Abstract:
A wide angle zoom lens, includes, from an object side in the following order: a first lens group that has a negative refractive power; a second lens group that has a positive refractive power; and a third lens group. When variable magnification is carried out from a wide angle end to a telephoto end, the first and second lens groups move while the third lens group remains fixed, so that an air space between the first lens group and the second lens group becomes narrow and that a space between the second lens group and the third lens group becomes wide. The first and second lens groups each include at least one positive lens and one negative lens. The third lens group includes one positive or negative meniscus lens whose convex surface faces an image surface. The following conditional expressions (1) and (2) are satisfied: 
       1.4≦| f 1|/ fw ≦1.8  (1); and 
       1.7≦ f 2/ fw ≦2.1  (2) 
     where f1 is a focal length of the first lens group of the wide angle zoom lens; f2 is a focal length of the second lens group of the wide angle zoom lens; and fw is a focal length of the entire system at the wide angle end of the wide angle zoom lens.

Description:
[0001]    This application claims benefit of Japanese Application No. 2009-029562 filed in Japan on Feb. 12, 2009, the contents of which are incorporated by this reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a zoom lens used in an image pickup device and the like and particularly to an interchangeable wide angle zoom lens used in a digital single lens camera and the like. 
         [0003]    As for the wide angle zoom lens of digital camera, an optical system having a long back focus is required in order to provide a space into which optical elements such as filters are inserted right in front of an electronic image pickup plane while securing a wide angle of view. As such an optical system, a retro-focus type optical system is widely applied that has a front group of negative refractive power and a rear group of positive refractive power. If a so-called oblique light incidence situation occurs in which an incident optical beam tilts with respect to an optical axis when striking the electronic image pickup plane after passing through the optical system, the amount of peripheral light decreases, and shading and color drift occur. Therefore, an optical system has been proposed to maintain telecentricity of the incident optical beam. Such a type of technology is disclosed in JP-A-4-217219, JP-A-6-94996, JP-A-11-52246, or JP-A-2005-37727. 
         [0004]    In recent years, a compact wide angle zoom lens of digital camera is required whose overall length of the lens is made shorter with a high variable magnification ratio. If an optical system has fewer lenses to achieve downsizing, it is difficult to design the optical system that maintains telecentricity while securing a wide angle and suppressing various types of aberration. However, in recent years, technologies have been proposed to correct distortion to some extent by using electric signals as the technologies of electronic image pickup elements advance. Therefore, it is not necessary to pay particular attention to distortion correction in correcting aberration. 
       SUMMARY OF THE INVENTION 
       [0005]    Provided is a compact wide angle zoom lens that corrects distortion to some extent, corrects various kinds of aberration such as chromatic aberration and field curvature appropriately, and secures a long back focus and telecentricity while having a wide angle of view of around 75 degrees at a wide angle end. 
         [0006]    A wide angle zoom lens, includes, from an object side in the following order: a first lens group that has a negative refractive power; a second lens group that has a positive refractive power; and a third lens group. When variable magnification is carried out from a wide angle end to a telephoto end, the first and second lens groups move while the third lens group remains fixed, so that an air space between the first lens group and the second lens group becomes narrow and that a space between the second lens group and the third lens group becomes wide. The first and second lens groups each include at least one positive lens and one negative lens. The third lens group includes one positive or negative meniscus lens whose convex surface faces an image surface. 
         [0007]    The zoom lens in which the negative first lens group, the positive second lens group, and the third lens group are arranged from the object side in that order has a wider picture-taking angle of view at the wide angle end, and is able to easily secure a longer back focus. Moreover, the number of lens groups that move when the variable magnification is carried out is small: Only the first and second lens groups move. Therefore, a lens frame is simple in structure and can be downsized easily. Furthermore, costs can be easily reduced because of a fewer number of lenses in an optical system compared with typical positive preceding zoom lens. 
         [0008]    In addition, the first and second lens groups use a combination of positive and negative lenses to correct various types of aberration such as chromatic aberration. The third lens group uses the meniscus lens whose convex surface faces the image surface to correct chiefly field curvature while reducing the decline in performance caused by eccentricity at the time of manufacturing. 
         [0009]    Moreover, the wide angle zoom lens of the present invention is so formed as to satisfy the following conditional expressions (1) and (2): 
         [0000]      1.4 ≦|f 1|/ fw≦ 1.8  (1); and 
         [0000]      1.7 ≦f 2/ fw≦ 2.1  (2) 
         [0010]    where f1 is a focal length of the first lens group of the zoom lens; 
         [0011]    f2 is a focal length of the second lens group of the zoom lens; 
         [0012]    and fw is a focal length of the entire system at the wide angle end of the zoom lens. 
         [0013]    The conditional expression (1) is a rule pertaining to the focal length of the first lens group, standardized by the focal length at the wide angle end. If the number exceeds the upper limit of the conditional expression (1), the negative refractive power of the first lens group declines. Accordingly, while it is easy to correct various types of aberration, the effect of the retro-focus type decreases and it becomes difficult to secure a substantial back focus. If the number goes below the lower limit of the conditional expression (1), the negative refractive power of the first lens group increases, and it becomes difficult to correct various types of aberration. 
         [0014]    The conditional expression (2) is a rule pertaining to the focal length of the second lens group, standardized by the focal length at the wide angle end. If the number exceeds the upper limit of the conditional expression (2), the positive refractive power of the second lens group declines and it becomes easy to correct various types of aberration. However, exceeding the upper limit is not preferable from a practical perspective because the amount of movement increases to secure the same ratio of the variable magnification and because the optical system as a whole increases in size. If the number goes below the lower limit of the conditional expression (2), the positive refractive power of the second lens group increases, and it becomes difficult to correct various types of aberration. 
         [0015]    Furthermore, the present invention described above preferably includes one of the following structures. 
         [0016]    In the wide angle zoom lens of the present invention, the third lens group preferably includes one positive meniscus lens whose convex surface faces an image surface and which has a weak positive refractive power. 
         [0017]    Thanks to the above configuration, while correcting field curvature and the like, it is possible to move the position of an exit pupil away from the image surface. Therefore, it becomes easier to secure a telecentric optical system and is advantageous to shading. 
         [0018]    Moreover, in the wide angle zoom lens of the present invention, only the first lens group preferably moves to the object side during focusing from a long distance object point to a short distance object point. 
         [0019]    Since the first lens group is moved to the object side during focusing from the long distance object point to the short distance object point, the change in field curvature or the like can be kept as low as possible. Therefore, it is easy to secure a high optical performance from the long distance object point to the short distance object point. Moreover, the configuration is formed by a simple frame structure and can therefore be realized by a small and low-cost mechanism. 
         [0020]    At the image side of the second lens group, there are two positive lenses, and the following conditional expression (3) is preferably satisfied: 
         [0000]      1.5 ≦f   2rp   /fw≦ 2.0  (3) 
         [0021]    where f 2rp  is a total focal length of the two positive lenses that are disposed at the image side of the second lens group. 
         [0022]    The conditional expression (3) is a rule pertaining to the total focal length of the two positive lenses that are disposed at the image side of the second lens group, standardized by the focal length at the wide angle end. If the number exceeds the upper limit of the conditional expression (3), the positive refractive power of the two positive lenses that are disposed at the image side of the second lens group declines. Accordingly, the angle of emergence of an optical beam around a screen increases at the wide angle end, and it becomes difficult to secure telecentricity. If the number goes below the lower limit of the conditional expression (3), the positive refractive power of the two positive lenses that are disposed at the image side of the second lens group increases. Therefore, field curvature occurs on a large scale at the minus side, and it becomes difficult to make correction. 
         [0023]    Moreover, in the wide angle zoom lens of the present invention, the second lens group, from the object side in the following order, includes a positive lens that has a convex surface facing the object; an aperture diaphragm; a cemented lens that has a negative refractive power and is made of a biconvex lens and a biconcave lens combined; a positive lens; and a positive lens. All the lens surfaces of the second lens group are preferably spherical. 
         [0024]    The second lens group contributes most to carrying out the variable magnification. Therefore, the second lens group has a large refractive power and is likely to cause aberration because the components vary with the manufacturing process. The use of aspheric lens is advantageous in correcting aberration. However, if a plastic aspheric lens is used, the problem is that aberration changes as temperature and humidity change. If a glass aspheric lens is used, the problem is that the lens radius increases as the image pickup element increases in size, leading to an increase in costs. Therefore, the above configuration is formed by only the glass spherical lenses. It is possible to have a stable optical performance during the manufacturing process, easily correct various types of aberration with a fewer number of lenses, and secure a low-cost, high optical performance. 
         [0025]    Moreover, in the wide angle zoom lens of the present invention, the first lens group preferably includes at least one aspheric surface. 
         [0026]    Thanks to the above configuration, it becomes easier to correct field curvature and barrel-shaped distortion, which are likely to occur when the retro-focus type optical system is downsized at the wide angle end. Therefore, the system can be small in size and secure a high optical performance. 
         [0027]    Moreover, in the wide angle zoom lens of the present invention, the first lens group includes a plastic aspheric lens, and the following conditional expression (4) is preferably satisfied: 
         [0000]      |φ12|≦0.04  (4) 
         [0028]    where φ12 is the refractive power of the plastic aspheric lens. 
         [0029]    The conditional expression (4) is a rule governing the refractive power of the plastic aspheric lens in the first lens group. 
         [0030]    In general, the plastic aspheric lens is inexpensive. However, one of the problems is that the performance is likely to vary depending on environmental changes. Since the conditional expression (4) is satisfied, the impact of temperature and humidity changes on the performance decline can be reduced. Therefore, a high optical performance can be secured. 
         [0031]    It is more preferable that: 
         [0000]      |φ12|≦0.02  (4)′ 
         [0032]    Moreover, in the wide angle zoom lens of the present invention, the first lens group preferably includes a hybrid aspheric lens produced by applying a thin resin layer to the surface of a glass lens and transforming the surface thereof into a aspheric surface. 
         [0033]    In general, the hybrid aspheric lens is inexpensive. Since the resin layer is thin, the change in performance rarely occurs even as environment changes. Therefore, it is possible to secure a high optical performance. 
         [0034]    Moreover, in the wide angle zoom lens of the present invention, the first lens group preferably includes a glass-molded aspheric lens. 
         [0035]    In general, the glass-molded aspheric lens has a higher refractive index than the plastic, and there is a relatively high degree of flexibility in the aspheric shape such as eccentricity ratio of wall thickness. Therefore, it is possible to reduce the number of lenses and secure a high optical performance. 
         [0036]    Moreover, in the wide angle zoom lens of the present invention, at least one of the surfaces of the meniscus lens whose convex surface faces the image surface of the third lens group preferably has an aspheric shape whose refractive power gradually decreases away from an optical axis. 
         [0037]    Thanks to the above configuration, it becomes easier to correct field curvature and barrel-shaped distortion, which are likely to occur when the retro-focus type optical system is downsized at the wide angle end. Therefore, the system can be small in size and secure a high optical performance. 
         [0038]    Moreover, in the wide angle zoom lens of the present invention, the following conditional expression (5) is preferably satisfied: 
         [0000]      20≦|( L 31 f+L 31 r )/( L 31 f−L 31 r )|≦50  (5) 
         [0039]    where L31f is the curvature radius of the object-side lens surface of the meniscus lens; and 
         [0040]    L31r is the curvature radius of the image-side lens surface of the meniscus lens. 
         [0041]    The conditional expression (5) is an expression governing the shape of the meniscus lens that constitutes the third lens group. If the number exceeds the upper limit of the conditional expression (5), field curvature occurs on a large scale at the plus side, and it becomes difficult to make correction. If the number goes below the lower limit of the conditional expression (5), field curvature occurs on a large scale at the minus side, and it becomes difficult to make correction. 
         [0042]    Moreover, in the wide angle zoom lens of the present invention, the following conditional expression (6) is preferably satisfied: 
         [0000]      0.8 ≦D   t2-3   /ft≦ 1.2  (6) 
         [0043]    where D t2-3  is an air space on an optical axis at the telephoto end between the lens surface that is closest to the image in the second lens group and the lens surface that is closest to the object in the third lens group; and 
         [0044]    ft is a focal length of the entire system at the telephoto end of the wide angle zoom lens. 
         [0045]    The conditional expression (6) is to govern the ratio of the air space on the optical axis at the telephoto end between the lens surface that is closest to the image in the second lens group and the lens surface that is closest to the object in the third lens group to the focal length of the entire system at the telephoto end of the zoom lens. If the number exceeds the upper limit of the conditional expression (6), the air space between the second lens group and the third lens group becomes longer with respect to the focal length of the entire system at the telephoto end. Therefore, the overall length of the lens becomes longer at the telephoto end when a picture is taken. The longer length is a disadvantage in downsizing. If the number goes below the lower limit of the conditional expression (6), the air space between the second lens group and the third lens group becomes shorter with respect to the focal length of the entire system at the telephoto end. Therefore, the overall length of the lens becomes shorter at the telephoto end when a picture is taken. The shorter length is an advantage in downsizing. However, the distance that the second lens group travels when the variable magnification is carried out from the wide angle end to the telephoto end decreases. The positive refractive power of the second lens group becomes too strong in order to secure the same ratio of the variable magnification, and it becomes difficult to correct various types of aberration. 
         [0046]    Moreover, in the wide angle zoom lens of the present invention, the following conditional expression (7) is preferably satisfied: 
         [0000]      1.0≦( SD 1+ SD 2+ SD 3)/ fw≦ 2.8  (7) 
         [0047]    where SD1 is the distance between the surface that is closest to the object and the surface that is closest to the image among the first lens group on an optical axis; 
         [0048]    SD2 is the distance between the surface that is closest to the object and the surface that is closest to the image among the second lens group on an optical axis; and 
         [0049]    SD3 is the distance between the surface that is closest to the object and the surface that is closest to the image among the third lens group on an optical axis. 
         [0050]    The conditional expression (7) is the one that has standardized the sum of distances on an optical axis of the first to third lens groups by the focal length at the wide angle end. If the number exceeds the upper limit of the conditional expression (7), the first to third lens groups increase in thickness on the optical axis. Exceeding the upper limit is not desirable in terms of portability because in a non-picture-taking state, when the overall length of the zoom lens becomes shorter (so-called retracted state) after every lens group moves toward the image from where the lens groups are positioned in a picture-taking state between the wide angle end and the telephoto end, the overall length of the zoom lens entire system becomes larger when being retracted. If the number goes below the lower limit of the conditional expression (7), the first to third lens groups decrease in thickness on the optical axis, and it becomes difficult to secure an enough number of lenses to correct aberration in each of the lens groups. 
         [0051]    Moreover, an image pickup device on which the wide angle zoom lens of the present invention is mounted preferably includes a driving means for moving the first, second, and third lens groups that constitute the zoom lens in the direction of the optical axis; and an image pickup element provided near an imaging plane of the optical system. At least one of the first, second, and third groups of the zoom lens preferably can move toward the image pickup element along the optical axis rather than toward the position of the picture-taking state between the wide angle end and the telephoto end, and stops closer to the image pickup element than to the position of the picture-taking state between the wide angle end and the telephoto end in the non-picture-taking state. 
         [0052]    According to the present invention, the adjustment of variable magnification of the zoom lens is performed by a mechanical correction method in which the first and second lens groups move in the direction of the optical axis. Even in the picture-taking state, the overall length of each lens group is kept as short as possible. In the non-picture-taking state, the first and second lens group move closer to the image pickup element than to the positions of the picture-taking state between the wide angle end and the telephoto end with the help of the zooming mechanism, thereby making it possible to further reduce the overall length (so-called retracted state). 
         [0053]    Moreover, in the image pickup device on which the wide angle zoom lens of the present invention is mounted, the following is preferable: A space is provided in advance between the lens surface that is closest to the image in the third lens group and the image pickup element so that in the non-picture-taking state, the first, second, and third lens groups that constitute the zoom lens can move toward the image pickup element along the optical axis rather than toward the position of the picture-taking state between the wide angle end and the telephoto end, and a control area is secured at the side of the image pickup element even when the entire lens frame is retracted. 
         [0054]    If the space at the side of the image pickup element is designed to be as narrow as possible, the overall length of the zoom lens entire system is reduced, the radius of the lens that is closest to the image increases, and the exit pupil comes closer to the image surface, posing a negative impact on shading. 
         [0055]    Therefore, according to the zoom lens of the present invention, in the picture-taking state, the exit pupil is substantially separated from the image surface to secure telecentricity even though the overall length becomes longer to a certain extent. A large space is provided in advance between the lens that is closest to the image and the image pickup element so that in the non-picture-taking state, the lens groups do not interfere with lowpass filters and the like that are positioned in front of the image pickup element even when each lens group is moved toward the image. Therefore, the image pickup device can be small in size when a picture is not taken, and at the same time, the optical performance is ensured when a picture is taken. 
         [0056]    Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
         [0057]    The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0058]      FIG. 1  is a cross-section view of an optical system according to a first embodiment of the present invention; 
           [0059]      FIG. 2  is a cross-section view of an optical system according to a second embodiment of the present invention; 
           [0060]      FIG. 3  is a cross-section view of an optical system according to a third embodiment of the present invention; 
           [0061]      FIGS. 4A to 4C  are diagrams illustrating various types of aberration in an infinite-distance focusing state of the optical system according to the first embodiment; 
           [0062]      FIGS. 5A to 5C  are diagrams illustrating various types of aberration in an infinite-distance focusing state of the optical system according to the second embodiment of the present invention; 
           [0063]      FIGS. 6A to 6C  are diagrams illustrating various types of aberration in an infinite-distance focusing state of the optical system according to the third embodiment of the present invention; 
           [0064]      FIG. 7  is a cross-section view of a lens interchangeable-type camera that uses a wide angle zoom lens of the present invention as an interchangeable lens; 
           [0065]      FIG. 8  is a front-side perspective view showing the exterior of a digital camera according to the present invention; 
           [0066]      FIG. 9  is a rear view of the digital camera illustrated in  FIG. 8 ; 
           [0067]      FIG. 10  is a transverse cross-section view of the digital camera illustrated in  FIG. 8 ; and 
           [0068]      FIG. 11  is a block diagram illustrating the configuration of internal circuits of major sections of the digital camera illustrated in  FIG. 8 . 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0069]    First to third embodiments of the present invention of wide angle zoom lens will be described. 
         [0070]      FIG. 1  is a cross-section view of a wide angle zoom lens according to a first embodiment of the present invention. 
         [0071]    As shown in  FIG. 1 , the wide angle zoom lens of the first embodiment includes, from an object side in the following order, a first lens group G 1  having a negative refractive power, a second lens group G 2  having a positive refractive power, and a third lens group G 3 . 
         [0072]    The first lens group G 1  includes, from the object side in the following order, a negative meniscus lens L 11  having a convex surface facing the object, a negative meniscus lens L 12  having a convex surface facing the object, and a positive meniscus lens L 13  having a convex surface facing the object. 
         [0073]    The second lens group G 2  includes, from the object side in the following order, a piano-convex positive lens L 21  having a convex surface facing the object, an aperture diaphragm S, a cemented lens having a biconvex positive lens L 22  and a biconcave negative lens L 23 , a positive meniscus lens L 24  having a convex surface facing an image, and a biconvex positive lens L 25 . 
         [0074]    The third lens group G 3  has one positive meniscus lens L 3  having a convex surface facing the image. 
         [0075]    The reference mark C represents a cover glass, and the reference mark I represents an image surface. 
         [0076]    Aspheric surfaces are used for the following four surfaces: both surfaces of the negative meniscus lens L 12  at the image side of the first lens group G 1 , and both surfaces of the positive meniscus lens L 3  of the third lens group G 3 . 
         [0077]      FIG. 2  is a cross-section view of a wide angle zoom lens according to a second embodiment of the present invention. 
         [0078]    As shown in  FIG. 2 , the wide angle zoom lens of the second embodiment includes, from an object side in the following order, a first lens group G 1  having a negative refractive power, a second lens group G 2  having a positive refractive power, and a third lens group G 3 . 
         [0079]    The first lens group G 1  includes, from the object side in the following order, a cemented lens including a negative meniscus lens L 11  having a convex surface facing the object and a negative meniscus lens L 12  having a convex surface facing the object, a negative meniscus lens L 13  having a convex surface facing the object, and a positive meniscus lens L 14  having a convex surface facing the object. 
         [0080]    The second lens group G 2  includes, from the object side in the following order, a piano-convex positive lens L 21  having a convex surface facing the object, an aperture diaphragm S, a cemented lens having a biconvex positive lens L 22  and a biconcave negative lens L 23 , a biconvex positive lens L 24 , and a biconvex positive lens L 25 . 
         [0081]    The third lens group G 3  has one positive meniscus lens L 3  having a convex surface facing the image. 
         [0082]    The reference mark C represents a cover glass, and the reference mark I represents an image surface. 
         [0083]    Aspheric surfaces are used for the following three surfaces: the surface that is closest to the image in the cemented lens of the first lens group G 1 , and both surfaces of the positive meniscus lens L 3  of the third lens group G 3 . 
         [0084]      FIG. 3  is a cross-section view of a wide angle zoom lens according to a third embodiment of the present invention. 
         [0085]    As shown in  FIG. 3 , the wide angle zoom lens of the third embodiment includes, from an object side in the following order, a first lens group G 1  having a negative refractive power, a second lens group G 2  having a positive refractive power, and a third lens group G 3 . 
         [0086]    The first lens group G 1  includes, from the object side in the following order, a negative meniscus lens L 11  having a convex surface facing the object and a positive meniscus lens L 12  having a convex surface facing the object. 
         [0087]    The second lens group G 2  includes, from the object side in the following order, a piano-convex positive lens L 21  having a convex surface facing the object, an aperture diaphragm S, a cemented lens having a biconvex positive lens L 22  and a biconcave negative lens L 23 , a biconvex positive lens L 24 , and a biconvex positive lens L 25 . 
         [0088]    The third lens group G 3  has one positive meniscus lens L 3  having a convex surface facing the image. 
         [0089]    The reference mark C represents a cover glass, and the reference mark I represents an image surface. 
         [0090]    Aspheric surfaces are used for the following three surfaces: the surface that is close to the image in the negative meniscus lens L 11  of the first lens group G 1 , and both surfaces of the positive meniscus lens L 3  of the third lens group G 3 . 
         [0091]    Numeric data of the first to third embodiments will be shown below. As to the numeric data of the first to third embodiments, r represents curvature radius of the lens surface; d represents lens thickness and an air space; Nd and νd represent a refractive index and Abbe number of the d-line (λ=587.6 nm); f represents a focal length; Fno represents the F number; ω represents a half angle of view (degree). 
         [0092]    On the specification list related to the description of the embodiments, the surfaces indicated by “aspheric surface” are aspheric surfaces. If the height perpendicular to an optical axis is represented by H, the amount of change in the direction of the optical axis as to the height H when the face top is the origin is represented by X(H), paraxial curvature radius is represented by r, the constant of the cone is represented by K, and the second-order, fourth-order, sixth-order, eighth-order, and tenth-order aspheric surface coefficients are represented by A2, A4, A6, A8, and A10, respectively, the shape of aspheric surface is represented by the following equation (a): 
         [0000]        X ( H )=( H   2   /r )/{1+[1−(1 +K )·( H   2   /r   2 )] 1/2   }+A 4 H   4   +A 6 H   6   +A 8 H   8   +A 10 H   10   (a) 
       Numeric Example 1 
       [0093]      
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
             
           
               
                   
               
               
                 Unit mm 
               
               
                   
               
             
             
               
                 Surface data 
               
             
          
           
               
                   
                 Surface Number 
                 r 
                 d 
                 nd 
                 νd 
               
               
                   
                   
               
               
                   
                  1 
                 622.057 
                 1.950 
                 1.77250 
                 49.60 
               
               
                   
                  2 
                 16.257 
                 1.163 
               
               
                   
                  3 (Aspheric) 
                 24.068 
                 2.000 
                 1.52542 
                 55.78 
               
               
                   
                  4 (Aspheric) 
                 13.129 
                 5.260 
               
               
                   
                  5 
                 20.929 
                 3.122 
                 1.84666 
                 23.78 
               
               
                   
                  6 
                 35.453 
                 variable 
               
               
                   
                  7 
                 21.231 
                 2.719 
                 1.83481 
                 42.71 
               
               
                   
                  8 
                 ∞ 
                 3.245 
               
               
                   
                  9 (Stop) 
                 ∞ 
                 0.900 
               
               
                   
                 10 
                 19.264 
                 3.390 
                 1.49700 
                 81.54 
               
               
                   
                 11 
                 −19.264 
                 1.615 
                 1.90366 
                 31.32 
               
               
                   
                 12 
                 19.264 
                 3.564 
               
               
                   
                 13 
                 −357.683 
                 2.113 
                 1.70154 
                 41.24 
               
               
                   
                 14 
                 −29.309 
                 1.672 
               
               
                   
                 15 
                 88.348 
                 2.652 
                 1.51823 
                 58.90 
               
               
                   
                 16 
                 −41.868 
                 variable 
               
               
                   
                 17 (Aspheric) 
                 −25.613 
                 2.200 
                 1.52542 
                 55.78 
               
               
                   
                 18 (Aspheric) 
                 −23.984 
                 11.300  
               
               
                   
                 19 
                 ∞ 
                 4.082 
                 1.51633 
                 64.14 
               
               
                   
                 20 
                 ∞ 
                 0.745 
               
               
                   
                 Image Surface 
                   
                 ∞ 
               
               
                   
                   
               
             
          
           
               
                 Aspheric data 
               
               
                   
               
               
                 3rd surface 
               
               
                 K = 2.0761, A4 = 9.4881E−05, A6 = −1.0061E−06, A8 = 5.5441E−09, 
               
               
                 A10 = −1.5976E−11 
               
               
                 4th surface 
               
               
                 K = 0.0000, A4 = 6.6885E−05, A6 = −1.3641E−06, A8 = 7.5439E−09, 
               
               
                 A10 = −3.0175E−11 
               
               
                 17th surface 
               
               
                 K = −3.3181, A4 = 5.1432E−05, A6 = −6.3896E−08, A8 = −1.7711E−10 
               
               
                 18th surface 
               
               
                 K = −1.9200, A4 = 8.7907E−05, A6 = −8.3092E−08, 
               
               
                 A8 = −1.11376E−10, A10 = 2.4384E−13 
               
               
                   
               
             
          
           
               
                 Zooming data 
               
             
          
           
               
                   
                   
                 wide angle 
                 intermediate 
                 telescopic 
               
               
                   
                   
               
               
                   
                 focal length 
                 14.29 
                 24.24 
                 41.19 
               
               
                   
                 F number 
                 3.63 
                 4.48 
                 5.76 
               
               
                   
                 half angle of view(°) 
                 41.6 
                 25.1 
                 14.9 
               
               
                   
                 image height 
                 11.45 
                 11.45 
                 11.45 
               
               
                   
                 d6 
                 30.536 
                 12.468 
                 1.810 
               
               
                   
                 d16 
                 12.365 
                 24.006 
                 43.987 
               
               
                   
                 BF 
                 16.13 
                 16.13 
                 16.13 
               
               
                   
                 lens total length 
                 96.59 
                 90.17 
                 99.49 
               
               
                   
                   
               
             
          
           
               
                 Zoom lens group data 
               
             
          
           
               
                 group 
                 starting surface 
                 focal length 
               
               
                   
               
               
                 1 
                 1 
                 −23.16 
               
               
                 2 
                 7 
                 27.14 
               
               
                 3 
                 17 
                 489.93 
               
               
                   
               
             
          
         
       
     
       Numeric Example 2 
       [0094]      
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
             
           
               
                   
               
               
                 Unit mm 
               
               
                   
               
             
             
               
                 Surface data 
               
             
          
           
               
                   
                 Surface Number 
                 r 
                 d 
                 nd 
                 νd 
               
               
                   
                   
               
               
                   
                  1 
                 146.182 
                 1.950 
                 1.77250 
                 49.60 
               
               
                   
                  2 
                 14.997 
                 0.200 
                 1.51940 
                 51.94 
               
               
                   
                  3 (Aspheric) 
                 12.346 
                 6.068 
               
               
                   
                  4 
                 63.423 
                 1.500 
                 1.69680 
                 55.53 
               
               
                   
                  5 
                 36.384 
                 1.051 
               
               
                   
                  6 
                 22.408 
                 3.147 
                 1.84666 
                 23.78 
               
               
                   
                  7 
                 40.445 
                 variable 
               
               
                   
                  8 
                 22.598 
                 2.635 
                 1.83481 
                 42.71 
               
               
                   
                  9 
                 ∞ 
                 3.257 
               
               
                   
                 10 (Stop) 
                 ∞ 
                 0.900 
               
               
                   
                 11 
                 18.271 
                 4.300 
                 1.49700 
                 81.54 
               
               
                   
                 12 
                 −20.672 
                 1.147 
                 1.90366 
                 31.32 
               
               
                   
                 13 
                 18.683 
                 4.903 
               
               
                   
                 14 
                 253.605 
                 1.966 
                 1.69680 
                 55.53 
               
               
                   
                 15 
                 −32.861 
                 0.199 
               
               
                   
                 16 
                 150.067 
                 2.122 
                 1.58144 
                 40.75 
               
               
                   
                 17 
                 −51.731 
                 variable 
               
               
                   
                 18 (Aspheric) 
                 −28.645 
                 2.200 
                 1.52542 
                 55.78 
               
               
                   
                 19 (Aspheric) 
                 −26.458 
                 11.300  
               
               
                   
                 20 
                 ∞ 
                 4.082 
                 1.51633 
                 64.14 
               
               
                   
                 21 
                 ∞ 
                 0.745 
               
               
                   
                 Image Surface 
                   
                 ∞ 
               
               
                   
                   
               
             
          
           
               
                 Aspheric data 
               
               
                   
               
               
                 3rd surface 
               
               
                 K = −0.4661, A4 = −1.1971E−05, A6 = −8.1617E−08, A8 = 2.2803E−10, 
               
               
                 A10 = −1.8905E−12 
               
               
                 18th surface 
               
               
                 K = −6.7655, A4 = 6.2372E−05, A6 = 2.9966E−07, A8 = −1.9333E−09 
               
               
                 19th surface 
               
               
                 K = −4.9999, A4 = 8.7669E−05, A6 = 3.2082E−07, A8 = −1.5923E−09, 
               
               
                 A10 = −6.2892E−13 
               
               
                   
               
             
          
           
               
                 Zooming data 
               
             
          
           
               
                   
                   
                 wide angle 
                 intermediate 
                 telescopic 
               
               
                   
                   
               
               
                   
                 focal length 
                 14.24 
                 24.42 
                 41.25 
               
               
                   
                 F number 
                 3.64 
                 4.49 
                 5.77 
               
               
                   
                 half angle of view(°) 
                 41.6 
                 25.0 
                 14.9 
               
               
                   
                 image height 
                 11.45 
                 11.45 
                 11.45 
               
               
                   
                 d7 
                 32.663 
                 13.035 
                 1.832 
               
               
                   
                 d17 
                 12.807 
                 24.506 
                 43.973 
               
               
                   
                 BF 
                 16.13 
                 16.13 
                 16.13 
               
               
                   
                 lens total length 
                 99.14 
                 91.21 
                 99.48 
               
               
                   
                   
               
             
          
           
               
                 Zoom lens group data 
               
             
          
           
               
                 group 
                 starting surface 
                 focal length 
               
               
                   
               
               
                 1 
                 1 
                 −24.24 
               
               
                 2 
                 8 
                 27.78 
               
               
                 3 
                 18 
                 489.93 
               
               
                   
               
             
          
         
       
     
       Numeric Example 3 
       [0095]      
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
             
           
               
                   
               
               
                 Unit mm 
               
               
                   
               
             
             
               
                 Surface data 
               
             
          
           
               
                   
                 Surface Number 
                 r 
                 d 
                 nd 
                 νd 
               
               
                   
                   
               
               
                   
                  1 
                 270.688 
                 1.951 
                 1.77250 
                 49.60 
               
               
                   
                  2 (Aspheric) 
                 11.214 
                 4.795 
               
               
                   
                  3 
                 18.737 
                 3.672 
                 1.84666 
                 23.78 
               
               
                   
                  4 
                 31.007 
                 variable 
               
               
                   
                  5 
                 21.110 
                 2.618 
                 1.83481 
                 42.71 
               
               
                   
                  6 
                 ∞ 
                 3.246 
               
               
                   
                  7 (Stop) 
                 ∞ 
                 0.900 
               
               
                   
                  8 
                 17.073 
                 3.840 
                 1.49700 
                 81.54 
               
               
                   
                  9 
                 −19.664 
                 0.907 
                 1.90366 
                 31.32 
               
               
                   
                 10 
                 17.527 
                 4.416 
               
               
                   
                 11 
                 120.767 
                 2.100 
                 1.69680 
                 55.53 
               
               
                   
                 12 
                 −29.574 
                 2.145 
               
               
                   
                 13 
                 101.163 
                 2.059 
                 1.62004 
                 36.26 
               
               
                   
                 14 
                 −79.934 
                 variable 
               
               
                   
                 15 (Aspheric) 
                 −23.233 
                 2.200 
                 1.58313 
                 59.38 
               
               
                   
                 16 (Aspheric) 
                 −22.235 
                 11.300  
               
               
                   
                 17 
                 ∞ 
                 4.082 
                 1.51633 
                 64.14 
               
               
                   
                 18 
                 ∞ 
                 0.745 
               
               
                   
                 Image Surface 
                   
                 ∞ 
               
               
                   
                   
               
             
          
           
               
                 Aspheric data 
               
               
                   
               
               
                 2nd surface 
               
               
                 K = −0.4735, A4 = −8.1098E−06, A6 = −7.8008E−08, A8 = 3.0281E−10, 
               
               
                 A10 = −3.2616E−12 
               
               
                 15th surface 
               
               
                 K = −0.5651, A4 = 3.9089E−05, A6 = 6.4383E−07, A8 = −1.9877E−09 
               
               
                 16th surface 
               
               
                 K = 0.0398, A4 = 6.9134E−05, A6 = 5.3315E−07, A8 = −6.7037E−10, 
               
               
                 A10 = −1.1823E−12 
               
               
                   
               
             
          
           
               
                 Zooming data 
               
             
          
           
               
                   
                   
                 wide angle 
                 intermediate 
                 telescopic 
               
               
                   
                   
               
               
                   
                 focal length 
                 14.28 
                 24.25 
                 41.22 
               
               
                   
                 F number 
                 3.64 
                 4.49 
                 5.77 
               
               
                   
                 half angle of view(°) 
                 41.6 
                 25.2 
                 14.9 
               
               
                   
                 image height 
                 11.45 
                 11.45 
                 11.45 
               
               
                   
                 d4 
                 29.769 
                 12.196 
                 1.834 
               
               
                   
                 d14 
                 9.781 
                 20.766 
                 39.603 
               
               
                   
                 BF 
                 16.13 
                 16.13 
                 16.13 
               
               
                   
                 lens total length 
                 90.52 
                 83.94 
                 92.41 
               
               
                   
                   
               
             
          
           
               
                 Zoom lens group data 
               
             
          
           
               
                 group 
                 starting surface 
                 focal length 
               
               
                   
               
               
                 1 
                 1 
                 −23.42 
               
               
                 2 
                 5 
                 25.95 
               
               
                 3 
                 15 
                 489.88 
               
               
                   
               
             
          
         
       
     
         [0096]      FIGS. 4 to 6  are diagrams illustrating various types of aberration in an infinite-distance focusing state of the wide angle end (W_INF), intermediate state (S_INF), and telephoto end (T_INF) of the optical systems of the first to third embodiments. As for spherical aberration and chromatic aberration of magnification, numbers are shown at each of the following wavelengths: 587.6 nm (d-line: Solid lines), 435.8 nm (g-line: Dotted lines), 486.1 nm 
         [0097]    (F-line: Alternate one long and one short dash lines), and 656.3 nm (C-line: Alternate one long and two short dash lines). As for astigmatism, the solid lines represent the sagittal image surfaces, and the dotted lines represent the meridional image surfaces. Incidentally, SA represents spherical aberration; AS represents astigmatism; DT represents distortion; CC represents chromatic aberration of magnification; FNO represents the F-number; FIY represents the height of the image. 
         [0098]    The following shows the numbers of the conditional expressions (1) to (7) according to each of the above-described embodiments. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Conditional 
                 1st 
                 2nd 
                 3rd 
               
               
                   
                 Exp. 
                 Embodiment 
                 Embodiment 
                 Embodiment 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 (1) 
                 1.622 
                 1.702 
                 1.640 
               
               
                   
                 (2) 
                 1.900 
                 1.950 
                 1.817 
               
               
                   
                 (3) 
                 1.793 
                 1.825 
                 1.678 
               
               
                   
                 (4) 
                 0.017 
                 — 
                 — 
               
               
                   
                 (5) 
                 30.455 
                 25.203 
                 45.555 
               
               
                   
                 (6) 
                 1.068 
                 1.066 
                 0.961 
               
               
                   
                 (7) 
                 2.630 
                 2.636 
                 2.440 
               
               
                   
                   
               
             
          
         
       
     
         [0099]      FIG. 7  is a cross-section view of a lens interchangeable-type camera that uses the wide angle zoom lens of the present invention and the small CCD, CMOS, or the like as image pickup element: The lens interchangeable-type camera serves as an electronic image pickup device.  FIG. 7  illustrates the lens interchangeable-type camera  1 , a picture-taking lens system  2  disposed in a lens barrel, a mounting section  3  of the lens barrel that allows the picture-taking lens system  2  to be mounted on or dismounted from the lens interchangeable-type camera  1 , and uses a the screw-type or bayonet-type mount or other types of mount. In the example here, the bayonet-type mount is used.  FIG. 7  also denotes an image pickup element plane  4 , a back monitor  5 , a finder image display element  6 , and a finder optical system  7 . 
         [0100]    As the picture-taking lens system  2  of the lens interchangeable-type camera  1  having the above configuration, the wide angle zoom lens of the present invention described in the above first to third embodiments may be used. 
         [0101]    According to the present invention described above, provided is the compact wide angle optical system that corrects, as an interchangeable lens suitable for lens interchangeable-type digital cameras, distortion to some extent; appropriately corrects various types of aberration, particularly chromatic aberration and field curvature; secures telecentricity; and has a fewer number of lenses. 
         [0102]      FIGS. 8 to 11  are schematic diagrams illustrating the configuration of an image pickup device in which the wide angle zoom lens is put in a picture-taking optical system  41 , according to the present invention.  FIG. 8  is a front-side perspective view showing the exterior of a digital camera  40 .  FIG. 9  is a rear view of the same.  FIG. 10  is a schematic transverse cross-section view illustrating the configuration of the digital camera  40 . Incidentally,  FIGS. 8 and 10  illustrate a picture-taking state of the picture-taking optical system  41  (non-retracted state).  FIG. 10  illustrates a non-picture-taking state of the picture-taking optical system  41  (retracted state). 
         [0103]    In the example here, the digital camera  40  includes such components as the picture-taking optical system  41  disposed on a picture-taking optical path  42 , a finder optical system  43  disposed on a finder optical path  44 , a shutter button  45 , a pop-up strobe  46 , and a liquid crystal display monitor  47 . When the shutter button  45  disposed on the upper side of the camera  40  is pressed, a picture is accordingly taken through the picture-taking optical system  41  including for example the lens of the first embodiment. The object image formed by the picture-taking optical system  41  is formed via a cover glass C on an image pickup plane (photoelectric conversion plane) of CCD  49  which is disposed near an imaging plane as image pickup element. The object image whose light is received by the CCD  49  is displayed as an electronic image through a processing means  51  on the liquid crystal display monitor  47  provided on the back of the camera or on a finder image display element  54 . A recording means  52  may be connected to the processing means  51  to record the electronic image that the camera has taken. 
         [0104]    When the non-picture-taking state comes after the image was taken, the picture-taking optical system  41  as a whole comes closer to the CCD  49  than to an infinite-distance focusing position and stops. Incidentally, in the non-picture-taking state, a space is provided in advance between the CCD  49  and the picture-taking optical system  41 &#39;s lens that is closest to the image, so that the picture-taking optical system  41  as a whole can move along an optical axis to the CCD  49  rather than toward the infinite-distance focusing position. Even when the entire lens frame is retracted, the control area is secured at the side of the CCD  49 . 
         [0105]    Incidentally, the recording means  52  may be provided separately from the processing means  51  or may be formed by a flexible disc, a memory card, MO, or the like to electronically record and write. The camera may be formed as a silver salt camera in which the CCD  49  is replaced by a silver salt film. 
         [0106]    Moreover, a finder eye lens  59  is disposed on the finder optical path  44 . The object image displayed on the finder image display element  54  is enlarged and adjusted in visibility by the finder eye lens  59  so that a viewer can more easily watch. The object image is then led to a viewer&#39;s eyeball E. Incidentally, a cover member  50  is provided at the emission side of the finder eye lens  59 . 
         [0107]      FIG. 11  is a block diagram illustrating the configuration of internal circuits of major sections of the above digital camera  40 . In the description below, the above processing means  51  includes, for example, a CDS/ADC section  24 , a temporary storage memory  17 , an image processing section  18 , and the like. The recording means  52  includes, for example, a storage medium section  19  and the like. 
         [0108]    As shown in  FIG. 11 , the digital camera  40  is equipped with an operation section  12 ; a control section  13 , which is connected to the operation section  12 ; an image pickup driving circuit  16  and the temporary storage memory  17 , which are connected to the control signal output ports of the control section  13  via buses  14  and  15 ; the image processing section  18 ; the storage medium section  19 ; a display section  20 ; and a setting information storage memory section  21 . 
         [0109]    The above temporary storage memory  17 , image processing section  18 , storage medium section  19 , display section  20 , and setting information storage memory section  21  are so formed as to be able to input or output data to each other via a bus  22 . The CCD  49  and the CDS/ADC section  24  are connected to the image pickup driving circuit  16 . 
         [0110]    The operation section  12  is equipped with various input buttons and switches and is a circuit that informs the control section about event information input from the outside (a camera user) through the input buttons and switches. The control section  13  is for example a central processing unit such as CPU, including a program memory (not shown). The control section  13  is a circuit that follows programs stored in the program memory to take overall control of the digital camera  40  in response to an instruction command input through the operation section  12  by the camera user. 
         [0111]    The CCD  49  receives light of the object image formed through the picture-taking optical system  41  of the present invention. The CCD  49  is driven and controlled by the image pickup driving circuit  16 . The CCD  49  is an image pickup element that converts the amount of light of the object image into electric signals on a per-pixel basis and outputs the electric signals to the CDS/ADC section  24 . 
         [0112]    The CDS/ADC section  24  is a circuit that amplifies the electric signals input from the CCD  49 , performs analog-to-digital conversion, and outputs to the temporary memory  17  the video raw data (Bayer data; referred to as RAW data hereinafter) on which only amplification and digital conversion have been performed. 
         [0113]    The temporary storage memory  17  is for example a buffer including SDRAM and the like and is a memory device that temporarily stores the RAW data output from the CDS/ADC section  24 . The image processing section  18  is a circuit that reads out the RAW data stored in the temporary storage memory  17  or the RAW data stored in the storage medium section  19  and performs a variety of electrical image processes, including distortion aberration correction, on the basis of an image-quality parameter specified by the control section  13 . 
         [0114]    The storage medium section  19  is a control circuit of a device on or from which a card- or stick-type recording medium including a flash memory and the like can be for example mounted and dismounted. The storage medium section  19  records and retains in the card- or stick-type flash memory the RAW data transferred from the temporary storage memory  17  and the image data obtained by image processing of the image processing section  18 . 
         [0115]    The display section  20  is equipped with the liquid crystal display monitor  47  and the finder image display element  54  and is a circuit that displays images, operation menus, and the like on the liquid crystal display monitor  47  and the finder image display element  54 . The setting information storage memory section  21  is equipped with a ROM section, in which various image-quality parameters are stored in advance, and a RAM section, which stores an image-quality parameter selected by the input operation of the operation section  12  from among the image-quality parameters read out from the ROM section. The setting information storage memory section  21  is a circuit that controls input to and output from the memories. 
         [0116]    With the digital camera  40  having the above configuration, the present invention provides an image pickup device that can correct distortion to some extent, appropriately correct various kinds of aberration, particularly chromatic aberration and field curvature, and secure telecentricity while using a compact wide angle optical system that has a fewer number of lenses. 
         [0117]    The present invention is applied not only to so-called compact digital cameras that take pictures of typical objects as described above but to surveillance cameras or the like that require wide angles of view.