JP7735149B2 - Zoom lens and imaging device - Google Patents

Description

The present invention relates to a zoom lens and an imaging device.

In recent years, there has been a demand for zoom lenses used in imaging devices such as surveillance cameras that are small, lightweight, and have high optical performance. Patent Document 1 discloses a two-group zoom lens that consists, from the object side to the image side, of a lens group with negative refractive power and a lens group with positive refractive power.

JP 2012-83706 A

The zoom lens disclosed in Patent Document 1 is disadvantageous in terms of compactness, light weight, and high optical performance.

The present invention aims to provide a zoom lens that is advantageous in terms of, for example, its small size, light weight, and high optical performance.

One aspect of the present invention is a zoom lens having, in order from the object side to the image side, a first lens group having negative refractive power and a second lens group having positive refractive power, the spacing between adjacent lens groups changing during zooming, the zoom lens having at least one plastic positive lens with an aspherical surface and at least one plastic negative lens with an aspherical surface, the first lens group having at least one positive lens and the second lens group having at least two negative lenses, and satisfying a predetermined conditional expression.

Other objects and features of the present invention are described in the following examples.

The present invention makes it possible to provide a zoom lens that is advantageous in terms of, for example, its small size, light weight, and high optical performance.

FIG. 1 is a cross-sectional view of a zoom lens at a wide-angle end in Example 1. 10A to 10C are aberration diagrams at the wide-angle end, at a middle zoom position, and at the telephoto end in Example 1. FIG. 10 is a cross-sectional view of a zoom lens at a wide-angle end in Example 2. 10A to 10C are aberration diagrams at the wide-angle end, at a middle zoom position, and at the telephoto end in Example 2. FIG. 10 is a cross-sectional view of a zoom lens at a wide-angle end in Example 3. 10A to 10C are aberration diagrams at the wide-angle end, at the intermediate zoom position, and at the telephoto end in Example 3. FIG. 10 is a cross-sectional view of a zoom lens at a wide-angle end in Example 4. 10A to 10C are aberration diagrams at the wide-angle end, at a middle zoom position, and at the telephoto end in Example 4. FIG. 1 is a diagram illustrating the configuration of an imaging device equipped with a zoom lens in each embodiment. FIG. 1 is a diagram illustrating the configuration of an imaging device equipped with a zoom lens in each embodiment. FIG. 1 is a diagram illustrating the configuration of an imaging device equipped with a zoom lens in each embodiment. 4A and 4B are explanatory diagrams illustrating the movement of lens groups of the zoom lens in Example 1. 10A and 10B are conceptual diagrams illustrating focus deviation due to temperature change in the zoom lens in Example 2.

The following describes in detail an embodiment of the present invention with reference to the drawings.

First, we will explain the use of plastic lenses, which is the purpose of each example. The advantage of using plastic lenses is that they can be molded to have aspherical surfaces, which is effective in correcting various aberrations other than chromatic aberration in lens design, allowing for shorter overall length and higher performance. Another advantage is that their specific gravity is about 1/3 to 1/5 of that of regular glass, making them effective for weight reduction. While the benefits of using plastic lenses are significant, plastic lenses have the characteristic of having a refractive index that is susceptible to change with temperature. The refractive index change (dn/dt) with respect to temperature is about 20 times greater than that of regular glass, making them susceptible to changes in refractive index due to temperature changes.

For example, the change in refractive index at the d line of Zeon Corporation's Zeonex E48R due to temperature is shown in Table 1 below.

As a result, if the temperature changes, especially during shooting, the refractive index of the plastic lens changes, causing a corresponding amount of focus shift and making it difficult to obtain good image quality. If the amount of focus shift is too large, the focus control range will be exceeded, making accurate focusing impossible. Furthermore, in situations where the temperature changes frequently, the focus tracking control will be burdened, making it difficult to obtain stable, high-quality image quality. Therefore, each embodiment aims to resolve the effects of temperature changes while taking advantage of the above-mentioned benefits of plastic lenses.

To reduce the effects of temperature changes, when plastic lenses are used, lenses with positive and negative powers are combined and appropriately positioned. Figure 13 is a conceptual diagram showing focus shift due to temperature changes in the zoom lens of Example 2, described below. Focusing on a plastic lens with positive power, as the temperature rises from the initial state (Figure 13(A)), the refractive index decreases, causing the focal position to shift toward the over-focus direction (Figure 13(C)). Conversely, if a plastic lens with negative power is used, as the temperature rises the focal position shifts toward the under-focus direction (Figure 13(B)).

As the temperature drops from the initial state, the directions of these focus shifts reverse. Therefore, each embodiment provides a configuration that cancels out the effects of the positive and negative powers. It also proposes more favorable conditions for achieving a wider angle and smaller size.

Next, the configurations of the zoom lenses of Examples 1 to 4 will be described with reference to Figures 1, 3, 5, and 7. Figures 1, 3, 5, and 7 are cross-sectional views of zoom lenses 1a to 1d of Examples 1 to 4, respectively. The zoom lenses of each Example are photographic lens systems used in imaging devices such as surveillance cameras, digital cameras, or video cameras. In each cross-sectional view, the left side is the subject side (object side), and the right side is the image side. The wide-angle end and telephoto end refer to zoom positions when the magnification-varying lens group is located at both ends of its mechanically movable range on the optical axis. The movement of each lens group from the wide-angle end to the telephoto end follows the trajectory indicated by the arrows (solid lines) in each cross-sectional view. The zoom lenses of each Example have a two-group configuration (Examples 1, 2, and 4) or a three-group configuration (Example 3). This configuration is ideal for achieving high optical performance from the wide-angle end to the telephoto end with a compact, bright FNO while correcting chromatic aberration.

In each cross-sectional view, the solid curve and dotted curve of the first lens group represent the movement locus for correcting image plane fluctuations when focusing on an object at infinity and a close-up object, respectively, from the wide-angle end to the telephoto end zoom position. For example, when focusing from an object at infinity to a close-up object at the telephoto end zoom position, the movement locus shown by arrow F is taken.

The aperture stop (aperture stop) SP is located on the object side of the second lens group and moves during zooming. This has the advantage of not interfering with the formation of the drive locus of multiple lens groups for zooming. However, the aperture stop SP may also be configured not to move during zooming (the aperture stop SP may be fixed). G is an optical block such as an optical filter or faceplate. IP is the image plane, which corresponds to the imaging surface of an imaging element (photoelectric conversion element) such as a CCD sensor or CMOS sensor when used as an imaging optical system.

Figures 2, 4, 6, and 8 are aberration diagrams of zoom lenses 1a to 1d of Examples 1 to 4, respectively. (A) shows the aberration diagram for zoom lenses 1a to 1d at the wide-angle end, (B) shows the aberration diagram for zoom lenses 1a to 1d at the intermediate zoom position, and (C) shows the aberration diagram for zoom lenses 1a to 1d at the telephoto end. In each aberration diagram, d and g indicate the d-line and g-line, respectively, and M and S indicate the meridian image plane and sagittal image plane, respectively. Chromatic aberration of magnification is shown using the g-line. Astigmatism is shown using M and S at the d-line, distortion is shown using the d-line, and chromatic aberration of magnification is shown using the g-line aberration for the d-line.

Next, the movement of the lens groups in the zoom lens of Example 1 will be described with reference to FIG. 12. FIG. 12 is an explanatory diagram of the movement of the lens groups. Varying magnification from the wide-angle end to the telephoto end is achieved by moving the first lens group L11 and the second lens group L12 independently of each other. Specifically, the second lens group moves monotonically from the image side to the object side to vary magnification, while at the same time, the first lens group moves toward the image side and then toward the object side, following a movement trajectory. To correct chromatic aberration while forming such a movement trajectory for each lens group, the configuration and power arrangement of each lens group are appropriately ensured. Focusing is handled by the first lens group L11. Furthermore, to simultaneously correct axial chromatic aberration and lateral chromatic aberration, the second lens group L12, where axial and off-axial light beams overlap, is configured to achieve the objectives of Example 1.

Next, the main features of the zoom lens in each embodiment will be described. The zoom lens in each embodiment has, in order from the object side to the image side, a first lens group with negative refractive power and a second lens group with positive refractive power. The spacing between adjacent lens groups changes during zooming. The zoom lens also has at least one plastic positive lens with an aspherical surface and at least one plastic negative lens with an aspherical surface. The plastic positive lens and plastic negative lens may each be lenses that constitute either lens group. The first lens group has at least one positive lens, and the second lens group has at least two negative lenses.

Let fpi be the focal length of the i-th plastic positive lens, fnj be the focal length of the j-th plastic negative lens, and define 1/fp and 1/fn as the following number A.

Let the Abbe number of at least one positive lens in the first lens group (if there are two or more positive lenses, at least one must satisfy this requirement) be νd1p, the focal length of the first lens group be f1, and the focal length of the second lens group be f2.

In this case, the following conditional expressions (1) to (3) are satisfied.

-1.30<fn/fp<-0.35...(1)
15.00<νd1p<21.00 (2)
-1.70<f1/f2<-0.80...(3)
Conditional formula (1) is a relational expression for a positive lens and a negative lens made of plastic material, and indicates the conditional formula for temperature focus correction when the environmental temperature changes. When a lens system is exposed to temperature changes, the overall focal position and resolving power change. This phenomenon is called thermal aberration. Correcting thermal aberration by selecting appropriate glass and power is called temperature compensation. Thermal aberration is caused by changes in refractive index and linear expansion coefficient due to temperature changes. Plastic materials have large changes in refractive index and linear expansion due to temperature changes, so they need to be appropriately corrected.

If the upper limit of conditional expression (1) is exceeded, the power of the positive lens made of plastic will be weaker than the power of the negative lens made of plastic, and the refractive index of the plastic material will decrease with increasing temperature, resulting in under-focus and making it difficult to eliminate thermal aberrations throughout the optical system. On the other hand, if the lower limit of conditional expression (1) is exceeded, the power of the positive lens made of plastic will be stronger than the power of the negative lens made of plastic, and the refractive index of the plastic material will decrease with increasing temperature, resulting in over-focus and making it difficult to eliminate thermal aberrations throughout the optical system.

Conditional formula (2) stipulates that a high-dispersion glass be used as a positive lens in a position that is effective for correcting chromatic aberration. The first lens group has negative power (refractive power), but there are more single lenses (or cemented lenses) with negative power within the first lens group than those with positive power. Therefore, chromatic aberration is actively generated and canceled by the positive lens so that the chromatic aberration generated by the negative lens is reversed, resulting in effective chromatic aberration correction.

This positive lens element can correct lateral chromatic aberration particularly effectively at the wide-angle end. Furthermore, because this positive lens element is located close to the aperture at the telephoto end, it can correct axial chromatic aberration particularly effectively. Exceeding the upper limit of conditional expression (2) is undesirable because chromatic aberration correction becomes insufficient. On the other hand, exceeding the lower limit of conditional expression (2) is undesirable because chromatic aberration correction becomes excessive.

Conditional expression (3) appropriately sets the relationship between the second lens group responsible for zooming and the first lens group responsible for image plane correction. Exceeding the upper limit of conditional expression (3) weakens the power of the second lens group responsible for zooming, while the power of the first lens group responsible for image plane correction tends to be strong. If the power of the second lens group is weakened too much, a longer movement path is required, which is undesirable for compactness. Furthermore, if the power of the first lens group becomes too strong, the effects of lateral chromatic aberration occur, which is undesirable. On the other hand, if the lower limit of conditional expression (3) is exceeded, the power of the second lens group responsible for zooming becomes stronger, while the power of the first lens group responsible for image plane correction becomes weaker. If the power of the second lens group is too strong, it becomes difficult to correct spherical aberration, which is undesirable. Furthermore, if the power of the first lens group becomes too weak, a larger distance must be provided between the moving groups, increasing the overall length, which is undesirable for compactness.

Preferably, the numerical ranges of conditional expressions (1) to (3) are set so as to satisfy at least one of the following conditional expressions (1a) to (3a):

-1.25<fn/fp<-0.40...(1a)
16.00<νd1p<20.00...(2a)
-1.60<f1/f2<-0.90...(3a)
More preferably, the numerical ranges of the conditional expressions (1) to (3) are set so as to satisfy at least one of the following conditional expressions (1b) to (3b).

-1.20<fn/fp<-0.42...(1b)
17.00<νd1p<18.00...(2b)
-1.50<f1/f2<-1.10...(3b)
In each embodiment, it is preferable that the first lens group has a plastic lens having negative power, and the focal length of the plastic lens having negative power in the first lens group is defined as f1n, and that the following conditional expression (4) be satisfied:

0.10<f1/f1n<0.40 (4)
Conditional expression (4) expresses the relationship between the focal length of the first lens group and the focal length of the lens in that group that is made of plastic and has negative power. If the upper limit of conditional expression (4) is exceeded, the power of the lens made of plastic and having negative power becomes too strong, resulting in a large difference between the lens center thickness and peripheral thickness, making molding difficult. On the other hand, if the lower limit of conditional expression (4) is exceeded, the power of the lens made of plastic and having negative power becomes too weak, reducing the contribution of thermal aberration correction due to temperature changes and making temperature focus correction difficult.

It is also preferable that the second lens group includes a plastic lens having positive power, and the following conditional expression (5) be satisfied, where f2p is the focal length of the plastic lens having positive power in the second lens group:

0.10<f2/f2p<0.58...(5)
Conditional expression (5) expresses the relationship between the focal length of the second lens group and the focal length of the lens in that group that is made of a plastic material and has positive power. If the upper limit of conditional expression (5) is exceeded, the power of the lens made of a plastic material and having positive power becomes too strong, resulting in a large difference between the lens's central thickness and its peripheral thickness, making molding difficult. On the other hand, if the lower limit of conditional expression (5) is exceeded, the power of the lens made of a plastic material and having positive power becomes too weak, reducing the contribution of thermal aberration correction due to temperature changes and making temperature focus correction difficult.

It is also preferable that the following conditional expression (6) be satisfied, where Z is the zoom ratio of the zoom lens (entire system) and M2 is the amount of movement (absolute value) of the second lens group from the wide-angle end to the telephoto end.

2.00<f2/(M2/Z)<3.50...(6)
Conditional expression (6) expresses the relationship between the amount of movement of the second lens group due to magnification change, the focal length, and the zoom ratio. Exceeding the upper limit of conditional expression (6) reduces the amount of movement of the second lens group due to magnification change, necessitating stronger power. This results in significant spherical aberration, which is undesirable for ensuring a bright FNO. On the other hand, exceeding the lower limit of conditional expression (6) increases the amount of movement of the second lens group due to magnification change too much, which is undesirable for reducing the overall length of the zoom lens.

Furthermore, it is preferable that the length (total lens length) of the zoom lens at the wide-angle end from the optical surface closest to the object at the wide-angle end to the image plane be TL, and that the following conditional expression (7) be satisfied:

0.20<M2/TL<0.50...(7)
Conditional expression (7) expresses the relationship between the amount of movement of the second lens due to magnification change and the length of the zoom lens from the optical surface closest to the object at the wide-angle end to the image plane. Exceeding the upper limit of conditional expression (7) results in excessively large fluctuations in the second lens group due to magnification change, which is undesirable for reducing the overall lens length. On the other hand, exceeding the lower limit of conditional expression (7) reduces the amount of fluctuations in the second lens group due to magnification change, necessitating stronger power. This results in significant spherical aberration, which is undesirable for ensuring a bright FNO.

It is also preferable that the average refractive index of all negative lenses in the first lens group, excluding the plastic lens, be 1Gn_NAve, and that the following conditional expression (8) be satisfied:

1.80<1Gn_NAve<1.95...(8)
Conditional expression (8) defines the material characteristics of the negative lenses in the first lens group that do not include lenses made of plastic. Exceeding the upper limit of conditional expression (8) is undesirable because it affects the Petzval sum balance of the entire lens system and increases field curvature. On the other hand, exceeding the lower limit of conditional expression (8) reduces the refractive index and increases the size of the lens system as the first lens group, which is undesirable from the perspective of compactness.

It is also preferable that the average Abbe number of all negative lenses in the second lens group be 2Gn_νAve, and that the following conditional expression (9) be satisfied:

15.00<2Gn_νAve<45.00 (9)
Conditional expression (9) defines the material characteristics of the negative lens in the second lens group. Exceeding the upper limit of conditional expression (9) undesirably results in excessive correction of chromatic aberration. On the other hand, exceeding the lower limit of conditional expression (9) undesirably results in insufficient correction of lateral chromatic aberration.

Furthermore, preferably, the difference between the largest and smallest Abbe numbers of the positive lenses in the second lens group is 2Gp_ν, and the following conditional expression (10) is satisfied:

38.00<2Gp_ν<65.00 (10)
Conditional expression (10) defines the material characteristics of the positive lens included in the second lens group. Exceeding the upper limit of conditional expression (10) undesirably results in overcorrection of chromatic spherical aberration for the g-line. On the other hand, exceeding the lower limit of conditional expression (10) undesirably results in undercorrection of chromatic spherical aberration for the g-line.

More preferably, the numerical ranges of conditional expressions (4) to (10) are set so as to satisfy at least one of the following conditional expressions (4a) to (10a):

0.15<f1/f1n<0.39 (4a)
0.10<f2/f2p<0.57...(5a)
2.30<f2/(M2/Z)<3.30...(6a)
0.25<M2/TL<0.45...(7a)
1.85<1Gn_NAve<1.94...(8a)
16.00<2Gn_νAve<44.00...(9a)
40.00<2Gp_ν<63.00...(10a)
More preferably, the numerical ranges of the conditional expressions (4) to (10) are set so as to satisfy at least one of the following conditional expressions (4b) to (10b).

0.18<f1/f1n<0.38...(4b)
0.20<f2/f2p<0.56...(5b)
2.60<f2/(M2/Z)<3.00...(6b)
0.30<M2/TL<0.42...(7b)
1.88<1Gn_NAve<1.92...(8b)
17.00<2Gn_νAve<43.00...(9b)
42.00<2Gp_ν<60.00...(10b)
The configuration of the zoom lens of each embodiment will be described in detail below.

First, the configuration of the zoom lens 1a in Example 1 will be described with reference to Figure 1. Figure 1 is a cross-sectional view of the zoom lens 1a. The zoom lens 1a has a three-group configuration consisting of, in order from the object side to the image side, a first lens group L11, a second lens group L12, and a third lens group L13 having positive refractive power.

The first lens group L11 consists, from the object side to the image side, of a biconcave negative lens G111, a negative lens G112 with a paraxial convex shape facing the object side, a meniscus positive lens G113 with a convex shape facing the object side, and a meniscus negative lens G114 with a convex shape facing the object side. The second lens group L12 consists, from the object side to the image side, of a meniscus positive lens G121 with a convex shape facing the object side, a biconvex positive lens G122, a biconcave negative lens G123, a biconvex positive lens G124, and a meniscus negative lens G125 with a concave shape facing the object side. The third lens group L13 consists of a biconvex positive lens G131. The meniscus positive lens G113 and the meniscus negative lens G114 are cemented together to form a cemented lens. Cemented lenses can effectively correct chromatic aberration by providing differences in Abbe numbers and refractive indices. The negative lens G112, the positive meniscus lens G121, and the negative meniscus lens G125 are aspheric plastic lenses.

Next, the configuration of zoom lens 1b in Example 2 will be described with reference to Figure 3. Figure 3 is a cross-sectional view of zoom lens 1b. Zoom lens 1b has a three-group configuration consisting of, in order from the object side to the image side, a first lens group L21, a second lens group L22, and a third lens group L23 having positive refractive power.

The first lens group L21 consists, from the object side to the image side, of a biconcave negative lens G211, a paraxially convex negative lens G212, a meniscus positive lens G213 with a convex surface toward the object side, and a meniscus negative lens G214 with a convex surface toward the object side. The second lens group L22 consists, from the object side to the image side, of a meniscus positive lens G221 with a convex surface toward the object side, a biconvex positive lens G222, a biconcave negative lens G223, a biconvex positive lens G224, and a meniscus negative lens G225 with a concave surface toward the object side. The third lens group L23 consists of a biconvex positive lens G231. The meniscus positive lens G213 and the meniscus negative lens G214 are cemented together to form a cemented lens. Cemented lenses can effectively correct chromatic aberrations by providing differences in Abbe numbers and refractive indices. The negative lens G212, the positive meniscus lens G221, and the negative meniscus lens G225 are plastic lenses with aspheric surfaces.

Next, the configuration of the zoom lens 1c in Example 3 will be described with reference to Figure 5. Figure 5 is a cross-sectional view of the zoom lens 1c. The zoom lens 1c has a two-group configuration consisting of, in order from the object side to the image side, a first lens group L31 and a second lens group L32.

The first lens group L31 consists, from the object side to the image side, of a biconcave negative lens G311, a meniscus negative lens G312 that is convex toward the object side in a paraxial direction, a biconvex positive lens G313, and a biconcave negative lens G314. The second lens group L32 consists, from the object side to the image side, of G321 to G326. G321 is a meniscus positive lens that is convex toward the object side, G322 is a biconvex positive lens, and G323 is a meniscus negative lens that is convex toward the object side. G324 is a biconvex positive lens, G325 is a meniscus negative lens that is concave toward the object side, and G326 is a meniscus positive lens that is convex toward the object side. The positive lens G313 and the negative lens G314 are cemented together to form a cemented lens. Cemented lenses can effectively correct chromatic aberration by providing differences in Abbe numbers and refractive indexes. The meniscus negative lens G312, the meniscus positive lens G321, and the meniscus negative lens G325 are plastic lenses with aspheric surfaces.

Next, the configuration of the zoom lens 1d in Example 4 will be described with reference to Figure 7. Figure 7 is a cross-sectional view of the zoom lens 1d. The zoom lens 1d has a three-group configuration consisting of, in order from the object side to the image side, a first lens group L41, a second lens group L42, and a third lens group L43 having negative refractive power.

The first lens group L41 consists, from the object side to the image side, of a biconcave negative lens G411, a meniscus negative lens G412 with a convex shape facing the object side in a paraxial direction, a biconvex positive lens G413, and a biconcave negative lens G414. The second lens group L42 consists, from the object side to the image side, of a biconvex positive lens G421, a biconvex positive lens G422, a meniscus negative lens G423 with a convex shape facing the object side, and a biconvex positive lens G424. The third lens group L43 consists of a meniscus negative lens G431 with a concave shape facing the object side and a meniscus negative lens G432 with a concave shape facing the object side. The positive lens G413 and the negative lens G414 are cemented together to form a cemented lens. Cemented lenses can effectively correct chromatic aberration by providing differences in Abbe numbers and refractive indices. The meniscus negative lens G412, the positive lens G421, and the meniscus negative lens G432 are plastic lenses with aspheric surfaces.

Next, we will explain the effect of temperature-induced focus correction using the zoom lens of Example 2. Table 2 shows the amount of change in focus movement for the plastic negative lens G212 and the meniscus positive lens G221. At a temperature of 25°C, the focus position is the same from the wide-angle end to the telephoto end, but the amount of shift varies depending on the zoom position due to changes in the refractive index of the plastic lens, and these conditions are shown. The impact of focus movement is particularly significant at the telephoto end, but the arrangement and conditions are such that this tends to be canceled out by the combination of the positive and negative lenses. As a result, the focus movement fluctuation range is kept constant from the wide-angle end to the telephoto end. This is not limited to this situation, and the amount of movement can be suppressed by adding additional plastic lenses, for example.

In each embodiment, the following configurations may be adopted. For example, the shapes and numbers of glass elements shown in each embodiment are not limited to those and may be modified as appropriate. Some lenses and lens groups may be moved so that they have a component perpendicular to the optical axis, thereby correcting image blur caused by vibrations such as camera shake. Distortion and chromatic aberration may also be corrected using electrical correction means. While focusing is performed using the first lens group, this is not limited to this, and other lens groups may also be used, and focusing may also be performed by moving the image sensor.

Numerical Examples 1 to 4 corresponding to Examples 1 to 4 are shown below. In each numerical example, the surface number of the ith surface is shown in order from the object side, r is the radius of curvature, d is the distance between the ith surface and the (i+1)th surface (lens thickness or air distance), and nd and vd are the refractive index and Abbe number of the material of the ith lens based on the d-line, respectively. Note that the Abbe number vd of a certain material is given by the following equation, 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:
νd=(Nd-1)/(NF-NC)
It is expressed as:

In each numerical example, d, focal length (mm), F-number, and half angle of view (°) are all values when the optical system (zoom lens) of each example is focused on an object at infinity. BF (back focus) is the distance on the optical axis from the final lens surface (the lens surface closest to the image) to the paraxial image plane, expressed as an air-equivalent length, and is the value when the glass block is not included. "Total lens length" is the distance on the optical axis from the frontmost lens surface (the lens surface closest to the object) of the zoom lens to the final surface plus the back focus. "Lens group" is not limited to cases where it is composed of multiple lenses, but also includes cases where it is composed of a single lens. In each numerical example, the two surfaces closest to the image are flat surfaces corresponding to the optical block G.

If the optical surface is aspherical, a symbol "*" is added to the right of the surface number. When the displacement in the optical axis direction at a position of optical axis height h is x with respect to the vertex of the surface, the aspherical shape is as follows:
x=(h ² /r)/[1+{1-(1+K)(h/r) ² } ^1/2 ]+A4 ・h ⁴ +A6・h ⁶ +A8・h ⁸
Here, r is the paraxial radius of curvature, K is the conic constant, and A4, A6, A8, A10, and A12 are the fourth-order, sixth-order, and eighth-order aspherical coefficients, respectively. For example, the notation "e-Z" means "10 ^-Z ."
The angle of view is the half angle of view (ω) value for the photographable angle of view taking distortion into consideration.

[Numerical Example 1]
Unit: mm

Surface data surface number rd nd νd
1 -32.482 0.60 1.90043 37.4
2 15.807 1.26
3* 21.502 0.80 1.53110 55.9
4* 12.653 0.15
5 13.433 1.90 1.95906 17.5
6 62.750 0.55 1.90043 37.4
7 19.252 (variable)
8 (Aperture) ∞ 0.10
9* 8.113 1.85 1.53110 55.9
10* 22.268 0.15
11 7.472 3.65 1.49700 81.5
12 -6.979 0.15
13 -17.328 0.45 1.75520 27.5
14 6.049 0.31
15 7.726 2.20 1.89190 37.1
16 -47.796 2.54
17* -4.451 0.70 1.53110 55.9
18* -15.114 (variable)
19 35.150 1.10 1.95906 17.5
20 -147.517 1.80
21 ∞ 1.00 1.51000 60.0
22∞0.89
Image plane ∞

Aspherical surface data No. 3
K = 0.00000e+000 A 4=-1.38911e-003 A 6= 2.76165e-005 A 8=-2.26532e-007
A10=-8.87005e-011

Side 4
K = 0.00000e+000 A 4=-1.37216e-003 A 6= 3.04798e-005 A 8=-2.72614e-007

9th page
K = 0.00000e+000 A 4=-3.76420e-004 A 6=-2.01537e-005 A 8=-1.12282e-006
A10=-4.03455e-008

Side 10
K = 0.00000e+000 A 4= 3.72199e-004 A 6= 1.57634e-005 A 8=-3.02485e-006
A10= 1.44343e-007

Page 17
K = 0.00000e+000 A 4=-9.97664e-004 A 6= 8.65293e-005 A 8=-5.54709e-006

Page 18
K = 0.00000e+000 A 4=-9.95094e-004 A 6= 5.67334e-005 A 8=-2.60511e-006

Various data Zoom ratio 4.90
Wide Angle Mid Telephoto Focal Length 4.95 14.60 24.25
F-number 2.04 3.53 5.04
Half angle of view 45.3 12.6 7.55
Image height 3.20 3.20 3.20
Lens length 38.85 33.61 38.85
BF 3.35 3.35 3.35

d7 16.64 3.49 0.80
d18 0.40 8.32 16.24

Zoom lens group data group Initial surface Focal length
1 1 -12.57
2 8 8.99
3 19 29.69

[Numerical Example 2]
Unit: mm

Surface data surface number rd nd νd
1 -27.510 0.60 1.90043 37.4
2 15.553 1.18
3* 35.393 0.80 1.53110 55.9
4* 12.011 0.29
5 15.349 1.68 1.95906 17.5
6 274.059 0.74 1.90043 37.4
7 40.138 (variable)
8 (Aperture) ∞ 0.10
9* 9.270 1.85 1.53110 55.9
10* 15.091 0.15
11 7.602 3.77 1.49700 81.5
12 -7.425 0.15
13 -20.874 0.45 1.76182 26.5
14 5.777 0.19
15 6.236 3.50 1.90525 35.0
16 -37.125 2.59
17 -4.833 0.45 1.90043 37.4
18 -11.402 (variable)
19 27.603 1.10 1.95906 17.5
20 -225.267 0.90
21 ∞ 0.50 1.52000 61.4
22∞2.12
Image plane ∞

Aspherical surface data No. 3
K = 0.00000e+000 A 4=-1.07518e-003 A 6= 3.00829e-005 A 8=-3.59433e-007
A10= 6.76416e-010

Side 4
K = 0.00000e+000 A 4=-1.10161e-003 A 6= 3.10058e-005 A 8=-3.62250e-007

9th page
K = 0.00000e+000 A 4=-5.15188e-004 A 6=-2.52871e-005 A 8=-1.98710e-006
A10= 5.48647e-008

Side 10
K = 0.00000e+000 A 4= 2.11472e-004 A 6=-2.87904e-006 A 8=-2.53751e-006
A10= 1.52964e-007

Various data Zoom ratio 4.67
Wide Angle Mid Telephoto Focal Length 4.95 14.03 23.10
F-number 1.90 3.24 4.81
Half angle of view 44.0 13.1 7.91
Image height 3.20 3.20 3.20
Lens length 38.84 33.94 38.85
BF 3.34 3.34 3.34

d7 15.51 3.02 0.35
d18 0.40 7.98 15.56

Zoom lens group data group Initial surface Focal length
1 1 -12.58
2 8 8.93
3 19 25.69

[Numerical Example 3]
Unit: mm

Surface data surface number rd nd νd
1 -20.735 0.60 1.91082 35.3
2 18.281 0.94
3* 21.435 0.80 1.53110 55.9
4* 12.215 0.44
5 16.500 1.90 1.95906 17.5
6 -110.523 0.50 1.90366 31.3
7 25.071 (variable)
8 (Aperture) ∞ 0.10
9* 8.411 1.85 1.53110 55.9
10* 312.581 0.15
11 10.770 3.65 1.49700 81.5
12 -9.491 0.15

13 24.311 0.45 1.95906 17.5
14 6.742 0.63
15 13.297 2.20 1.80810 22.8
16 -55.066 2.54
17* -8.899 0.70 1.53110 55.9
18* 78.183 0.71
19 15.753 1.10 1.72825 28.5
20 32.959 (variable)
21 ∞ 0.50 1.52000 61.4
22∞0.10
Image plane ∞

Aspherical surface data No. 3
K = 0.00000e+000 A 4=-1.56767e-003 A 6= 2.68801e-005 A 8=-3.57384e-008
A10=-9.69653e-010

Side 4
K = 0.00000e+000 A 4=-1.57874e-003 A 6= 3.20799e-005 A 8=-1.78537e-007

9th page
K = 0.00000e+000 A 4=-5.37390e-004 A 6=-1.15485e-005 A 8=-1.41081e-006
A10=-2.51415e-008

Side 10
K = 0.00000e+000 A 4= 2.04033e-004 A 6= 3.75107e-006 A 8=-2.42379e-006
A10= 3.15273e-008

Page 17
K = 0.00000e+000 A 4=-1.62955e-003 A 6=-2.62824e-005 A 8=-1.19267e-006

Side 18
K = 0.00000e+000 A 4=-1.51173e-003 A 6=-3.38248e-006 A 8= 4.82601e-007

Various data Zoom ratio 4.30
Wide Angle Mid Telephoto Focal Length 4.90 12.98 21.05
F-number 2.04 3.21 4.40
Half angle of view 48.4 14.4 8.74
Image height 3.20 3.20 3.20
Lens length 38.83 33.07 36.84
BF 3.33 9.98 16.64

d7 16.10 3.69 0.80
d20 2.90 9.55 16.21

Zoom lens group data group Initial surface Focal length
1 1 -10.89
2 8 8.97

[Numerical Example 4]
Unit: mm

Surface data surface number rd nd νd
1 -28.121 0.60 1.90366 31.3
2 15.944 1.13
3* 16.743 0.80 1.53110 55.9
4* 10.628 0.42
5 15.957 1.90 1.95906 17.5
6 -218.620 0.50 1.89190 37.1
7 23.126 (variable)
8 (Aperture) ∞ 0.10
9* 10.094 1.85 1.53110 55.9
10* -116.660 0.15
11 8.233 3.54 1.49700 81.5
12 -9.458 0.21
13 138.285 0.45 1.95906 17.5
14 9.050 1.16
15 14.795 1.93 1.80810 22.8
16 -34.196 (variable)
17 -7.437 0.60 1.49700 81.5
18 -16.708 0.82
19* -10.720 1.10 1.53110 55.9
20* -31.374 (variable)
21 ∞ 0.50 1.52000 61.4
22∞0.20
Image plane ∞

Aspherical surface data No. 3
K = 0.00000e+000 A 4=-1.95946e-003 A 6= 4.13917e-005 A 8=-3.05660e-007
A10=-2.27906e-009

Side 4
K = 0.00000e+000 A 4=-1.98762e-003 A 6= 4.93090e-005 A 8=-5.46167e-007

9th page
K = 0.00000e+000 A 4=-4.06064e-004 A 6=-8.44879e-006 A 8=-1.37935e-006
A10= 2.19280e-008

Side 10
K = 0.00000e+000 A 4= 2.12076e-004 A 6= 5.36167e-006 A 8=-1.73335e-006
A10= 5.67389e-008

Page 19
K = 0.00000e+000 A 4=-3.01869e-003 A 6=-8.28083e-005 A 8=-3.88417e-006
A10= 2.25051e-007

Page 20
K = 0.00000e+000 A 4=-2.34441e-003 A 6=-4.40007e-005 A 8= 2.32504e-006

Various data Zoom ratio 4.30
Wide Angle Mid Telephoto Focal Length 4.90 12.93 21.07
F-number 2.04 3.25 4.50
Half angle of view 46.7 14.4 8.72
Image height 3.20 3.20 3.20
Lens length 38.84 32.03 35.04
BF 3.34 9.31 15.28

d7 16.70 3.85 0.80
d16 1.54 1.62 1.71
d20 2.81 8.78 14.75

Zoom lens group data group Initial surface Focal length
1 1 -11.49
2 8 8.03
3 17 -14.36

Table 3 shows the relationship between the above-mentioned conditional expressions and the respective numerical examples.

Next, with reference to Figures 9 to 11, an imaging device (surveillance camera) using the zoom lens of each embodiment as an imaging optical system will be described. Figures 9 to 11 are structural diagrams of the imaging device in each embodiment. In Figures 9 to 11, 16 denotes an imaging optical system configured with any of the zoom lenses of Examples 1 to 4. In Figures 9 and 11(B), 15 denotes a dome cover that protects the imaging optical system 16. The dome cover 15 is molded with a thickness of approximately several millimeters from a plastic material such as polymethyl methacrylate (PMMA) or polycarbonate (PC). Therefore, when designing an imaging device that is intended to include a dome cover, the influence of the dome cover 15 (focal length and material) can be taken into consideration in the design, and various aberrations can be corrected. In Figure 10, 17 denotes a flat protective cover that protects the imaging optical system 16.

In Figures 11(A) and (B), 11a and 11b denote surveillance camera bodies. Figure 11(B) shows an example in which a dome cover 15 is attached to the surveillance camera body 11b and the camera is mounted on a ceiling. The surveillance camera body 11b is installed on the ceiling with the dome cover 15 facing downward. 12 denotes an imaging element (photoelectric conversion element) such as a CCD sensor or CMOS sensor built into the surveillance camera body 11a or 11b, which receives the subject image formed by the imaging optical system 16 (capturing the image formed by the zoom lens). 13 denotes memory that records information corresponding to the subject image photoelectrically converted by the imaging element 12. 14 denotes a network cable for transferring the subject image photoelectrically converted by the imaging element 12. Note that the zoom lenses in each embodiment are not limited to surveillance cameras and can also be used in other imaging devices, such as video cameras and digital cameras. Additionally, the imaging device of each embodiment may include a circuit that electrically corrects either distortion or lateral chromatic aberration, or both, along with the zoom lens of any of embodiments 1 to 5.

Each embodiment can provide a zoom lens and an imaging device that are advantageous in terms of, for example, compact size, light weight, and high optical performance.

The above describes preferred embodiments of the present invention, but the present invention is not limited to these embodiments and various modifications and variations are possible within the scope of the invention.

1a, 1b, 1c, 1d: zoom lenses L11, L21, L31, L41: first lens group L12, L22, L32, L42: second lens group

Claims (11)

A zoom lens having, in order from the object side to the image side, a first lens group having negative refractive power and a second lens group having positive refractive power,
The spacing between adjacent lens groups changes during zooming,
the zoom lens has at least one plastic positive lens having an aspherical surface and at least one plastic negative lens having an aspherical surface;
the first lens group has at least one positive lens;
the second lens group has at least two negative lenses;
The focal length of the i-th plastic positive lens is defined as fpi, and the focal length of the j-th plastic negative lens is defined as fnj,

year,
The focal length of the first lens group is f1, the focal length of the second lens group is f2,
-1.30<fn/fp<-0.35
-1.70<f1/f2<-0.80
The conditional expression is satisfied,
The at least one positive lens in the first lens group has an Abbe number νd1p, and
15.00<νd1p<21.00
1. A zoom lens comprising a positive lens that satisfies the following condition: The first lens group has a focal length of f1n,
0.10<f1/f1n<0.40
2. The zoom lens according to claim 1, further comprising a plastic negative lens that satisfies the following condition: The second lens group has a focal length of f2p,
0.10<f2/f2p<0.58
3. The zoom lens according to claim 1, further comprising a plastic positive lens element that satisfies the following condition: Let Z be the zoom ratio, and M be the amount of movement of the second lens group from the wide-angle end to the telephoto end,
2.00<f2/(M2/Z)<3.50
4. The zoom lens according to claim 1, wherein the following condition is satisfied: Let M2 be the amount of movement of the second lens group from the wide-angle end to the telephoto end, and TL be the length of the zoom lens at the wide-angle end from the optical surface closest to the object to the image plane,
0.20<M2/TL<0.50
5. The zoom lens according to claim 1, wherein the following condition is satisfied: The average refractive index of all negative lenses in the first lens group excluding the plastic lens is set to 1Gn_NAve,
1.80<1Gn_NAve<1.95
6. The zoom lens according to claim 1, wherein the following condition is satisfied: Let 2Gn_νAve be the average Abbe number of the negative lenses in the second lens group,
15.00<2Gn_νAve<45.00
7. The zoom lens according to claim 1, wherein the following condition is satisfied: The difference between the largest Abbe number and the smallest Abbe number among the Abbe numbers of the positive lenses in the second lens group is defined as 2Gp_ν,
38.00<2Gp_ν<65.00
8. The zoom lens according to claim 1, wherein the following condition is satisfied: 9. The zoom lens according to claim 1, wherein the zoom lens comprises, in order from the object side to the image side, the first lens group, the second lens group, and a third lens group having positive or negative refractive power. A zoom lens according to any one of claims 1 to 8, characterized in that the zoom lens is composed of, in order from the object side to the image side, the first lens group and the second lens group. a zoom lens according to any one of claims 1 to 10;
an image sensor for capturing an image formed by the zoom lens.