JP7757486B2 - Zoom lens and imaging device - Google Patents

Description

The present invention relates to a zoom lens suitable for imaging devices such as video cameras, digital still cameras, and surveillance cameras.

Patent Documents 1 and 2 disclose wide-angle zoom lenses that change magnification by moving the first lens group closest to the object.

Patent No. 5777592 Japanese Patent Application Laid-Open No. 2018-189733

However, in the zoom lenses disclosed in Patent Documents 1 and 2, the long back focus 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 use multiple large-diameter lenses to achieve a wide angle and correct distortion, which tends to increase the weight of the first lens group. As a result, it becomes difficult to suppress the increase in aberration caused by decentering when the first lens group moves.

The present invention provides a compact zoom lens that has a wide angle yet can suppress aberration fluctuations when changing magnification.

A zoom lens according to one aspect of the present invention comprises, 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 zooming. The first lens group includes an aspherical lens A and does not move during zooming, and 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 in the first lens group closest to the object to the lens surface in the first lens group closest to the image, 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
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.

The present invention provides a compact zoom lens that has a wide angle and can suppress aberration fluctuations when changing magnification.

FIG. 1 is a cross-sectional view of a zoom lens according to a first embodiment. 5A and 5B are longitudinal aberration diagrams at the wide-angle end of the zoom lens of Example 1. 4A and 4B are longitudinal aberration diagrams of the zoom lens of Example 1 at an intermediate zoom position. 5A and 5B are longitudinal aberration diagrams at the telephoto end of the zoom lens of Example 1. FIG. 10 is a cross-sectional view of a zoom lens according to a second embodiment. 10A and 10B are longitudinal aberration diagrams at the wide-angle end of the zoom lens of Example 2. 10A and 10B are longitudinal aberration diagrams of the zoom lens of Example 2 at an intermediate zoom position. 10A and 10B are longitudinal aberration diagrams at the telephoto end of the zoom lens of Example 2. FIG. 10 is a cross-sectional view of a zoom lens according to a third embodiment. 10A and 10B are longitudinal aberration diagrams at the wide-angle end of the zoom lens of Example 3. 10A and 10B are longitudinal aberration diagrams of the zoom lens of Example 3 at an intermediate zoom position. 10A and 10B are longitudinal aberration diagrams at the telephoto end of the zoom lens of Example 3. FIG. 10 is a cross-sectional view of a zoom lens according to a fourth embodiment. 10A and 10B are longitudinal aberration diagrams at the wide-angle end of the zoom lens of Example 4. 10A and 10B are longitudinal aberration diagrams of the zoom lens of Example 4 at an intermediate zoom position. 10A and 10B are longitudinal aberration diagrams at the telephoto end of the zoom lens of Example 4. FIG. 1 is a schematic diagram of an imaging device equipped with zoom lenses according to first to fourth embodiments.

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

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 magnification (zooming) or focusing is defined as a lens group, and in the figures they are designated by 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 possible in the optical axis direction during zooming. In each cross-sectional view, arrows indicate the movement trajectory of the lens group as it moves during zooming from the wide-angle end to the telephoto end, and also indicate the direction of movement of the lens group as it moves during focusing from an object at infinity to a close-up object.

The zoom lens in 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 onwards) with positive refractive power overall, and magnification is changed by changing the spacing between adjacent lens groups. The first lens group B1 includes an aspherical lens and does not move during magnification changes. An aperture stop SP is located between the first lens group B1 and the second lens group B2. Note that the rear group only needs to have positive refractive power overall; for example, in each embodiment, the second lens group B2 has positive refractive power, but it may also have negative refractive power. Furthermore, it is sufficient for the aspherical lens to have at least one of the object-side and image-side lens surfaces have an aspherical shape.

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

The zoom lens of Example 1 shown in Figure 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. When 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.

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. When the refractive indexes at the d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm) of the Fraunhofer lines are Nd, NF, and NC, respectively, the Abbe number vd is given by
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.

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

If the lens surface is aspherical, a * 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 aspherical coefficients of each order.
x=(h ² /R)/[1+{1-(1+k)(h/R) ² } ^1/2
+A4 x ^h4 +A6 x ^h6 +A8 x ^h8 +A10 x ^h10
+A12 x ^h12 +A14 x ^h14
It should be noted that "e±XX" in the conic constant and aspherical coefficient means "×10 ^±XX ". The above explanation regarding Numerical Example 1 also applies to other Numerical Examples described later.

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 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

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. When 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 greater than in Example 1, and the focal length at the wide-angle end is longer.

A specific numerical example of Example 2 is shown below 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

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. When 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 telephoto end are longer than in Example 1, while maintaining the same zoom ratio.

A specific numerical example of Example 3 is shown below 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 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

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 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. When 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 position of the aspherical lens is changed from that of Example 3. By placing the aspherical lens closer to the object side, the light beam at each image height is effectively separated on the aspherical surface, resulting in a stronger aspherical effect.

A specific numerical example of Example 4 is shown below as Numerical Example 4. Figures 14, 15, and 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 lenses of each embodiment will be described in more detail. The first lens unit B1 in each embodiment has a strong negative refractive power, contributing to the wide-angle zoom lens as a whole. The rear group, consisting of the second lens unit B2 through the fourth lens unit B4 or the fifth lens unit B5, has a positive refractive power as a whole and condenses the light beam diverged by the first lens unit B1. If the first lens unit B1 has a strong refractive power to achieve a wide-angle zoom, 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 unit 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, an aspherical lens is disposed as the second lens closer to the image than the first lens. These aspherical lenses have an aspherical shape in which the central portion thereof along the optical axis has a strong negative refractive power, the negative refractive power weakens from the central portion toward the periphery, and the refractive power reverses from negative to positive at the periphery.

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 unit B1 (the distance on the optical axis from the foreground to the surface closest to the image in the first lens unit 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 the values defined by conditional formula (1), the thickness of the first lens unit B1 will be too large or too small. If the thickness is too large, 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, which is undesirable. On the other hand, if the thickness is too small, the refractive power (the reciprocal of the focal length) required for the lenses constituting the first lens unit B1, which requires a strong refractive power in a compact wide-angle lens, will be too strong, which is undesirable, as distortion will increase to an amount that is difficult to correct.

Moreover, the first lens unit B1 satisfies the following conditional expression (2).
−0.17≦f1/TDw≦−0.05 (2)
In conditional expression (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 expression (2) represents a condition that the first lens unit B1 must satisfy in order 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 expression (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 expression (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.

In addition, in each embodiment, the first lens group B1 is substantially composed of four or more lenses having refractive power. The term "lens" here 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 widening the angle of view will be too strong, making it difficult to suppress distortion even with an aspherical surface. As a result, it becomes necessary to increase the diameter of the first lens, which is not suitable for miniaturizing the zoom lens. Furthermore, because the light beam diameter in the first lens group B1 increases at the telephoto end, if the number of lenses is small, such as three or fewer, it becomes difficult to suppress fluctuations in spherical aberration and coma at the telephoto end.

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

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

In Examples 1 to 3, the third lens element in the first lens group B1, which is closer to the image than the second lens element, 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 element (Examples 1 and 2) or the second lens element (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 element is an aspherical lens with negative refractive power at the periphery.

The negative fourth lens and positive fifth lens located closer to the image than the third lens in the first lens group B1 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 image-side lens surface of the fourth lens has a concave shape. 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 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.

The zoom lens of each embodiment configured as described above satisfies conditional expressions (1) and (2), thereby suppressing aberration fluctuations during zooming while remaining compact and wide-angle.

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 impart the function of correcting image plane fluctuations to another moving group, and this function is imparted 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 becomes 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 fluctuation in spherical aberration increases. 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 chromatic aberration of magnification increase. To suppress these issues, it is necessary to use multiple lenses to ensure that aberrations within the specific moving group are well corrected. In this case, the thickness of the specific moving group increases, resulting in an increase in the overall lens length, and the use of multiple glass lenses increases 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 become large, making it necessary to increase the spacing between the lens groups, which will result in an increase in the overall lens length, which is undesirable.

Furthermore, during zooming, it is preferable that at least one of the multiple moving groups (hereinafter referred to as the reciprocating group) move to trace 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 zoom 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 image plane variation is corrected by moving the first lens group to trace a reciprocating locus.

In contrast, if the first lens group B1 is left stationary during zooming, as in each embodiment, it is possible to finely divide the other lens groups and correct image plane fluctuations without causing any of the lens groups to trace a reciprocating trajectory. However, in this case, it is necessary to provide many cams to drive these lens groups in a small space, making the cam layout difficult.

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 in place of the first lens group B1. In each embodiment, the final lens group moves in a reciprocating motion that is convex toward the image side during zooming to correct image plane variation. If image plane variation 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 expand toward the telephoto end, resulting in greater variation in spherical aberration due to changes in the spacing between lens groups. This would require a larger number of moving groups, but it would be difficult to provide a large number of cams in a compact zoom lens. Therefore, the final lens group is preferably the reciprocating group.

Moreover, 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 realize a compact ultra-wide-angle lens, it is necessary to narrow the spacing between these moving groups. For this reason, it is preferable to make the final lens group a reciprocating group, as this makes it easier to ensure the amount of movement required to trace a reciprocating trajectory.

It is preferable that the first lens unit B1 satisfy the following conditional expression (4):

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 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 small, making it difficult to obtain the full wide-angle effect of the aspherical lens, which is undesirable. In this case, it is possible to achieve a wide-angle by imparting refractive power to another lens, but using a spherical lens for that other lens will result in significant distortion. Furthermore, using another aspherical lens to achieve a wide-angle result increases 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 large, making it difficult to correct distortion due to the aspherical effect, which is undesirable.

In the zoom lens of each embodiment, at least one aspherical lens is arranged 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 arranged closest to the image side among the multiple aspherical lenses included in the zoom lens.

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 unit B1 has the function of suppressing distortion and 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 positioned closer to the image side than the aperture stop ST. This aspherical lens also functions to counteract distortion generated in the first lens unit B1 at the wide-angle end and to provide a significant lift-up effect for upper rays at intermediate image heights at the telephoto end.

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 chromatic aberration of magnification, 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.

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 unit B2. The second lens unit B2 mainly functions to converge the light beam diverged by the first lens unit B1. If |fw/f2| exceeds the upper limit of conditional expression (6), the refractive power of the second lens unit B2 becomes too weak to sufficiently converge the light beam diverged by the first lens unit B1, which is undesirable. If |fw/f2| falls below the lower limit of conditional expression (6), the refractive power of the second lens unit 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.

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 the conditional expressions (1) to (6) of the first to fourth numerical examples are summarized below.

Figure 17 shows a digital still camera as an imaging device that uses the zoom lens of each of the above examples as its 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 the 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.

By using the zoom lenses of each embodiment, a compact camera with high performance can be obtained.

The camera may be a single-lens reflex camera with a quick-turn mirror, or a mirrorless camera without a quick-turn mirror.

The embodiments described above are merely representative examples, and various modifications and variations are possible when implementing the present invention.

B1 First lens unit B2 Second lens unit B3 Third lens unit B4 Fourth lens unit B5 Fifth lens unit IP Image plane

Claims (12)

A zoom lens comprising: a first lens group having negative refractive power ; and a rear group having positive refractive power as a whole, which is made up of four lens groups, arranged in this order from the object side to the image side; wherein the spacing between adjacent lens groups changes during zooming,
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: The zoom lens of claim 1, wherein the first lens group is composed of four or more lenses. A zoom lens according to claim 1 or 2, characterized in that two or more lens groups in the rear group move during magnification variation. 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. The zoom lens according to claim 4, wherein the aspherical lens A has a positive refractive power in the peripheral portion. 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: A zoom lens according to claim 6, wherein the moving lens group moves to one of the object side and the image side and then to the other side when varying magnification between the wide-angle end and the telephoto end. A zoom lens according to claim 6 or 7, wherein the moving lens group is the lens group in the rear group closest to the image side. 9. The zoom lens according to claim 1, further comprising an aspherical lens arranged closer to the image side than the aperture stop. 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: 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: 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.