Abstract:
A zoom lens includes, in the order from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power. The first and third lens units are stationary. The second lens unit is moved in one direction to vary the magnification. The fourth lens unit is moved so as to compensate for the shift of an image plane resulting from the variation of the magnification, and focusing is performed by moving the fourth lens unit. The third lens unit has a bi-convex lens and a negative meniscus lens having a strong concave surface facing the object side, and the fourth lens unit has a negative meniscus lens having a strong concave surface facing the image side, a bi-convex lens and a positive lens having a strong convex surface facing the object side.

Description:
This application is a continuation of application Ser. No. 07/740,303 filed Aug. 5, 1991, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to zoom lenses and, more particularly, to zoom lenses of compact size and light weight at an F-number of 1.8 to 2.0 with still as high a zoom ratio as about 8, while having a good optical performance over the entire zooming range, suited to cameras for photography, video cameras, etc. 
     2. Description of the Related Art: 
     Among the zoom lenses of a relatively high range with a large relative aperture used in still cameras or video cameras, there has been the so-called 4-unit zoom lens. This 4-unit type of zoom lens comprises, from front to rear, a first lens unit for focusing, a second lens for varying the magnification, a third lens unit for compensating for the image shift resulting from variation of the magnification, and a fourth lens unit for use in balancing the focal length of the entire system and correcting aberrations. In the 4-unit zoom lens, a configuration employed is that for the zooming purpose, two lens units, and, for the focusing purpose, another one lens unit, totaling three lens units, are made movable. For this reason, there is a tendency to have a relatively complicated mounting mechanism for such a zoom lens. 
     Also, when focusing to shorter object distances, the first lens unit moves toward the object side. Accordingly, admission of a sufficient off-axial light beam to enter the zoom lens results in another tendency for the diameter of the front lens members to increase largely. 
     For these reasons, there have been many previous proposals for zoom lenses in which a lens unit other than the first lens unit is employed in focusing, or which utilize the so-called rear focus method, in, for example, Japanese Laid-Open Patent Applications Nos. Sho 62-24213, Sho 63-123009 and Hei 2-55308. 
     By the way, in the above-described publications, the zoom lenses disclosed have a zoom ratio of 6 to 8 or thereabout. However, the number of constituent lens elements is relatively large. An attempt to use a smaller number of lens elements in constructing the zoom lens of a range of about 8 leads to a tendency to increase the amount of movement of the fourth lens unit from an infinitely distant object to a closest object, which in turn increases the variation of aberrations with focusing. So, this increased range of variation of aberrations becomes difficult to correct. As the related art of the present invention, there are U.S. Pat. No. 5,009,492 and U.S. patent application Ser. No. 647,495 filed on Jan. 29, 1991, now U.S. Pat. No. 5,241,421, issued on Aug. 31, 1993. 
     SUMMARY OF THE INVENTION 
     With such a problem in mind, the invention has, despite simultaneous occurrence of a high range and promotion to a compact form by making ever smaller the optical effective diameter and ever shorter the total lens length, to realize an aperture ratio of about F1.8-2.0. In addition, variation of aberrations with zooming over the high range and variation of aberrations with focusing are well corrected. Thus, the invention proposes a rear focus type zoom lens of compact form and light weight. 
     The zoom lens of the invention comprises, in the order from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein the first and third lens units are made stationary, variation of the magnification is performed by moving the second lens unit in one direction, the fourth lens unit is made to move so as to compensate for the shift of an image plane resulting from the variation of the magnification, and focusing is performed by moving the fourth lens unit. The aforesaid third lens unit has a bi-convex lens and a negative meniscus lens having a strongly refractive concave surface facing the object side. The aforesaid fourth lens unit has a negative meniscus lens having a strongly refractive concave surface facing the image side, a bi-convex lens and a positive lens having a strongly refractive convex surface facing the object side. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 and FIG. 4 are sectional views of numerical examples 1 and 3 of zoom lenses of the invention. 
     FIGS. 2(A), 2(B) and 2(C), FIGS. 3(A), 3(B) and (C), FIGS. 5(A), 5(B) and 5(C) and FIGS. 6(A), 6(B) and (C) are graphs of various aberrations of the numerical examples 1 to 4 of the invention respectively. 
     In the graphs of aberrations, FIGS. 2(A), 3(A), 5(A) and 6(A) are the graphs of aberrations in the wide angle end, FIGS. 2(B), 3(B), 5(B) and 6(B) in the middle, and FIGS. 2(C), 3(C), 5(C) and 6(C) in the telephoto end. 
     In the figures, I, II, III and IV are respectively the first, second, third and fourth lens units. ΔM is the meridional image surface and ΔS is the sagittal image surface, d is the d-line, g is the g-line, and SP is a stop. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a lens sectional view of the numerical example 1 of the invention. In the figure, I is the first lens unit of positive refractive power, II is the second lens unit of negative refractive power, which moves monotonously toward the image side during variation of the magnification from the wide-angle end to the telephoto end, III is the third lens unit of positive refractive power, IV is the fourth lens unit of positive refractive power, which moves on an optical axis while having a locus of convex shape toward the object side covering from the wide-angle end to the telephoto end in order to keep the image plane that moves with variation of the magnification at a constant position and which also moves along the optical axis for focusing, SP is a fixed stop or diaphram arranged in the third lens unit. It is to be noted that the first and third lens units are stationary during variation of the magnification and during focusing. 
     In the present embodiment, during focusing, the first lens unit is not moved, but remains always stationary. Thus, the increase of the lens diameter, which would be caused by the forward movement, is prevented. During focusing, instead of the first lens unit, the fourth lens unit that is part of the magnification varying system is made to move. Therefore, the number of movable lens units is reduced. Thus, the mechanism is simplified in structure. Also, while letting the fourth lens unit have both of the functions of varying the magnification and focusing, the fourth lens unit is made to move within a space between the third lens unit and the fourth lens unit. Therefore, an efficient utilization of the space within the lens system is performed. In such a manner, a shortening of the total lens length is achieved. 
     Further, by constructing the third lens unit with a bi-convex lens and a negative meniscus lens having a strongly refractive concave surface facing the object side, spherical aberration and coma are well corrected. By constructing the fourth lens unit with a negative meniscus lens having a strongly refractive concave surface facing the image side, a bi-convex lens and a positive lens having a strongly refractive convex surface facing the object side, variation of aberration due to focusing from a closest object to an infinitely distant object, particularly curvature of field and astigmatism, are corrected. 
     The zoom lens the invention aims at can be achieved by the above-described form and construction and arrangement. Further, in view of aberration correction, letting the radius of curvature of the j-th lens surface in the i-th lens unit be denoted by R i ,j, the j-th lens thickness or air separation in the i-th lens unit by D i ,j, and the focal length in the wide-angle end by fw, it is preferred to satisfy the following conditions: 
     
         -1.75&lt;R.sub.3,1 /R.sub.3,3 &lt;-1.10                          (1) 
    
     
         0.7&lt;R.sub.4,3 /R.sub.4,5 &lt;1.0                              (2) 
    
     
         0.07&lt;D.sub.3,2 /fw&lt;0.12                                    (3) 
    
     
         0.01&lt;D.sub.4,2 /fw&lt;0.03                                    (4) 
    
     The inequalities of condition (1) are concerned with the ratio of the radius of curvature of the first lens surface, when counted from the object side, in the third lens unit to the radius of curvature of the third lens surface in the third lens unit, being the condition chiefly for properly correcting spherical aberration. When the lower limit is exceeded, the spherical aberration becomes under-corrected. Also, when the upper limit is exceeded, it becomes over-corrected. 
     The inequalities of condition (2) are concerned with the ratio of the radius of curvature of the third lens surface, when counted from the object side, in the fourth lens unit to the radius of curvature of the fifth lens surface in the fourth lens unit, being a condition for correcting curvature of field and coma in good balance. When the lower limit is exceeded, the curvature of field becomes under-corrected and, further, large inward coma is produced. Also, when the upper limit is exceeded, the curvature of field becomes over-corrected and, further, large outward coma is produced, which becomes difficult to correct well. Also, the variation of astigmatism due to focusing from a closest object to an infinitely distant object becomes large. 
     The inequalities of condition (3) are a condition chiefly for correcting spherical aberration and astigmatism in good balance. When the lower limit is exceeded, the astigmatism becomes large. Also, when the upper limit is exceeded, spherical aberrations of higher order are produced. 
     The inequalities of condition (4) are a condition chiefly for correcting coma and distortion. When the lower limit is exceeded, outward coma is produced and it becomes difficult to correct well. Also, when the upper limit is exceeded, distortion of barrel type is produced in the wide-angle end, which is no good. 
     Next, numerical examples 1 and 2 of the invention are shown. In the numerical examples 1 and 2, Ri is the radius of curvature of the i-th lens surface in the order from the object side, Di is the i-th lens thickness and air separation from the object side, and Ni and νi are respectively the refractive index and Abbe number of the glass of the i-th lens in the order from the object side. R22 and R23 are a face plate, filter or the like. 
     Also, the relationship of the above-described inequalities of conditions (1) to (4) and the various numerical values in the numerical examples 1 and 2 is shown in Table 1. 
     
         ______________________________________Numerical Example 1 (FIGS. 1, 2(A), 2(B) and 2(C):F = 1-7.67 FNo 1:2.05-2.83 2ω = 47.9°-6.6°______________________________________R1 = 7.8663      D1 = 0.1667                 N1 = 1.80518                             ν1 = 25.4R2 = 3.3829      D2 = 0.7222                 N2 = 1.51633                             ν2 = 64.1R3 = -8.0915      D3 = 0.0278R4 = 2.5531      D4 = 0.4028                 N3 = 1.60311                             ν3 = 60.7R5 = 6.2798      D5 = Vari-      ableR6 = 16.6730      D6 = 0.1111                 N4 = 1.69680                             ν4 = 55.5R7 = 1.0768      D7 = 0.4891R8 = -1.5016      D8 = 0.0972                 N5 = 1.69680                             ν5 = 55.5R9 = 1.5016      D9 = 0.3750                 N6 = 1.84666                             ν6 = 23.9R10 = -14.9686      D10 = Vari-      ableR11 = Stop D11 = 0.2778R12 = 2.7842      D12 = 0.3333                 N7 = 1.63854                             ν7 = 55.4R13 = -4.5959      D13 = 0.0832R14 = -2.1560      D14 = 0.1111                 N8 = 1.80518                             ν8 = 25.4R15 = -3.2269      D15 = Vari-      ableR16 = 4.3594      D16 = 0.1111                 N9 = 1.84666                             ν9 = 23.9R17 = 1.9151      D17 = 0.0193R18 = 2.1825      D18 = 0.3472                 N10 = 1.51633                             ν10 = 64.1R19 = -3.9148      D19 = 0.0208R20 = 2.7545      D20 = 0.2778                 N11 = 1.51633                             ν11 = 64.1R21 = -219.0174      D21 = 0.6944R22 = ∞      D22 = 0.8333                 N12 = 1.51633                             ν12 = 64.1R23 = ∞______________________________________ 
    
     
         ______________________________________Separations During ZoomingFocal Length       1.00         2.02   7.67______________________________________D5          0.20         1.24   2.50D10         2.93         1.89   0.63D15         1.34         1.04   1.64______________________________________ 
    
     
         ______________________________________Numerical Example 2 (FIGS. 3(A), 3(B) and 3(C)):F = 1-7.72 FNo 1:1.85-2.82 2ω = 47.9°-6.6°______________________________________R1 = 7.9022      D1 = 0.1667                 N1 = 1.80518                             ν1 = 25.4R2 = 3.3235      D2 = 0.7500                 N2 = 1.51633                             ν2 = 64.1R3 = -7.8306      D3 = 0.0278R4 = 2.5029      D4 = 0.4028                 N3 = 1.60311                             ν3 = 60.7R5 = 5.9356      D5 = Vari-      ableR6 = 18.1868      D6 = 0.1111                 N4 = 1.69680                             ν4 = 55.5R7 = 1.0676      D7 = 0.5517                 N5 = 1.69680                             ν5 = 55.5R8 = -1.5000      D8 = 0.0972R9 = 1.5000      D9 = 0.3750                 N6 = 1.84666                             ν6 = 23.9R10 = -12.0392      D10 = Vari-      ableR11 = Stop D11 = 0.2778R12 = 3.0007      D12 = 0.3750                 N7 = 1.63854                             ν7 = 55.4R13 =  -4.2307      D13 = 0.0898R14 = -2.2185      D14 = 0.1111                 N8 = 1.80518                             ν8 = 25.4R15 = -3.2104      D15 = Vari-      ableR16 = 4.1662      D16 = 0.1111                 N9 = 1.84666                             ν9 = 23.9R17 = 1.8677      D17 = 0.0716R18 = 2.0582      D18 = 0.3889                 N10 = 1.51633                             ν10 = 64.1R19 = -4.0391      D19 = 0.0208R20 = 2.4136      D20 = 0.2778                 N11 = 1.51633                             ν11 = 64.1R21 = 12.6334      D21 = 0.6944R22 = ∞      D22 = 0.8333                 N12 = 1.51633                             ν12 = 64.1R23 = ∞______________________________________ 
    
     
         ______________________________________Separations During ZoomingFocal Length       1.00         2.01   7.72______________________________________D5          0.19         1.23   2.49D10         2.93         1.89   0.63D15         1.44         1.13   1.74______________________________________ 
    
     
                       (TABLE 1)______________________________________         Numerical ExampleConditions      1        2______________________________________(1) R.sub.3,1 /R.sub.3,3           -1.29    -1.35(2) R.sub.4,3 /R.sub.4,5           0.79     0.85(3) D.sub.3,2 /fw           0.083    0.090(4) D.sub.4,2 /fw           0.019    0.018______________________________________ 
    
     Next, another embodiment which has some improvements on the above-described embodiment is described on the basis of the lens sectional view of FIG. 4. 
     In the present embodiment, the fundamental form of each lens is the same as in the above-described embodiment, but what is characteristic is in the point that the first lens and the second lens in the fourth lens unit IV are cemented together. Accordingly, lateral chromatic aberration particularly in the telephoto end is corrected well. 
     Further, it is desirable that, letting the radius of curvature of the j-th lens surface in the i-th lens unit be denoted by R i ,j, the j-th lens thickness or air separation in the i-th lens unit by D i ,j, and the focal length in the wide-angle end by fw, the following conditions are satisfied: 
     
         -1.75&lt;R.sub.3,1 /R.sub.3,3 &lt;-1.10                          (5) 
    
     
         1.30&lt;R.sub.4,2 /fw&lt;1.90                                    (6) 
    
     
         5.60&lt;(R.sub.4,1 +R.sub.4,4)/fw&lt;7.40                        (7) 
    
     
         0.07&lt;D.sub.3,2 /fw&lt;0.12                                    (8) 
    
     The inequalities of condition (5) are concerned with the ratio of the radius of curvature of the first lens surface, when counted from the object side, in the third lens unit to the radius of curvature of the third lens surface in the third lens unit, being a condition chiefly for properly correcting spherical aberration. When the lower limit is exceeded, the spherical aberration becomes under-corrected. Also, when the upper limit is exceeded, it becomes over-corrected. 
     The inequalities of condition (6) are concerned with the radius of curvature of the second or cemented lens surface, when counted from the object side, in the fourth lens unit, being a condition for correcting chromatic aberrations in good balance. When the lower limit is exceeded, the longitudinal chromatic aberration in the telephoto end becomes over-corrected. Also, when the upper limit is exceeded, the lateral chromatic aberration in the wide-angle end becomes under-corrected. 
     The inequalities of condition (7) are concerned with the radius of curvature of the first lens surface, when counted from the object side, in the fourth lens unit and the radius of curvature of the fourth lens surface in the fourth lens unit, being a condition chiefly for correcting curvature of field and distortion. When the radius of curvature becomes small beyond the lower limit, large distortion of the barrel type is produced in the wide-angle end. Also, when the radius of curvature becomes large beyond the upper limit, the image surface curves in such a manner as to become convex toward the object side, which becomes difficult to correct well. 
     The inequalities of condition (8) are a condition chiefly for correcting spherical aberration and astigmatism in good balance. When the lower limit is exceeded, the astigmatism becomes large. Also, when the upper limit is exceeded, spherical aberrations of higher order are produced. 
     Next, numerical examples 3 and 4 of the invention are shown. 
     Also, the relationship of the above-described inequalities of conditions (5) to (8) and the various numerical values in the numerical examples 3 and 4 is shown in Table-2. 
     
         ______________________________________Numerical Example 3 (FIGS. 4, 5(A), 5(B) and 5(C)):F = 1-7.71 FNo 1:2.05-2.83 2ω = 47.9°-6.6°______________________________________R1 = 8.0315      D1 = 0.1667                 N1 = 1.80518                             ν1 = 25.4R2 = 3.3249      D2 = 0.7500                 N2 = 1.51633                             ν2 = 64.1R3 = -7.7696      D3 = 0.0278R4 = 2.5010      D4 = 0.4028                 N3 = 1.60311                             ν3 = 60.7R5 = 5.9842      D5 = Vari-      ableR6 = 18.3188      D6 = 0.1111                 N4 = 1.69680                             ν4 = 55.5R7 = 1.0726      D7 = 0.4902R8 = -1.4958      D8 = 0.0972                 N5 = 1.69680                             ν5 = 55.5R9 = 1.4958      D9 = 0.3889                 N6 = 1.84666                             ν6 = 23.9R10 = -13.6255      D10 = Vari-      ableR11 = Stop D11 = 0.2778R12 = 3.1523      D12 = 0.3333                 N7 = 1.65844                             ν7 = 50.9R13 =  -4.3235      D13 = 0.0862R14 = -2.0668      D14 = 0.1111                 N8 = 1.80518                             ν8 = 25.4R15 = -2.8224      D15 = Vari-      ableR16 = 3.8221      D16 = 0.1111                 N9 = 1.84666                             ν9 = 23.9R17 = 1.5873      D17 = 0.3889                 N10 = 1.51633                             ν10 = 64.1R18 = -4.1634      D18 = 0.0208R19 = 2.8249      D19 = 0.2778                 N11 = 1.51633                             ν11 = 64.1R20 = 33.6607      D20 = 0.6944R21 = ∞      D21 = 0.8333                 N12 = 1.51633                             ν12 = 64.1R22 = ∞______________________________________ 
    
     
         ______________________________________Separations During ZoomingFocal Length       1.00         2.01   7.71______________________________________D5          0.21         1.25   2.51D10         2.93         1.89   0.63D15         1.46         1.16   1.77______________________________________ 
    
     
         ______________________________________Numerical Example 4 (FIGS. 6(A), 6(B) and 6(C)):F = 1-7.73 FNo 1:1.85-2.81 2ω = 47.9°-6.6°______________________________________R1 = 8.1576      D1 = 0.1667                 N1 = 1.80518                             ν1 = 25.4R2 = 3.3137      D2 = 0.7500                 N2 = 1.51633                             ν2 = 64.1R3 = -7.6997      D3 = 0.0278R4 = 2.4969      D4 = 0.4028                 N3 = 1.60311                             ν3 = 60.7R5 = 6.0283      D5 = Vari-      ableR6 = 20.6889      D6 = 0.1111                 N4 = 1.69680                             ν4 = 55.5R7 = 1.0619      D7 = 0.5365                 N5 = 1.69680                             ν5 = 55.5R8 = -1.4844      D8 = 0.0972R9 = 1.4844      D9 = 0.3889                 N6 = 1.84666                             ν6 = 23.9R10 = -10.9709      D10 = Vari-      ableR11 = Stop D11 = 0.2778R12 = 3.1915      D12 = 0.3750                 N7 = 1.65844                             ν7 = 50.9R13 =  -4.1837      D13 = 0.0892R14 = -2.2205      D14 = 0.1111                 N8 = 1.80518                             ν8 = 25.4R15 = -3.1754      D15 = Vari-      ableR16 = 3.8793      D16 = 0.1111                 N9 = 1.84666                             ν9 = 23.9R17 = 1.5979      D17 = 0.4167                 N10 = 1.51633                             ν10 = 64.1R18 = -4.1745      D18 = 0.0208R19 = 2.4664      D19 = 0.2778                 N11 = 1.51633                             ν11 = 64.1R20 = 12.2820      D20 = 0.6944R21 = ∞      D21 = 0.8333                 N12 = 1.51633                             ν12 = 64.1R22 = ∞______________________________________ 
    
     
         ______________________________________Separations During ZoomingFocal Length       1.00         2.01   7.73______________________________________D5          0.21         1.25   2.51D10         2.93         1.89   0.63D15         1.49         1.19   1.80______________________________________ 
    
     
                       (TABLE 2)______________________________________          Numerical ExampleCondition        1        2______________________________________(5) R.sub.3,1 /R.sub.3,3            -1.53    -1.44(6) R.sub.4,2 /fw            1.59     1.60(7) (R.sub.4,1 + R.sub.4,4)/fw            6.65     6.35(8) D.sub.3,2 /fw            0.086    0.089______________________________________ 
    
     According to the present invention, a zoom lens of compact form and light weight with an F-number of about 1.8-2.0 and still as high a zoom ratio as about 8, having a good optical performance over the entire high range of variation of the magnification, suited to the camera for photography, video camera, etc, can be achieved.