JP4189257B2 - Zoom lens system - Google Patents

Description

[0001]
【Technical field】
The present invention relates to a zoom lens system mainly used in an electronic still camera (digital camera) and including a wide angle region (half angle of view of 30 ° or more) with a zoom ratio (magnification ratio) of about 2 to 3 times.
[0002]
[Prior art and its problems]
In recent years, the need for miniaturization and high definition of digital cameras has increased, and the pixels of CCD image sensors have been miniaturized. Therefore, the photographing lens of the digital camera is required to have a high resolution. In addition, a long back focus is required to arrange the filters. Further, an optical system for a color CCD is required to have so-called telecentricity in which light emitted from the last lens surface is incident on the imaging surface as perpendicularly as possible to prevent shading and color misregistration.
[0003]
As a compact zoom lens system for a digital camera, a negative lens leading type (negative lead type) lens system (telephoto type) is often used when the zoom ratio is about 2 to 3 times. These lens systems can widen the short focal length and reduce the size of the lens system, especially the diameter of the front lens (most object side lens). Suitable for zooming. In addition, since the exit pupil position needs to be sufficiently far from the image plane, a so-called three-group zoom lens system including three negative and positive components in order from the object side is often used. Such a three-group zoom lens system is proposed in, for example, Japanese Patent Application Laid-Open Nos. 10-213745 and 10-170826.
[0004]
However, in Japanese Patent Laid-Open No. 10-213745, the number of lenses is reduced and the size is reduced. However, the front lens diameter and the entire lens length are large with respect to the focal length, and it cannot be said that the size reduction is sufficiently achieved. . Also, in Japanese Patent Laid-Open No. 10-170826, a telecentric optical system that achieves miniaturization is proposed, but since the number of components is as large as seven, the retractable storage length of the lens system becomes long and the camera becomes large. There's a problem. In order to reduce the size of the camera body, a zoom lens system for a compact retractable camera is required to have a small front lens diameter and a small total lens length, as well as a small thickness for each lens group. In general, if the number of components is reduced in order to reduce the size of the lens system and the group thickness, the degree of difficulty in correcting aberrations increases. In order to satisfactorily correct various aberrations over the entire zooming range while reducing the size, an appropriate refractive power arrangement and lens configuration of each lens group is required.
[0005]
OBJECT OF THE INVENTION
The present invention provides a telephoto type three-group zoom lens system, which is a small zoom lens system for a digital camera including a wide angle region (half angle of view of 30 ° or more) with a zoom ratio of about 2 to 3 times. Objective.
[0006]
SUMMARY OF THE INVENTION
The zoom lens system of the present invention includes, in order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. When zooming from the short focal length end to the long focal length end, the distance between the first lens group and the second lens group is decreased, and the distance between the second lens group and the third lens group is increased. The first lens group is composed of one negative lens and one positive lens in order from the object side, and the second lens group is composed of one positive lens and one negative lens in order from the object side. The lens group is composed of one positive lens, and satisfies the following conditional expressions (1), (2), (3), and (5) .
(1) 0.4 <(fw · ft) ^1/2 /|f1|<0.8 (f1 <0)
(2) 0.7 <(fw · ft) 1/2 /f2<1.4
(3) 0.4 <(fw · ft) ^1/2 /f3<0.9
(5) νs ≦ 21.3
However,
fw: total focal length at the short focal length end,
ft: total system focal length at the long focal length end,
fi: focal length of the i-th lens group (i = 1 to 3),
νs: Abbe number of the lens on the image side of the second lens group,
It is.
[0007]
The most image-side lens in the second lens group is preferably composed of a lens having a concave surface directed to the image side, and further preferably satisfies the following conditional expression (4).
(4) 0.4 <| Rs | / fw <0.8
However,
Rs: radius of curvature of the image side surface of the lens having a concave surface on the image side,
It is.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In the zoom lens system of the present invention, as shown in the simplified movement diagram of FIG. 13, in order from the object side, the negative first lens group 10, the aperture stop S, the positive second lens group 20, and the positive third lens group. It consists of a lens group 30. In the three-group zoom lens, during the zooming from the short focal length end (W) to the long focal length end (T), the distance between the first lens group 10 and the second lens group 20 decreases, and the second lens group 20 And the distance between the third lens group 30 is increased. The diaphragm S moves together with the second lens group 20. Focusing is performed by the first lens group 10. CG is a cover glass (parallel flat plate) such as an infrared cut filter positioned in front of the image sensor.
[0009]
Conditional expression (1) defines the range of the focal length of the first lens group with respect to the intermediate focal length ((fw · ft) ^1/2 ).
When the lower limit of conditional expression (1) is exceeded, the negative refractive power of the first lens group 10 becomes small, and the back focus at the short focal length end is insufficient. In addition, widening the angle becomes difficult.
If the negative refractive power of the first lens group 10 increases beyond the upper limit of conditional expression (1), the refractive power of each group becomes strong, making it difficult to correct aberrations, and good imaging performance cannot be obtained. Further, since the back focus becomes long, the total lens length is increased.
[0010]
Conditional expression (2) defines the range of the focal length of the second lens group 20 with respect to the intermediate focal length. The positive second lens group 20 is responsible for the main zooming action of the present zoom lens system, and it is necessary to set an appropriate refractive power.
If the positive refractive power of the second lens group 20 decreases beyond the lower limit of the conditional expression (2), the amount of movement when obtaining a zoom ratio of about 3 times increases, so that the total lens length at the long focal length end is reduced. It will be long.
When the positive refractive power of the second lens group 20 increases beyond the upper limit of conditional expression (2), the refractive power of each group becomes strong, making it difficult to correct aberrations, and good imaging performance cannot be obtained.
[0011]
Conditional expression (3) defines the range of the focal length of the third lens group 30 with respect to the intermediate focal length. The positive third lens group 30 mainly serves to ensure telecentricity by making the exit pupil position farther from the image plane.
If the lower limit of conditional expression (3) is exceeded, the positive refracting power of the third lens group 30 becomes small, so that the exit pupil position approaches the image plane at the short focal length end, and telecentricity cannot be maintained.
If the positive refractive power of the third lens group 30 increases beyond the upper limit of the conditional expression (3), the positive refractive power of the second lens group 20 decreases relatively, so that the total lens length at the long focal length end Will increase. Further, the field curvature aberration and astigmatism at the long focal length end deteriorate, and good imaging performance cannot be obtained.
[0012]
The most image side lens of the second lens group 20 is preferably composed of a lens having a strong divergence on the image side, and this concave lens preferably satisfies the conditional expression (4).
Conditional expression (4) defines the range of the radius of curvature of the concave surface closest to the image side of the second lens group 20 with respect to the total focal length at the short focal length end, and the lens system can be downsized at the short focal length end. And to ensure telecentricity.
If the curvature radius of this surface is reduced beyond the lower limit of conditional expression (4), the total lens length can be shortened, but the exit pupil position becomes too close to the image plane, and telecentricity is lost.
When the curvature radius of this surface increases beyond the upper limit of conditional expression (4), the distance between the second lens group 20 and the third lens group 30 increases to maintain the telecentricity, and the total lens length increases.
[0013]
In order to shorten the length when retracted, it is necessary to reduce the number of lenses in each lens group. Specifically, the first lens group 10 is composed of two negative and positive lenses in order from the object side, and the second lens group 20 responsible for the main zooming function is composed of two positive and negative elements in order from the object side, and is telecentric. The third lens group 30 that has the role of securing the property is composed of one positive lens.
[0014]
Furthermore, if at least one surface of the second lens having a positive refractive power in the first lens group is aspheric, it is possible to satisfactorily correct off-axis aberrations such as distortion, coma and astigmatism at the short focal length end. It becomes possible to do. The aspherical lens is made of glass or plastic, but the cost can be reduced by using a plastic lens.
[0015]
Further, as a result of the second lens group 20 having two positive and negative lenses, the refractive power of one positive lens becomes strong, and it becomes difficult to suppress aberration fluctuations such as spherical aberration accompanying zooming. both preferable to the one surface aspherical less when a double-sided aspherical, spherical aberration varying due to zooming, it is possible to reduce the coma aberration. Furthermore, conditional expression (5) is a condition relating to the Abbe number that the negative lens on the image side of the second lens group 20 should satisfy. If the lower limit of conditional expression (5) is exceeded, fluctuations in axial and magnification chromatic aberration due to zooming increase, and good imaging performance cannot be maintained.
[0016]
If the positive lens closest to the object side in the second lens group 20 is provided with an aspherical surface, the imaging performance is further improved. By making at least one of the object side surface or the image side surface an aspheric surface whose positive refractive power becomes weaker toward the periphery, the variation in spherical aberration can be reduced in the entire focal length range, and the coma on the long focal length side can be reduced. It becomes possible to correct aberrations more satisfactorily. The aspheric lens can be made of either glass or plastic, but is preferably a glass lens because of its strong refractive power.
[0017]
Next, specific examples will be described. In the various aberration diagrams and tables, the d-line, g-line, and C-line in the chromatic aberration (axial chromatic aberration) diagram and magnification chromatic aberration diagram represented by spherical aberration are the aberrations for each wavelength, S is sagittal, and M is meridional. , FNO is the F number, f is the focal length of the entire system, W is the half angle of view (°), fB is the back focus, r is the radius of curvature, d is the lens thickness or lens spacing, Nd is the refractive index of the d-line, νd Indicates the Abbe number.
In the embodiment, the last plane-parallel plate (surface numbers 12 and 13) represents a filter such as a cover glass or a low-pass filter, and can be arranged at an arbitrary position between the image-side lens and the image plane in terms of optical performance.
A rotationally symmetric aspherical surface is defined by the following equation.
x = cy2 / [1+ [1- (1 + K) c2y2] 1/2] + A4y4 + A6y6 + A8y8 + A10y10 + A12y12 ...
(Where x is an aspherical shape, c is a curvature, y is a height from the optical axis, K is a conical coefficient, A4, A6, A8, A10... Are aspherical coefficients of each order)
[0018]
[Example 1]
FIG. 1 is a lens configuration diagram of Embodiment 1 of the zoom lens system of the present invention. A first lens group 10 includes, in order from the object side, a negative lens 11 and a positive lens 12, and a second lens group 20 includes In order from the object side, the positive lens 21 and a negative meniscus lens 22 having a concave surface facing the image side are included, and the third lens group 30 is a positive single lens. CG is a cover glass (filters) positioned in front of the image sensor. The stop is disposed at a position 0.7 (front side of the object) 0.7 on the fifth surface, and moves together with the second lens group 20 during zooming. 2, 3 and 4 are diagrams showing various aberrations at the short focal length end, intermediate focal length, and long focal length end of this zoom lens system, respectively, and Table 1 shows numerical data thereof.
[0019]
(Table 1)
FNO. = 1: 2.7-3.6-5.2
f = 7.25-12.10-20.64 (zoom ratio = 2.85)
W = 32.9-20.4-12.3
fB = 0.00-0.00-0.00
Surface NO. R d Nd νd
1 51.813 0.90 1.77250 49.6
2 7.251 2.72
3 * 20.298 2.40 1.80518 25.4
4 * 83.771 16.95‐8.15‐2.7
5 * 7.327 2.50 1.72916 54.7
6 * -18.987 0.10
7 16.947 3.73 1.92286 21.3
8 4.634 4.80-9.64-18.1
9 23.922 2.80 1.72916 54.7
10 -24.519 2.73
11 ∞ 1.51 1.51633 64.1
12 ∞ 0.50
13 ∞ 0.50 1.51633 64.1
14 ∞
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00):
Surface No. K A4 A6 A8
3 0.00 -0.27398 × 10 ^-4 -0.14171 × 10 ^-5 0.88000 × 10 ^-7
4 0.00 -0.21862 × 10 ^-3 0.51925 × 10 ^-6 0.00
5 0.00 -0.32331 × 10 ^-3 -0.54123 × 10 ^-5 0.99049 × 10 ^-7
6 0.00 0.28664 × 10 ^-3 -0.47358 × 10 ^-5 0.68722 × 10 ^-7
[0020]
[Example 2]
FIG. 5 shows a lens configuration diagram of Embodiment 2 of the zoom lens system of the present invention. FIGS. 6, 7 and 8 respectively show a short focal length end, an intermediate focal length, and a long focal length of the zoom lens system. Various aberration diagrams at the distance end, Table 2 shows numerical data. The basic lens configuration is the same as that of the first embodiment. The stop is disposed at a position 0.7 (front side of the object) 0.7 on the fifth surface, and moves together with the second lens group 20 during zooming.
[0021]
(Table 2)
FNO. = 1: 2.7-3.4-5.3
f = 8.00-12.50-24.03 (zoom ratio = 3.00)
W = 30.4-19.8-10.6
fB = 0.00-0.00-0.00
Surface NO. R d Nd νd
1 35.725 0.90 1.83481 42.7
2 7.646 2.17
3 * 13.231 2.40 1.84666 23.8
4 * 25.272 17.99‐9.74‐2.70
5 * 7.110 2.50 1.77250 49.6
6 -28.219 0.20
7 * 18.266 3.50 1.92286 21.3
8 4.880 4.92‐8.97‐19.37
9 30.098 2.80 1.69680 55.5
10 -21.514 2.95
11 ∞ 1.51 1.51633 64.1
12 ∞ 0.50
13 ∞ 0.50 1.51633 64.1
14 ∞
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00):
Surface No. K A4 A6 A8
3 0.00 -0.88427 × 10 ^-4 -0.16679 × 10 ^-5 0.43529 × 10 ^-7
4 0.00 -0.23870 × 10 ^-3 -0.62834 × 10 ^-6 0.00
^{5 0.00 -0.11983 × 10 -3 -0.25840 ×} 10 -5 0.00
7 0.00 -0.45509 × 10 ^-3 -0.86460 × 10 ^-5 0.16360 × 10 ^-6
[0022]
[Example 3]
FIG. 9 shows a lens configuration diagram of Embodiment 3 of the zoom lens system of the present invention. FIGS. 10, 11 and 12 respectively show a short focal length end, an intermediate focal length, and a long focal length of the zoom lens system. Various aberration diagrams at the distance end and Table 3 are numerical data. The basic lens configuration is the same as that of the first embodiment. The stop is disposed at a position 0.67 in front of the fifth surface (object side), and moves together with the second lens group 20 during zooming.
[0023]
(Table 3)
FNO. = 1: 2.7-3.5-4.8
f = 5.70-9.00-14.30 (zoom ratio = 2.51)
W = 32.9-21.7-14.1
fB = 2.70-2.70-2.70
Surface NO. R d Nd νd
1 16.171 0.90 1.77250 49.6
2 4.553 1.70
3 * 8.879 1.60 1.84666 23.8
4 * 13.209 11.05-6.14-3.00
5 * 4.433 2.00 1.69350 53.2
6 * -18.477 0.20
7 7.137 1.00 1.92286 21.3
8 3.193 3.14-6.99-13.17
9 75.707 2.00 1.58913 61.2
10 -10.659 0.80
11 ∞ 1.50 1.51633 64.1
12 ∞ 0.50
13 ∞ 0.50 1.51633 64.1
14 ∞
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00):
Surface No. K A4 A6 A8
^{3 0.00 -0.83561 × 10 -3 0.22557 ×} 10 -4 -0.78955 × 10 -
4 0.00 -0.16518 × 10 ^-2 0.38674 × 10 ^-4 -0.24551 × 10 ^-5
5 0.00 -0.12186 × 10 ^-2 -0.15219 × 10 ^-4 0.00
6 0.00 0.82006 × 10 ^-3 0.20843 × 10 ^-4 0.00
[0024]
Table 4 shows values for each conditional expression in each example.
(Table 4)
Example 1 Example 2 Example 3
Conditional expression (1) 0.677 0.683 0.734
Conditional expression (2) 1.017 1.078 0.957
Conditional expression (3) 0.732 0.753 0.564
Conditional expression (4) 0.643 0.610 0.560
Condition (5) 21.3 21.3 21.3
[0025]
【The invention's effect】
According to the present invention, it is possible to obtain a small zoom lens system for a digital camera which is a telephoto type three-group zoom lens and has a zoom ratio of about 2 to 3 times.
[Brief description of the drawings]
FIG. 1 is a lens configuration diagram of Embodiment 1 of a zoom lens system according to the present invention.
2 is a diagram illustrating various aberrations at a short focal length end of the lens configuration in FIG. 1; FIG.
3 is a diagram illustrating various aberrations at an intermediate focal length of the lens configuration in FIG. 1; FIG.
4 is a diagram illustrating various aberrations at the long focal length end of the lens configuration in FIG. 1; FIG.
FIG. 5 is a lens configuration diagram of Example 2 of a zoom lens system according to the present invention.
6 is a diagram illustrating various aberrations at the short focal length end of the lens configuration in FIG. 5. FIG.
7 is a diagram illustrating various aberrations at an intermediate focal length of the lens configuration in FIG. 5. FIG.
8 is a diagram illustrating various aberrations at the long focal length end of the lens configuration in FIG. 5. FIG.
FIG. 9 is a lens configuration diagram of Example 3 of the zoom lens system according to the present invention.
10 is a diagram illustrating various aberrations at the short focal length end of the lens configuration in FIG. 9. FIG.
FIG. 11 is a diagram illustrating various aberrations at the intermediate focal length of the lens configuration in FIG. 9;
12 is a diagram showing various aberrations at the long focal length end of the lens configuration in FIG. 9. FIG.
FIG. 13 is a simplified movement diagram of a zoom lens system according to the present invention.

Claims (2)

In order from the object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power, and from the short focal length end to the long focal length During zooming to the distance end, the distance between the first lens group and the second lens group is decreased, and the distance between the second lens group and the third lens group is increased,
The first lens group is composed of one negative lens and one positive lens in order from the object side, and the second lens group is composed of one positive lens and one negative lens in order from the object side. The lens group consists of one positive lens,
A zoom lens system characterized by satisfying the following conditional expressions (1), (2), (3) and (5):
(1) 0.4 <(fw · ft) ^1/2 /|f1|<0.8 (f1 <0)
(2) 0.7 <(fw · ft) ^1/2 /f2<1.4
(3) 0.4 <(fw · ft) ^1/2 /f3<0.9
(5) νs ≦ 21.3
However,
fw: total focal length at the short focal length end,
ft: total system focal length at the long focal length end,
fi: focal length of the i-th lens group (i = 1 to 3),
νs: Abbe number of the lens on the image side of the second lens group. 2. The zoom lens system according to claim 1, wherein the most image side lens of the second lens group is a lens having a concave surface directed to the image side, and satisfies the following conditional expression (4). (4) 0.4 <| Rs | / fw <0.8
However,
Rs: radius of curvature of the image side surface of the meniscus lens.

Images