2(B) and 2(C))F = 1-7.5 FNo = 1:2.07-2.70 2ω = 50.9°-7.3°______________________________________R1 = 8.293 D1 = 0.1585 N1 = 1.80518                              ν1 = 25.4R2 = 3.212 2 = 0.7017  N2 = 1.51633                              ν2 = 64.1R3 = -7.244      D3 = 0.0226R4 = 2.643 D4 = 0.3735 N3 = 1.69680                              ν3 = 55.5R5 = 8.515 D5 = Vari-      ableR6 = 5.457 D6 = 0.0792 N4 = 1.77250                              ν4 = 49.6R7 = 1.046 D7 = 0.3327R8 = -1.254      D8 = 0.0792 N5 = 1.69680                              ν5 = 55.5R9 = 1.255 D9 = 0.2943 N6 = 1.84666                              ν6 = 23.9R10 = 21.076      D10 = Vari-      ableR11 = -2.415      D11 = 0.905 N7 = 1.69680                              ν7 = 55.5R12 = -16.551      D12 =  Vari-      ableR13 = 6.146      D13 = 0.3282                  N8 = 1.65844                              ν8 = 50.9R14 = -2.310      D14 = 0.1698R15 = Stop D15 = 0.2264R16 = 3.354      D16 = 0.3169                  N9 = 1.62374                              ν9 = 47.1R17 = -4.677      D17 = 0.0949R18 = -2.126      D18 = 0.1019                  N10 = 1.84666                              ν10 = 23.9R19 = -41.309      D19 = 0.0170R20 = 2.570      D20 = 0.2943                  N11 = 1.63854                              ν11 = 55.4R21 = -7.703      D21 = 1.5181R22 = 102.170      D22 = 0.0792                  N12 = 1.83400                              ν12 = 37.2R23 = 1.346      D23 = 0.0865R24 = 4.212      D24 = 0.2151                  N13 = 1.51633                              ν13 = 64.1R25 = -2.881      D25 = 0.0170R26 = 1.362      D26 = 0.2943                  N14 = 1.57099                              ν14 = 50.8R27 = -154.621      D27 = 0.5659R28 = ∞      D28 = 0.6791                  N15 = 1.51633                              ν15 = 64.1R29 = ∞______________________________________Variable   Focal LengthSeparation 1.0          2.4    77.5______________________________________D5         0.11         1.29   2.11D10        2.28         0.72   0.34D12        0.18         0.56   0.12______________________________________Numerical Example 2 (FIGS. 3(A), 3(B) and 3(C))F = 1-7.5 FNo = 1:2.07-2.70 2ω = 50.8°-7.3°______________________________________R1 = 8.244 D1 = 0.1582 N1 = 1.80518                              ν1 = 25.4R2 = 3.189 D2 = 0.7232 N2 = 1.51633                              ν2 = 64.1R3 = -7.262      D3 = 0.0226R4 = 2.653 D4 = 0.3842 N3 = 1.69680                              ν3 = 55.5R5 = 8.741 D5 = Vari-      ableR6 = 5.670 D6 = 0.0791 N4 = 1.77250                              ν4 = 49.6R7 = 1.038 D7 = 0.3386R8 = -1.258      D8 = 0.0791 N5 = 1.69680                              ν5 = 55.5R9 = 1.258 D9 = 0.2938 N6 =  1.84666                              ν6 = 23.9R10 = 25.443      D10 = Vari-      ableR11 = -2.426      D11 = 0.0904                  N7 = 1.69680                              ν7 = 55.5R12 = -17.306      D12 = Vari-      ableR13 = 5.417      D13 = 0.3277                  N8 = 1.65844                              ν8 = 50.9R14 = -2.422      D14 = 0.1695R15 = Stop D15 = 0.2034R16 = 3.137      D16 = 0.3164                  N9 = 1.62374                              ν9 = 47.1R17 = -5.485      D17 = 0.1028R18 = -2.097      D18 = 0.1017                  N10 = 1.84666                              ν10 = 23.9R19 = -36.647      D19 = 0.0169R20 = 2.519      D20 = 0.2938                  N11 = 1.63854                              ν11 = 55.4R21 = -7.300      D21 = 1.5284R22 = 98.119      D22 = 0.0791                  N12 = 1.83400                              ν12 = 37.2R23 = 1.287      D23 = 0.0882R24 = 3.990      D24 = 0.2147                  N13 = 1.51633                              ν13 = 64.1R25 = -2.868      D25 = 0.0169R26 = 1.291      D26 = 0.2938                  N14 = 1.57099                              ν14 = 50.8R27 = 108.960      D27 = 0.5650R28 = ∞      D28 = 0.6780                  N15 = 1.51633                              ν15 = 64.1R29 = ∞______________________________________Variable   Focal LengthSeparation 1.0          2.4    7.5______________________________________D5         0.11         1.29   2.10D10        2.27         0.71   0.35D12        0.18         0.56   0.12______________________________________Numerical Example 3 (FIGS. 4(A), 4(B) and 4(C)F = 1-7.5 FNo = 1:2.07-2.70 2ω = 50.8°-7.3°______________________________________R1 = 8.204 D1 = 0.1582 N1 = 1.80518                              ν1 = 25.4R2 = 3.206 D2 = 0.7006 N2 = 1.51633                              ν2 = 64.1R3 = -7.385      D3 = 0.0226R4 = 2.648 D4 = 0.3729 N3 = 1.69680                              ν3 = 55.5R5 = 8.786 D5 = Vari-      ableR6 = 6.232 D6 = 0.0791 N4 = 1.77250                              ν4 = 49.6R7 = 1.062 D7 = 0.3367R8 = -1.283      D8 = 0.0791 N5 = 1.69680                              ν5 = 55.5R9 = 1.283 D9 = 0.3051 N6 = 1.84666                              ν6 = 23.9R10 = 19.406      D10 = Vari-      ableR11 = -2.432      D11 = 0.0904                  N7 = 1.69680                              ν7 = 55.5R12 = -17.832      D12 = Vari-      ableR13 = 6.246      D13 = 0.3277                  N8 = 1.65844                              ν8 = 50.9R14 = -2.291      D14 = 0.1695R15 = Stop D15 = 0.2260R16 = 3.439      D16 = 0.3164                  N9 = 1.62374                              ν9 = 47.1R17 = -4.698      D17 = 0.0979R18 = -2.106      D18 = 0.1017                  N10 = 1.84666                              ν10 = 23.9R19 = -31.449      D19 = 0.0169R20 = 2.521      D20 = 0.2938                  N11 = 1.63854                              ν11 = 55.4R21 = -7.659      D21 = 1.5106R22 = 45.911      D22 = 0.0791                  N12 = 1.83400                              ν12 = 37.2R23 = 1.136      D23 = 0.0890R24 = 4.423      D24 = 0.2147                  N13 = 1.51633                              ν13 = 64.1R25 = -2.990      D25 = 0.0169R26 = 1.357      D26 = 0.3051                  N14 = 1.57099                              ν14 = 50.8R27 = -81.822      D27 = 0.5650R28 = ∞      D28 = 0.6780                  N15 = 1.51633                              ν15 = 64.1R29 = ∞______________________________________Variable   Focal LengthSeparation 1.0          2.4    7.5______________________________________D5         0.11         1.29   2.10D10        2.30         0.76   0.39D12        0.18         0.55   0.12______________________________________Numerical Example 4 (FIGS. 5(A), 5(B) and 5(C)F = 1-7.5 FNo = 1:2.07-2.70 2ω = 50.8°-7.3°______________________________________R1 = 7.458 D1 = 0.1582 N1 = 1.80518                              ν1 = 25.4R2 = 2.982 D2 = 0.6893 N2 = 1.51633                              ν2 = 64.1R3 = -7.265      D3 = 0.0226R4 = 2.464 D4 = 0.3955 N3 = 1.69680                              ν3 = 55.5R5 = 8.512 D5 = Vari-      ableR6 = 8.147 D6 = 0.0791 N4 = 1.77250                              ν4 = 49.6R7 = 0.986 D7 = 0.3181R8 = -1.208      D8 = 0.0791 N5 = 1.69680                              ν5 = 55.5R9 = 1.208 D9 = 0.3051 N6 = 1.84666                              ν6 = 23.9R10 = 22.233      D10 = Vari- -                  ableR11 = -2.262      D11 = 0.0904                  N7 = 1.69680                              ν7 = 55.5R12 = -17.044      D12 = Vari-      ableR13 = 7.056      D13 = 0.3503                  N8 = 1.65844                              ν8 = 50.9R14 = -2.043      D14 = 0.1695R15 = Stop D15 = 0.2260R16 = 3.044      D16 = 0.3164                  N9 = 1.62374                              ν9 = 47.1R17 = -5.888      D17 = 0.1099R18 = -2.097      D18 = 0.1017                  N10 = 1.84666                              ν10 =  23.9R19 = -35.931      D19 = 0.0169R20 = 2.530      D20 = 0.2938                  N11 = 1.63854                              ν11 = 55.4R21 = -7.620      D21 = 1.5569R22 = 120.444      D22 = 0.0791                  N12 = 1.83400                              ν12 = 37.2R23 = 1.335      D23 = 0.0814R24 = 3.688      D24 = 0.2147                  N13 = 1.51633                              ν13 = 64.1R25 = -2.804      D25 = 0.0169R26 = 1.310      D26 = 0.3051                  N14 = 1.53172                              ν14 = 48.9R27 = -405.406      D27 = 0.5650R28 = ∞      D28 = 0.6780                  N15 = 1.51633                              ν15 = 64.1R29 = ∞______________________________________Variable   Focal LengthSeparation 1.0          2.5    7.5______________________________________D5         0.12         1.20   1.92D10        2.09         0.66   0.35D12        0.18         0.52   0.12______________________________________ 
    
     
                       TABLE-1______________________________________Con-dition               Numerical ExampleNo.   Factor         1       2     3     4______________________________________(1)  ##STR1##      0.044   0.044 0.044 0.038(2)   | F2/FT |                0.12    0.12  0.12  0.10(3)   | F3/FT |                0.55    0.54  0.54  0.50(4)  ##STR2##      1.34    1.33  1.32  1.31(5)  ##STR3##      0.72    0.69  0.73  0.70(6)  ##STR4##      0.32    0.31  0.30  0.31______________________________________ 
    
     According to the invention, by setting forth the rules of design for each lens unit as has been described before, the total length of the complete lens is shortened and the whole lens system is simplified, while still permitting the optical performance to be well maintained throughout the entire zooming range. The invention has thus achieved a 4-unit zoom lens of as high a range as 8 in the simple form suited to the photographic camera, video camera, etc.