Zoom lens and imaging device

Abstract

To provide a compact, wide-angle zoom lens which minimizes aberration variation while zooming.SOLUTION: An zoom lens provided herein comprises a first lens group having negative refractive power and a rear group including multiple lens groups and having positive refractive power as a whole, arranged in order from the object side to the image side, and is configured such that distances between adjacent lens groups change while zooming. The first lens group includes an aspherical lens and is stationary while zooming. The zoom lens satisfies conditions expressed as: 0.10≤skw / TD1≤0.50 and -0.17≤f1 / TDw≤-0.05, where TD1 represents an optical axial distance from a lens surface of the first lens group on the most object side to a lens surface of the first lens group on the most image side, skw represents an optical axial distance from a lens surface of the zoom lens on the most image side at the wide-angle end to the image plane, TDw represents an optical axial distance from a lens surface on the most object side at the wide-angle end to the image plane, and f1 represents a focal length of the first lens group.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a zoom lens suitable for use in imaging devices such as video cameras, digital still cameras, and surveillance cameras. [Background technology]

[0002] As a wide-angle zoom lens, Patent Documents 1 and 2 disclose zoom lenses that change magnification by moving the first lens group closest to the object. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5777592 [Patent Document 2] Japanese Patent Application Publication No. 2018-189733 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the zoom lenses disclosed in Patent Documents 1 and 2, the long back focal length results in a large diameter first lens group, and the weight of the first lens group that moves when changing magnification increases. Moreover, wide-angle lenses tend to use multiple large-diameter lenses to achieve a wide angle and correct distortion, which increases the weight of the first lens group. As a result, it becomes difficult to suppress an increase in aberrations due to decentering when the first lens group moves.

[0005] The present invention provides a compact zoom lens that has a wide angle and is capable of suppressing aberration fluctuations during zooming. [Means for solving the problem]

[0006] The zoom lens according to one aspect of the present invention comprises a first lens group having negative refractive power, arranged in order from the object side to the image side. and , four Lens group It consists ofRear group with overall positive refractive power and The first lens group is an aspherical lens, and the spacing between adjacent lens groups changes during magnification. A It includes a lens that is immovable when zooming. The rear group is composed of, arranged in order from the object side to the image side, a second lens group with positive refractive power, a third lens group with positive refractive power, a fourth lens group with negative refractive power, and a fifth lens group with positive refractive power. The distance on the optical axis from the lens surface closest to the object in the first lens group to the lens surface closest to the image in the first lens group is TD1, the distance on the optical axis from the lens surface closest to the image in the zoom lens at the wide-angle end to the image plane is skw, the distance on the optical axis from the lens surface closest to the object in the wide-angle end to the image plane is TDw, and the focal length of the first lens group is f1. The focal length of the aspherical lens closest to the object among the aspherical lenses included in the zoom lens is fa1. When 0.10≦skw / TD1≦0.50 -0.17≦f1 / TDw≦-0.05 0.01≦|f1 / fa1|≦0.278 The present invention is characterized in that the following conditions are satisfied: An imaging device including the zoom lens described above also constitutes another aspect of the present invention. [Effects of the Invention]

[0007] According to the present invention, a compact zoom lens is provided that has a wide angle and is capable of suppressing aberration fluctuations during zooming. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a cross-sectional view of a zoom lens according to a first embodiment. [Figure 2] 4A and 4B are longitudinal aberration diagrams at the wide-angle end of the zoom lens of Example 1. [Figure 3] 4A and 4B are longitudinal aberration diagrams of the zoom lens of Example 1 at an intermediate zoom position. [Figure 4] 4A and 4B are longitudinal aberration diagrams at the telephoto end of the zoom lens of Example 1. [Figure 5] FIG. 10 is a cross-sectional view of a zoom lens according to a second embodiment. [Figure 6] 10A and 10B are longitudinal aberration diagrams at the wide-angle end of the zoom lens of Example 2. [Figure 7] 10A and 10B are longitudinal aberration diagrams of the zoom lens of Example 2 at an intermediate zoom position. [Figure 8] 10A and 10B are longitudinal aberration diagrams at the telephoto end of the zoom lens of Example 2. [Figure 9] FIG. 10 is a cross-sectional view of a zoom lens according to a third embodiment. [Figure 10] 10A and 10B are longitudinal aberration diagrams at the wide-angle end of the zoom lens of Example 3. [Figure 11] 10A and 10B are longitudinal aberration diagrams of the zoom lens of Example 3 at an intermediate zoom position. [Figure 12] 10A and 10B are longitudinal aberration diagrams at the telephoto end of the zoom lens of Example 3. [Figure 13] FIG. 10 is a cross-sectional view of a zoom lens according to a fourth embodiment. [Figure 14] 10A and 10B are longitudinal aberration diagrams at the wide-angle end of the zoom lens of Example 4. [Figure 15] 10A and 10B are longitudinal aberration diagrams of the zoom lens of Example 4 at an intermediate zoom position. [Figure 16] 10A and 10B are longitudinal aberration diagrams at the telephoto end of the zoom lens of Example 4. [Figure 17] FIG. 1 is a schematic diagram of an imaging device equipped with a zoom lens according to any one of Examples 1 to 4. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Figures 1, 5, 9, and 13 show cross sections at the wide-angle end of zoom lenses according to embodiments 1, 2, 3, and 4, respectively. The zoom lenses according to each embodiment are used as imaging lenses in imaging devices such as digital still cameras, broadcast cameras, silver halide film cameras, and surveillance cameras.

[0010] In each cross-sectional view, the left side is the object side (magnification conjugate side) and the right side is the image side (reduction conjugate side). In the following description, a group of one or more lenses whose spacing between adjacent lens surfaces changes during zooming or focusing is defined as a lens group, and the lenses are designated in the figures with the symbols Bi (i = 1, 2, 3, ...) in order from the object side. Furthermore, the wide-angle end and telephoto end refer to the zoom states (zoom positions) when the lens group is located at either end of the range of mechanical movement of the lens group in the optical axis direction during zooming. In each cross-sectional view, arrows indicate the movement locus of the lens group during zooming from the wide-angle end to the telephoto end, and also indicate the movement direction of the lens group during focusing from an object at infinity to a close-up object.

[0011] The zoom lens of each embodiment is composed of, arranged in order from the object side to the image side, a first lens group B1 with negative refractive power and a rear group including multiple lens groups (B2, etc.) with overall positive refractive power, and magnification is varied by changing the spacing between adjacent lens groups. The first lens group B1 includes an aspherical lens and does not move during magnification variation. An aperture stop SP is disposed between the first lens group B1 and the second lens group B2. Note that the rear group only needs to have overall positive refractive power; for example, in each embodiment, the second lens group B2 has positive refractive power, but it may also have negative refractive power. Furthermore, the aspherical lens only needs to have at least one of the object-side and image-side lens surfaces with an aspherical shape.

[0012] The aperture stop SP determines (limits) the light flux at the maximum F-number (Fno). IP is the image plane (reduced conjugate plane). The image plane IP is where the imaging surface of a solid-state image sensor (photoelectric conversion element) such as a CCD sensor or CMOS sensor or the photosensitive surface of silver halide film is located. [Example]

[0013] 1 is composed of, arranged in order from the object side to the image side, a first lens unit B1 with negative refractive power, an aperture stop SP, a second lens unit B2 with positive refractive power, a third lens unit B3 with positive refractive power, and a fourth lens unit B4 with positive refractive power. During zooming from the wide-angle end to the telephoto end, the first lens unit B1 remains stationary, while the aperture stop SP and the second and third lens units B2 and B3 move toward the object side, and the fourth lens unit B4 moves toward the image side and then moves toward the object side.

[0014] A specific numerical example of Example 1 is shown below as Numerical Example 1. The surface number (m) indicates the order of the optical surface counted from the object side, and the radius of curvature d (mm) indicates the spacing (distance) on the optical axis between the mth surface and the (m+1)th surface. nd and vd respectively indicate the refractive index at the d-line of the optical material between the mth surface and the (m+1)th surface and the Abbe number based on the d-line. The Abbe number vd is given by: where Nd, NF, and NC are the refractive indices at the d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm) of the Fraunhofer lines, respectively. νd=(Nd-1) / (NF-NC) In the numerical examples, the radius of curvature d, focal length (mm), F-number, and half angle of view (°) are all values ​​when the zoom lens is focused on an object at infinity.

[0015] BF represents back focus (mm). Back focus is the distance on the optical axis from the final lens surface (the lens surface closest to the image) of a zoom lens to the paraxial image plane, expressed as an air-equivalent length. The total lens length is the distance on the optical axis from the front lens surface (the lens surface closest to the object) of a zoom lens to the final lens surface, plus the back focus.

[0016] If the lens surface is aspherical, an * symbol is added to the right of the surface number. The aspherical shape is expressed as follows: x is the displacement from the vertex of the surface in the optical axis direction, h is the height from the optical axis in a direction perpendicular to the optical axis, R is the paraxial radius of curvature, k is the conic constant, and A4, A6, A8, A10, and A12 are the aspherical coefficients of each order. x=(h 2 / R) / [1+{1-(1+k)(h / R)2} 1 / 2 +A4×h 4 +A6×h 6 +A8×h 8 +A10×h 10 +A12×h 12 +A14×h 14 In addition, "e±XX" in the conic constant and aspherical coefficient is expressed as "×10 ±XX The above explanation regarding Numerical Example 1 also applies to other numerical examples described later.

[0017] 2, 3, and 4 respectively show longitudinal aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration) of the zoom lens of Example 1 (Numerical Example 1) at the wide-angle end, the intermediate zoom position, and the telephoto end when focused on an object at infinity. In the spherical aberration diagrams, Fno is the F-number, the solid line indicates spherical aberration at the d-line (wavelength 587.56 nm), and the two-dot chain line indicates spherical aberration at the g-line (wavelength 435.8 nm). In the astigmatism diagrams, S indicates astigmatism at the sagittal image plane, and M indicates astigmatism at the meridional image plane. The distortion diagrams show distortion at the d-line. The chromatic aberration diagrams show chromatic aberration of magnification at the g-line. ω is the half angle of view (°) calculated by paraxial calculation. The explanations regarding these longitudinal aberration diagrams are the same for the other numerical examples. [Numerical Example 1] Unit: mm Surface number rd nd νd 1* 32.281 3.200 1.58313 59.4 2* 10.895 11.867 3 216.443 1.500 1.72916 54.7 4 16.149 5.587 5* 28.079 2.200 1.58313 59.4 6 27.754 4.647 7 -48.42 1.300 1.43875 94.7 8 22.216 0.149 9 22.682 5.319 1.8061 40.9 10 -260.356 (variable) 11 (Aperture) ∞ (Variable) 12 13.959 1.200 1.98612 16.5 13 10.571 8.714 1.51742 52.4 14 -15.912 0.150 15 -16.913 1.000 2.001 29.1 16 13.222 5.105 1.92286 18.9 17 -44.663 (variable) 18 21.182 1.200 2.0509 26.9 19 13.872 9.195 1.497 81.5 20 -81.685 0.150 21 27.775 1.200 1.874 35.3 22 17.112 13.905 1.497 81.5 23 -18.318 0.023 24* -20.347 1.750 1.854 40.4 25* 1715.285 (variable) 26 58.567 5.895 1.497 81.5 27 -118.782 (variable) Image plane ∞ K A4 A6 S8 1st page 9.646440E-03 -2.459300E-05 2.684610E-08 -7.279870E-12 2nd side -9.726320E-01 -1.742970E-05 -1.186640E-07 -3.100590E-10 5th side 2.587180E+00 -3.131390E-05 -1.693380E-07 3.708740E-10 Page 24 0.000000E+00 1.174230E-04 -1.713920E-06 1.443100E-08 Page 25 0.000000E+00 1.349530E-04 -1.595560E-06 1.344000E-08 A10 A12 A14 1st side -2.763590E-14 3.565180E-17 -1.635890E-20 2nd side 3.389220E-12 -8.158310E-15 6.484920E-18 5th side 1.008230E-12 -1.666670E-14 0.000000E+00 Page 24 -6.625540E-11 1.356740E-13 0.000000E+00 Page 25 -6.864650E-11 1.928800E-13 -2.218970E-16 Wide-angle Mid-range Telephoto d10 22.688 12.499 2.310 d11 2.192 2.649 3.106 d17 3.198 2.741 2.283 d25 0.732 12.943 13.900 d27 16.865 14.843 24.075 Wide-angle Mid-range Telephoto Focal length 9.28 13.05 17.45 F-number 4.12 4.12 4.12 Half angle of view (°) 64.01 58.13 51.11 Image height 19.04 20.99 21.64 Lens total length 130.93 130.93 130.93 BF 16.86 14.84 24.07 [Example]

[0018] The zoom lens of Example 2 shown in Figure 5 is composed of, arranged in order from the object side to the image side, a first lens unit B1 with negative refractive power, an aperture stop SP, a second lens unit B2 with positive refractive power, a third lens unit B3 with positive refractive power, a fourth lens unit B4 with negative refractive power, and a fifth lens unit B5 with positive refractive power. During zooming from the wide-angle end to the telephoto end, the first lens unit B1 remains stationary, while the aperture stop SP and the second to fourth lens units B2 to B4 move toward the object side, and the fifth lens unit B5 moves toward the image side and then moves toward the object side. In this Example, the zoom ratio is larger than in Example 1, and the focal length at the wide-angle end is longer.

[0019] Below, specific numerical examples of Example 2 are shown as Numerical Example 2. Figures 6, 7, and 8 respectively show longitudinal aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration) of the zoom lens of Example 2 (Numerical Example 2) at the wide-angle end, at the intermediate zoom position, and at the telephoto end when focused on an object at infinity. [Numerical Example 2] Unit: mm Surface number rd nd νd 1 50 3.000 1.854 40.4 2 39.83 10.000 3* 41.15 3.500 1.854 40.4 4* 16.556 15.911 5* -16336.1 2.300 1.854 40.4 6* 35.734 7.432 7 -84.771 1.200 1.497 81.5 8 32.858 5.939 1.85025 30.1 9 -327.525 (variable) 10 (Aperture) ∞ (Variable) 11 24.363 1.000 1.92286 18.9 12 14.242 5.076 1.673 38.1 13 -248.151 1.489 14 -44.713 1.000 2.001 29.1 15 20.881 6.128 1.92286 18.9 16 -66.998 (variable) 17 73.451 2.500 1.72 46 18 -115.01 0.500 2.001 29.1 19 -358.744 0.100 20 26.905 4.729 1.43875 94.7 21 -39.813 (variable) 22 184.892 1.000 1.95375 32.3 23 20.618 0.100 24 18.329 4.735 1.43875 94.7 25 78.232 1.000 26 20.034 5.830 1.497 81.5 27 209.719 1.000 2.00069 25.5 28 45.787 3.248 29* -34.602 1.500 1.854 40.4 30* -62.553 (variable) 31 96.633 6.020 1.6968 55.5 32 -223.073 (variable) Image plane ∞ K A4 A6 S8 Third page 0.000000E+00 -5.474450E-06 2.174360E-09 -2.320770E-13 4th side -7.532190E-01 -7.061300E-06 -2.706320E-08 9.359870E-12 5th side 0.000000E+00 1.151910E-06 0.000000E+00 0.000000E+00 6th side -3.190850E-01 1.536690E-05 4.108790E-08 -6.551010E-11 Page 29 0.000000E+00 -3.572950E-06 3.807150E-07 -1.689380E-09 Page 30 0.000000E+00 3.585550E-05 4.066510E-07 -1.301500E-09 A10 A12 A14 3rd side 0.000000E+00 0.000000E+00 0.000000E+00 4th side 0.000000E+00 0.000000E+00 0.000000E+00 5th side 0.000000E+00 0.000000E+00 0.000000E+00 6th side 3.179170E-13 0.000000E+00 0.000000E+00 Page 29 0.000000E+00 0.000000E+00 0.000000E+00 Page 30 0.000000E+00 0.000000E+00 0.000000E+00 Wide-angle Mid-range Telephoto d 9 29.495 19.486 6.825 d10 1.722 1.000 1.000 d16 4.777 3.859 3.027 d21 0.100 0.294 1.093 d30 0.524 16.433 21.353 d32 22.000 17.546 25.321 Wide-angle Mid-range Telephoto Focal length 11.33 16.1 23.42 F-number 4.12 4.12 4.12 Half angle of view (°) 59.84 52.13 42.73 Image height 19.5 20.7 21.64 Lens length 154.86 154.86 154.86 BF 22 17.55 25.32 [Example]

[0020] The zoom lens of Example 3 shown in Figure 9 is composed of, arranged in order from the object side to the image side, a first lens unit B1 with negative refractive power, an aperture stop SP, a second lens unit B2 with positive refractive power, a third lens unit B3 with positive refractive power, a fourth lens unit B4 with negative refractive power, and a fifth lens unit B5 with positive refractive power. During zooming from the wide-angle end to the telephoto end, the first lens unit B1 remains stationary, while the aperture stop SP and the second to fourth lens units B2 to B4 move toward the object side, and the fifth lens unit B5 moves toward the image side and then moves toward the object side. In this Example, the focal lengths at the wide-angle end and the telephoto end are longer than in Example 1, while maintaining the same zoom ratio.

[0021] Below, specific numerical examples of Example 3 are shown as Numerical Example 3. Figures 10, 11, and 12 respectively show longitudinal aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration) of the zoom lens of Example 3 (Numerical Example 3) at the wide-angle end, at the intermediate zoom position, and at the telephoto end when focused on an object at infinity. [Numerical Example 3] Unit: mm Surface number rd nd νd 1* 34.361 3.500 1.58313 59.4 2* 15.668 12.466 3 64.307 2.000 1.79017 47.9 4 21.301 8.000 5* -378.564 2.300 1.854 40.4 6* 44.343 5.121 7 -121.54 1.200 1.497 81.5 8 17.608 6.174 1.738 32.3 9 147.845 (variable) 10 (Aperture) ∞ (Variable) 11 23.145 1.000 1.92286 18.9 12 15.429 4.630 1.58267 46.4 13 -31.499 0.931 14 -26.608 1.000 2.001 29.1 15 25.087 3.161 1.92286 18.9 16 -60.338 (variable) 17 41.651 7.064 1.53775 74.7 18 -22.08 1.000 2.001 29.1 19 -31.132 0.500 20 23.145 5.013 1.43875 94.7 21 96.359 (variable) 22 40.19 1.000 1.95375 32.3 23 17.301 8.507 1.43875 94.7 24 -9789.11 3.234 25* -46.294 1.500 1.854 40.4 26* -104.92 (variable) 27 58.489 3.591 1.48749 70.2 28 328.174 (variable) Image plane ∞ K A4 A6 S8 1st page 0.000000E+00 -7.519290E-06 3.759650E-09 -8.975440E-13 2nd side -7.372740E-01 -6.074870E-06 -3.042300E-08 2.258740E-12 5th side 0.000000E+00 9.035100E-06 0.000000E+00 0.000000E+00 6th side 9.940200E+00 2.236370E-05 6.653020E-09 5.642050E-10 25th page 0.000000E+00 5.879050E-05 -3.272030E-07 2.915710E-10 Page 26 0.000000E+00 8.543210E-05 -2.663570E-07 4.261750E-10 A10 A12 A14 1st page 0.000000E+00 0.000000E+00 0.000000E+00 2nd side 0.000000E+00 0.000000E+00 0.000000E+00 5th side 0.000000E+00 0.000000E+00 0.000000E+00 6th side -2.290530E-12 0.000000E+00 0.000000E+00 Page 25 0.000000E+00 0.000000E+00 0.000000E+00 Page 26 0.000000E+00 0.000000E+00 0.000000E+00 Wide-angle Mid-range Telephoto d 9 15.923 8.142 1.748 d10 1.000 1.000 1.000 d16 6.162 3.831 1.990 d21 0.100 0.473 1.326 d26 0.524 12.263 5.295 d28 20.241 18.241 32.589 Wide-angle Mid-range Telephoto Focal length 10.3 14.81 20 F-number 4.12 4.12 4.12 Half angle of view (°) 62.16 54.43 47.25 Image height 19.5 20.7 21.64 Lens length 126.84 126.84 126.84 BF 20.24 18.24 32.59 [Example]

[0022] The zoom lens of Example 4 shown in Figure 13 is composed of, arranged in order from the object side to the image side, a first lens group B1 with negative refractive power, an aperture stop SP, a second lens group B2 with positive refractive power, a third lens group B3 with positive refractive power, a fourth lens group B4 with negative refractive power, and a fifth lens group B5 with positive refractive power. During zooming from the wide-angle end to the telephoto end, the first lens group B1 remains stationary, while the aperture stop SP and the second to fourth lens groups B2 to B4 move toward the object side, and the fifth lens group B5 moves toward the image side and then moves toward the object side. In this Example, the position of the aspherical lens is changed from that of Example 3. By locating the aspherical lens closer to the object side, light beams at each image height are effectively separated on the aspherical surface, resulting in a more effective aspherical effect.

[0023] Below, specific numerical examples of Example 4 are shown as Numerical Example 4. Fig. 14, Fig. 15, and Fig. 16 respectively show longitudinal aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration) of the zoom lens of Example 4 (Numerical Example 4) at the wide-angle end, at the intermediate zoom position, and at the telephoto end when focused on an object at infinity. [Numerical Example 4] Unit: mm Surface number rd nd νd 1* 41.065 3.500 1.58313 59.4 2* 17.01 16.000 3* 37.732 2.500 1.854 40.4 4* 20.085 5.316 5 31.559 2.500 1.7725 49.6 6 21.056 6.627 7 618.895 1.200 1.43875 94.7 8 20.522 7.357 1.85478 24.8 9 37.088 (variable) 10 (Aperture) ∞ (Variable) 11 25.136 3.285 1.48749 70.2 12 -66.08 1.395 13 -19.039 1.000 2.00069 25.5 14 23.427 5.377 1.89286 20.4 15 -23.066 (variable) 16 52.275 5.856 1.53775 74.7 17 -19.024 1.000 1.85478 24.8 18 -35.889 0.500 19 31.983 6.270 1.43875 94.7 20 -41.66 (variable) 21 65.837 1.000 1.95375 32.3 22 21.349 7.044 1.43875 94.7 23 -98.48 3.077 24* -144.615 1.500 1.854 40.4 25* 70.128 (variable) 26 63.634 5.467 1.85896 22.7 27 250.761 (variable) Image plane ∞ K A4 A6 S8 1st page 0.000000E+00 -6.189500E-06 3.040060E-09 -8.745770E-13 Second side -9.872120E-01 4.221470E-06 -1.940650E-08 1.104740E-11 Third side 0.000000E+00 -9.958810E-06 2.359090E-09 4.605520E-13 4th side 0.000000E+00 -9.788770E-06 1.206740E-08 -9.955650E-12 Page 24 0.000000E+00 -3.299340E-05 -4.474840E-08 -4.026380E-10 Page 25 0.000000E+00 -5.570740E-06 -3.066370E-08 1.700400E-11 A10 A12 A14 1st page 0.000000E+00 0.000000E+00 0.000000E+00 2nd side 0.000000E+00 0.000000E+00 0.000000E+00 3rd side 0.000000E+00 0.000000E+00 0.000000E+00 4th side 0.000000E+00 0.000000E+00 0.000000E+00 Page 24 0.000000E+00 0.000000E+00 0.000000E+00 Page 25 0.000000E+00 0.000000E+00 0.000000E+00 Wide-angle Mid-range Telephoto d 9 15.918 6.750 1.000 d10 1.000 2.000 1.000 d15 4.261 2.750 2.057 d20 0.100 1.046 3.341 d25 0.524 12.368 16.027 d27 20.000 16.889 18.378 Wide-angle Mid-range Telephoto Focal length 10.3 14.8 20 F-number 4.12 4.12 4.12 Half angle of view (°) 62.16 54.44 47.25 Image height 19.5 20.7 21.64 Lens length 129.57 129.57 129.57 BF 20 16.89 18.38 Next, the zoom lens of each embodiment will be described in more detail. The first lens group B1 in each embodiment has a strong negative refractive power, contributing to the wide-angle of the entire zoom lens. The rear group, consisting of the second lens group B2 through the fourth lens group B4 or the fifth lens group B5, has a positive refractive power as a whole and condenses the light beam diverged by the first lens group B1. If the first lens group B1 has a strong refractive power to achieve a wide-angle, significant distortion will occur at positions where off-axis incident light rays are high. Therefore, in each embodiment, an aspherical lens that effectively corrects distortion is disposed in the first lens group B1 to suppress this distortion. Specifically, in Examples 1, 3, and 4, an aspherical lens is disposed as the first lens closest to the object, and in Example 2, it is disposed as the second lens closer to the image than the first lens. These aspherical lenses have a strong negative refractive power at the center where the optical axis passes, and the negative refractive power weakens from the center to the periphery, with the refractive power reversing from negative to positive at the periphery.

[0024] The first lens unit B1 satisfies the following conditional expression (1). 0.10≦skw / TD1≦0.50 (1) In conditional formula (1), TD1 represents the thickness of the first lens group B1 (the distance on the optical axis from the foreground to the surface closest to the image in the first lens group B1), and skw represents the air-equivalent length (back focus BF) from the final lens surface to the image plane at the wide-angle end. If skw / TD1 falls outside the range of conditional formula (1), the thickness of the first lens group B1 will be too large or too small. Too large a thickness is undesirable because the diameter of the first lens increases due to the wide angle of view, making it impossible to achieve a compact and lightweight zoom lens. On the other hand, too small a thickness is undesirable because the refractive power (the reciprocal of the focal length) required for the lenses constituting the first lens group B1, which requires strong refractive power in a compact wide-angle lens, becomes too strong, increasing distortion to an extent that is difficult to correct.

[0025] Moreover, the first lens unit B1 satisfies the following conditional expression (2). -0.17≦f1 / TDw≦-0.05 (2) In conditional formula (2), TDw represents the overall length of the zoom lens at the wide-angle end (total lens length), and f1 represents the focal length of the first lens unit B1. Conditional formula (2) represents a condition that the first lens unit B1 must satisfy to achieve a wider angle of view for the zoom lens. If TDw increases so that f1 / TDw falls below the lower limit of conditional formula (2), light rays are bent gently, which is desirable from the perspective of suppressing distortion. However, this is undesirable because it increases the overall lens length and increases the diameter of the first lens. If f1 / TDw exceeds the upper limit of conditional formula (2), the refractive power of the first lens unit B1 becomes too strong, making it difficult to simultaneously suppress aberrations at the wide-angle end and suppress aberration fluctuations during zooming, which is undesirable.

[0026] In each embodiment, the first lens group B1 is substantially composed of four or more lenses having refractive power. The term "lens" as used herein refers to an optical element having refractive power, and does not include elements without refractive power, such as parallel plates. If the first lens group B1 is composed of three or fewer lenses, the refractive power of the negative lens responsible for achieving a wide angle becomes too strong, making it difficult to suppress distortion even with an aspherical surface. As a result, the diameter of the first lens must be increased, which is unsuitable for miniaturizing the zoom lens. Furthermore, because the beam diameter in the first lens group B1 becomes large at the telephoto end, if the number of lenses is small (three or fewer), it becomes difficult to suppress fluctuations in spherical aberration and coma at the telephoto end.

[0027] In contrast, in each embodiment, as described above, an aspherical lens with a convex peripheral portion is used for the first lens (embodiments 1, 3, and 4) or the second lens (embodiment 2), thereby widening the angle of the zoom lens and suppressing distortion at the periphery. The second lens in embodiments 1, 3, and 4 and the first lens in embodiment 2 do not have an aspherical shape, and their main role is to widen the angle of the zoom lens.

[0028] The first lens group B1 has an aspherical surface with a convex periphery, which cancels out distortion throughout the entire zoom lens. This allows the second lens group B2, which is made up of spherical lenses, to have strong refractive power.

[0029] In Examples 1 to 3, the third lens in the first lens group B1, which is closer to the image side than the second lens, has an aspherical shape and serves to flatten the image surface at the wide-angle end and to suppress coma at the telephoto end. At the wide-angle end, the first lens (Examples 1 and 2) or the second lens (Example 2) suppresses distortion at high image heights, but instead the image surface moves in the under direction. In order to flatten this image surface, the third lens is an aspherical lens with negative refractive power in the peripheral portion.

[0030] The negative fourth lens and the positive fifth lens in the first lens group B1, which are located closer to the image than the third lens, have a positive composite focal length, which has the effect of canceling out distortion occurring in the first lens group B1 and lateral chromatic aberration occurring from the first lens to the third lens. The fourth lens also has a concave lens surface on the image side. This is because the fourth lens is positioned at a high position where the marginal ray of the axial light beam is likely to generate spherical aberration, and it is therefore necessary to suppress the generation of this spherical aberration. A low-dispersion negative lens is used for the fourth lens, and a high-dispersion positive lens is used for the fifth lens. This is to enhance the correction effect of lateral chromatic aberration.

[0031] The zoom lens of each embodiment configured as described above satisfies conditional expressions (1) and (2), and is therefore capable of suppressing aberration fluctuations during zooming while being small and having a wide angle.

[0032] It is preferable that the zoom lens of each embodiment satisfies the above conditional expressions (1) and (2), and also satisfies at least one of the following conditional expressions (3) to (6). First, it is preferable that at least one (hereinafter referred to as a specific moving group) of a plurality of lens groups that move during zooming (hereinafter referred to as a moving group) satisfies the following conditional expression (5). 01≦|fw / fc|<1.00 (3) In conditional expression (3), fc represents the focal length of the specific moving group. In a zoom lens in which the first lens group moves during zooming, movement of the first lens group can correct (compensate) for fluctuations in the image plane position during zooming (hereinafter referred to as image plane fluctuations). However, when the first lens group B1 is kept stationary during zooming, as in each embodiment, it is necessary to provide another moving group with the function of correcting image plane fluctuations, and this function is provided to the specific moving group that satisfies conditional expression (3). If |fw / fc| exceeds the upper limit of conditional expression (3), the refractive power of the specific moving group will be too strong, resulting in excessively large aberration fluctuations when correcting image plane fluctuations, which is undesirable. Furthermore, if the specific moving group is located at a position where the marginal ray of the on-axis light beam is high, the amount of spherical aberration fluctuation will increase. Furthermore, if the specific moving group is located at a position where the off-axial chief ray is high, fluctuations in image plane fluctuations and lateral chromatic aberration will increase. To suppress these issues, it is necessary to use multiple lenses to ensure that the aberrations within the specific moving group are well corrected. In this case, the thickness of the specific moving group will increase, lengthening the overall lens length, and the use of multiple glass lenses will increase costs. If |fw / fc| falls below the lower limit of conditional expression (3), the refractive power of the specific moving group will be too weak, and the amount of movement of the specific moving group to correct image plane fluctuations will increase, making it necessary to increase the spacing between the lens groups. As a result, the overall lens length will increase, which is undesirable.

[0033] Furthermore, during zooming, it is preferable that at least one of the multiple moving groups (hereinafter referred to as the reciprocating group) moves to describe a reciprocating locus that is convex toward the object side or convex toward the image side. In wide-angle zoom lenses with a high zoom ratio, multiple moving groups with positive refractive power move toward the object side almost as one unit to perform zooming from the wide-angle end to the telephoto end. During this operation, the amount of image plane variation relative to the amount of movement of each of the multiple moving groups is not constant, so the image plane variation is corrected by moving the first lens group to describe a reciprocating locus.

[0034] On the other hand, when the first lens unit B1 is left stationary during zooming as in each embodiment, it is possible to correct image plane fluctuations by dividing the other lens units into smaller units and preventing any of the lens units from tracing a reciprocating locus. In this case, however, it is necessary to provide many cams for driving these lens units in a small space, making the cam layout difficult.

[0035] For this reason, in each embodiment, a final lens group (the fourth lens group B4 or the fifth lens group B5 closest to the image) that moves in a reciprocating motion is provided instead of the first lens group B1. In each embodiment, the final lens group moves in a reciprocating motion convex toward the image side during zooming to correct image plane fluctuations. If image plane fluctuations were corrected by moving a lens group located near the aperture stop, the diameter of the ray bundle on the optical axis near the aperture stop would increase toward the telephoto end, resulting in greater fluctuations in spherical aberration due to changes in the spacing between lens groups. This would require a larger number of moving groups, but providing a large number of cams is difficult in a compact zoom lens. Therefore, the final lens group is preferably used as the reciprocating lens group.

[0036] Furthermore, ultra-wide-angle lenses generally have a configuration in which multiple moving groups closer to the image side than the aperture stop move almost as one unit during zooming, but to achieve a compact ultra-wide-angle lens, it is necessary to narrow the spacing between these moving groups, and for this reason it is preferable to make the final lens group the reciprocating group, which makes it easier to ensure the amount of movement required to describe a reciprocating locus.

[0037] It is preferable that the first lens unit B1 satisfies the following conditional expression (4).

[0038] 0.01≦|f1 / fa1|<1.00 (4) In conditional expression (4), fa1 represents the focal length of the aspherical lens closest to the object among the one or more aspherical lenses included in the first lens group B1. If |f1 / fa1| falls below the lower limit of conditional expression (4), the refractive power of the aspherical lens in the first lens group B1 becomes too weak, preventing the wide-angle effect of the aspherical lens from being fully achieved, which is undesirable. In this case, it is possible to achieve a wider angle by imparting refractive power to another lens, but using a spherical lens for that other lens would result in significant distortion. Furthermore, using another aspherical lens to achieve a wider angle would require increasing the number of aspherical lenses in the zoom lens, resulting in increased costs. If |f1 / fa1| exceeds the upper limit of conditional expression (4), the refractive power of the aspherical lens in the first lens unit B1 becomes too strong, making it difficult to correct distortion due to the aspherical effect, which is undesirable.

[0039] In the zoom lens of each embodiment, at least one aspherical lens is disposed closer to the image side than the aperture stop ST. Specifically, the lens closest to the image side in the second lens group counting from the image side (the third lens group B3 in Embodiment 1, and the fourth lens group B4 in Embodiments 2 to 4) is an aspherical lens, and is the aspherical lens disposed closest to the image side among the multiple aspherical lenses included in the zoom lens.

[0040] This aspherical lens has the function of flattening the image surface at the wide-angle end and the function of suppressing flare of marginal rays at intermediate image heights at the telephoto end. The first lens group B1 has the function of suppressing distortion and the function of flattening the image surface at the wide-angle end, but at the telephoto end, the light beam diameter becomes thicker, causing flare at intermediate image heights. To counteract this flare at the telephoto end, in each embodiment, an aspherical lens with negative refractive power is located closer to the image side than the aperture stop ST. Furthermore, this aspherical lens functions to counteract distortion generated in the first lens group B1 at the wide-angle end and also functions as a lens that provides a significant lift-up effect to upper rays at intermediate image heights at the telephoto end.

[0041] It is preferable that the aspherical lens arranged on the image side (closest to the image side) of the aperture stop ST satisfies the following conditional expression (5). 0.01≦|fw / fa2|<1.00 (5) In conditional expression (5), fa2 is the focal length of the aspherical lens arranged closest to the image. If |fw / fa2| exceeds the upper limit of conditional expression (5), the refractive power of the aspherical lens becomes too strong, resulting in significant lateral chromatic aberration, which is undesirable. If |fw / fa2| falls below the lower limit of conditional expression (5), the refractive power of the aspherical lens becomes too weak, making it impossible to correct flare of upper light rays generated in the under-focus direction by the first lens unit B1 at the telephoto end, which is also undesirable.

[0042] It is also preferable that the second lens unit B2 satisfies the following conditional expression (6). 0.01≦|fw / f2|<1.00 (6) In conditional expression (6), f2 is the focal length of the second lens group B2. The second lens group B2 primarily functions to converge the light beam diverged by the first lens group B1. If |fw / f2| exceeds the upper limit of conditional expression (6), the refractive power of the second lens group B2 becomes too weak to sufficiently converge the light beam diverged by the first lens group B1, which is undesirable. If |fw / f2| falls below the lower limit of conditional expression (6), the refractive power of the second lens group B2, which is positioned at a position on the optical axis where the light beam diverges, becomes too strong, which makes it difficult to correct spherical aberration and axial chromatic aberration, which is undesirable.

[0043] It is more preferable that the numerical ranges of the conditional expressions (1) to (6) be as follows: 0.15≦TD1 / skw≦0.48 (1a) -0.16≦TTDw / f1≦-0.06 (2a) 0.03≦|fw / fc|≦0.50 (3a) 0.10≦|fa1 / f1|≦0.50 (4a) 0.05≦|fa2 / fw|≦0.50 (5a) 0.05≦|f2 / fw|≦0.50 (6a) It is even more preferable to set the numerical ranges of the conditional expressions (1) to (6) as follows: 0.28≦TD1 / skw≦0.46 (1b) -0.15≦TTDw / f1≦-0.08 (2b) 0.06≦|fw / fc|≦0.15 (3b) 0.20≦|fa1 / f1|≦0.65 (4b) 0.08≦|fa2 / fw|≦0.47 (5b) 0.10≦|f2 / fw|≦0.21 (6b) The values ​​of conditional expressions (1) to (6) in Numerical Examples 1 to 4 are summarized below.

[0044] [Table 5]

[0045] 17 shows a digital still camera as an imaging device that uses the zoom lens of each of the above-described embodiments as an imaging optical system. 20 denotes the camera body, and 21 denotes the imaging optical system configured with any of the zoom lenses of Examples 1 to 4. 22 denotes a solid-state imaging element such as a CCD sensor or CMOS sensor that is built into the camera body 20 and captures an optical image (subject image) formed by the imaging optical system 21. 23 denotes a recording unit that records image data generated by processing the imaging signal from the imaging element 22, and 24 denotes a rear display that displays the image data.

[0046] By using the zoom lens of each embodiment, a small-sized camera with high performance can be obtained.

[0047] The camera may be a single-lens reflex camera having a quick-turn mirror, or may be a mirrorless camera having no quick-turn mirror.

[0048] The embodiments described above are merely representative examples, and various modifications and alterations are possible to each embodiment when implementing the present invention. [Explanation of symbols]

[0049] B1 First lens group B2 Second lens group B3 Third lens group B4 4th lens group B5 5th lens group IP image plane

Claims

1. A zoom lens comprising, arranged in order from the object side to the image side, a first lens group having negative refractive power and a rear group consisting of four lens groups having positive refractive power as a whole, wherein the spacing between adjacent lens groups changes during magnification variation, the first lens group includes an aspherical lens A and does not move during zooming; the rear group comprises, arranged in order from the object side to the image side, a second lens group having positive refractive power, a third lens group having positive refractive power, a fourth lens group having negative refractive power, and a fifth lens group having positive refractive power; Let TD1 be the distance on the optical axis from the lens surface closest to the object in the first lens group to the lens surface closest to the image in the first lens group, skw be the distance on the optical axis from the lens surface closest to the image at the wide-angle end of the zoom lens to the image plane, TDw be the distance on the optical axis from the lens surface closest to the object at the wide-angle end to the image plane, f1 be the focal length of the first lens group, and fa1 be the focal length of the aspherical lens closest to the object among the aspherical lenses included in the zoom lens. 0.10≦skw / TD1≦0.50 -0.17≦f1 / TDw≦-0.05 0.01≦|f1 / fa1|≦0.278 A zoom lens characterized by satisfying the following conditions:

2. 2. The zoom lens according to claim 1, wherein the first lens group is made up of four or more lenses.

3. 3. A zoom lens according to claim 1, wherein two or more lens groups in the rear group move during zooming.

4. 4. The zoom lens according to claim 1, wherein the aspherical lens A has a negative refractive power whose absolute value decreases from the center to the periphery.

5. 5. The zoom lens according to claim 4, wherein the aspherical lens A has a positive refractive power in the peripheral portion.

6. When the focal length of the moving lens unit in the rear group that moves during magnification variation is fc, 0.01≦|fw / fc|<1.00 6. The zoom lens according to claim 1, wherein the following condition is satisfied:

7. 7. The zoom lens according to claim 6, wherein the moving lens group moves to one of the object side and the image side and then moves to the other side during zooming between the wide-angle end and the telephoto end.

8. 8. A zoom lens according to claim 6, wherein the moving lens group is the lens group in the rear group closest to the image side.

9. 9. The zoom lens according to claim 1, further comprising an aspherical lens arranged closer to the image side than the aperture stop.

10. When the focal length of the aspherical lens arranged closest to the image among the aspherical lenses included in the zoom lens is fa2, 0.01≦|fw / fa2|<1.00 10. The zoom lens according to claim 1, wherein the following condition is satisfied:

11. When the focal length of the lens unit in the rear group located closest to the object side is f2, 0.01≦|fw / f2|<1.00 11. The zoom lens according to claim 1, wherein the following condition is satisfied:

12. a zoom lens according to any one of claims 1 to 11; and an image sensor for capturing an image of an object through the zoom lens.

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