JP5592159B2 - Magnification optical system and imaging device - Google Patents

Description

  The present invention relates to a variable magnification optical system and an imaging apparatus, and more specifically, a variable magnification optical system that can be used for a video camera, an electronic still camera, and the like, and can be suitably used particularly for a surveillance camera application, and the variable magnification optical system. The present invention relates to an imaging apparatus provided.

  Conventionally, surveillance cameras are used for crime prevention and recording purposes. Such an optical system for a surveillance camera is required to be small and inexpensive, to have a large aperture ratio and to have high optical performance so that a subject can be specified even under low illumination conditions. In recent years, the demand for surveillance cameras with a zooming function has increased, and zooming optical systems are becoming mainstream.

  Conventionally known zoom lenses for cameras include those described in Patent Documents 1 and 2 below. In Patent Document 1, a first lens group having a negative refractive power in a three-lens configuration and a second lens group having a positive refractive power in a three-lens configuration are arranged in order from the object side. A zoom lens in which both positive and negative plastic lenses are arranged in the lens group and the second lens group is described. In Patent Document 2, a negative first lens group and a positive second lens group are arranged in order from the object side, and both positive and negative plastic lenses are used as the second lens group or the first lens. A zoom lens arranged in the first lens group and the second lens group is described.

  Further, as a conventionally proposed variable power optical system for a surveillance camera, for example, there is one described in Patent Document 3 below. In Patent Document 3, a negative first lens group, a stop, and a positive second lens group are arranged in order from the object side, and at least one positive plastic lens and at least one negative lens group are arranged. A variable power optical system in which a plastic lens is arranged in a second lens group is described.

Japanese Patent No. 3333473 Japanese Patent No. 2856464 JP 2008-310133 A

  Currently, high-performance lenses with a wide angle and a large aperture ratio are the mainstream lenses for surveillance cameras, and in recent years, there has been a strong demand for cost reduction. One method for promoting cost reduction is the use of plastic lenses. However, although the optical systems described in Patent Documents 1 and 2 have a configuration in which plastic lenses are frequently used, there are problems such as a large F value, a small angle of view, and a small zoom ratio. In addition, all of the optical systems described in Patent Documents 1 to 3 have a plastic lens disposed closest to the image side. The light flux that passes through the lens on the most image side is relatively thin, and plastic lenses are more susceptible to dust due to static electricity than glass lenses, so if dust adheres to the lens surface on the optical path of this plastic lens, There is a problem that the influence on the image is large.

  A plastic lens is advantageous in that it is cheaper and lighter than a glass lens and has a high degree of freedom in shape. For this reason, it has been considered difficult to adopt plastic lenses for surveillance camera lenses that are used outdoors with large temperature changes.

  The present invention has been made in view of the above circumstances, and a variable power optical system capable of suppressing cost deterioration and reducing costs while maintaining small size, wide angle, large aperture ratio and high performance, and cost reduction An object of the present invention is to provide an imaging apparatus provided with the variable magnification optical system.

The variable magnification optical system of the present invention includes, in order from the object side, a first lens group having a negative refractive power, a stop, and a second lens group having a positive refractive power, and the first lens group and the second lens group. A zooming optical system that zooms by changing an interval in the optical axis direction of a lens group, and the first lens group includes a negative meniscus lens, a negative lens, and a positive lens in order from the object side. The second lens group is composed of four lenses, and an aspheric plastic lens having a positive refractive power is disposed on the most object side of the second lens group, and an aspheric plastic lens having a negative refractive power is the second lens group. is arranged from the object side of the lens unit second, the focal length of the aspherical plastic lens having a positive refractive power on the most object side in the second lens group and fpp, second from the object side in the second lens group aspheric flop having a negative refractive power arranged to The focal length of the stick lens is fpn, when the focal length of the entire system at the wide angle end is set to fw, the following conditional expressions (1 B), is characterized in satisfying the (2 B).
3.8 <fpp / fw <5.0 (1 B )
−11.0 <fpn / fw < −9.0 (2 B )

In the case of using a plastic lens for cost reduction, it is conceivable to reduce the refractive power of the plastic lens as one method for suppressing the performance change due to the plastic lens when the temperature changes. However, with this method, it is difficult to realize a compact, wide angle, large aperture ratio, high performance optical system that is the object of the present invention. In view of this, the variable magnification optical system of the present invention is configured to include both a plastic lens having a positive refractive power and a plastic lens having a negative refractive power so as to cancel the influence of temperature changes. This eliminates the need to weaken the refractive power of the plastic lens, increases the degree of freedom in design, and enables favorable aberration correction with a small number of lenses. Furthermore, the variable magnification optical system of the present invention realizes high optical performance while maintaining a small size, wide angle, and large aperture ratio by setting a suitable range of refractive power of positive and negative plastic lenses. .

In the variable power optical system of the present invention, when the νdpp an Abbe number at the d-line of the aspherical plastic lens having a positive refractive power on the most object side in the second lens group satisfies the following conditional expression (3) It is preferable.
νdpp> 54.0 (3)

In the variable power optical system of the present invention, when an Abbe number at the d-line of the aspherical plastic lens having a negative refractive power arranged from the object side in the second lens unit to the second and Nyudpn, the following conditional expression It is preferable to satisfy (4).
νdpn <29.0 (4)

In the variable power optical system of the present invention, the object-side surface of the aspherical plastic lens having a negative refractive power arranged from the object side in the second lens unit to the second is a concave in the vicinity of the optical axis, It is preferable to have an inflection point within the effective diameter. The “effective diameter” referred to here is the outermost (in the direction away from the optical axis) of the light beams included in these light beams when an axial light beam and an off-axis light beam are incident to the maximum diameter based on the specifications. This is the diameter determined by the ray height of the side) rays. As specifications, the F value, the angle of view, the image height, the diameter of a stop that blocks a predetermined light beam, and the like can be considered.

The “aspheric plastic lens” in the present invention is a lens made of a plastic material, and at least one of the object side surface and the image side surface is an aspherical lens. The sign of the refractive power of the lens in the present invention described above is considered in the paraxial region for an aspheric lens. The focal lengths of conditional expressions ( 1B) and (2B) are in the paraxial region.

  Note that the number of lenses described above is the number of lenses that are constituent elements. For example, the number of lenses in a cemented lens in which a plurality of single lenses of different materials are cemented is represented by the number of single lenses constituting the cemented lens.

  An image pickup apparatus according to the present invention includes the variable magnification optical system according to the present invention described above.

The zoom optical system of the present invention includes, in order from the object side, a negative first lens group, a stop, and a positive second lens group, and an interval between the first lens group and the second lens group in the optical axis direction. In the variable magnification optical system configured to change the magnification by changing the first lens group, the lens shape and power arrangement of the first lens group are suitably set, and the second lens group has positive and negative aspheric plastic lenses. These positive and negative aspheric plastic lenses are effectively arranged, and the power of the positive and negative aspheric plastic lenses in this state is suitably set, so the small, wide angle, large aperture ratio In addition, while maintaining high performance, performance degradation due to temperature change can be suppressed, and cost reduction can be realized.

  Since the imaging device of the present invention includes the variable magnification optical system of the present invention, it can be configured to be small and inexpensive, can capture images with a wide angle of view, and can achieve high image quality even in environments with low illuminance and large temperature changes. Can be obtained.

1A and 1B are cross-sectional views showing lens configurations at the wide-angle end and the telephoto end of the variable magnification optical system according to Example 1 of the present invention, respectively. 2A and 2B are cross-sectional views showing lens configurations at the wide-angle end and the telephoto end of the variable magnification optical system according to Example 2 of the present invention, respectively. 3A and 3B are cross-sectional views showing lens configurations at the wide-angle end and the telephoto end of the variable magnification optical system according to Example 3 of the present invention, respectively. 4A and 4B are cross-sectional views showing lens configurations at the wide-angle end and the telephoto end of the variable magnification optical system according to Example 4 of the present invention, respectively. FIGS. 5A to 5H are graphs showing aberrations of the variable magnification optical system according to Example 1 of the present invention. FIGS. 6A to 6H are graphs showing aberrations of the variable magnification optical system according to Example 2 of the present invention. FIGS. 7A to 7H are diagrams showing aberrations of the variable magnification optical system according to Example 3 of the present invention. 8A to 8H are aberration diagrams of the variable magnification optical system according to Example 4 of the present invention. 1 is a schematic configuration diagram of an imaging apparatus according to an embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, a variable magnification optical system according to the first embodiment of the present invention will be described with reference to FIGS. 1 (A) and 1 (B). FIGS. 1A and 1B are cross-sectional views showing a configuration example of a variable magnification optical system according to the first embodiment of the present invention, and correspond to a variable magnification optical system of Example 1 described later. ing.

  The variable magnification optical system according to the first embodiment of the present invention has a first lens group G1 having a negative refractive power, an aperture stop St, and a positive refractive power in order from the object side along the optical axis Z. The second lens group G2 is provided, and zooming is performed by changing the distance in the optical axis direction between the first lens group G1 and the second lens group G2, and the first lens group G1 is negative in order from the object side. The second lens group G2 includes a meniscus lens, a negative lens, and a positive lens, and includes a single aspheric plastic lens having a positive refractive power and a single aspheric plastic lens having a negative refractive power. It is configured.

Further, in the variable magnification optical system according to the first embodiment of the present invention, an aspheric plastic lens having a positive refractive power is disposed on the most object side of the second lens group G2, and the positive lens of the second lens group G2 is arranged as described above. The focal length of the aspheric plastic lens having refractive power is denoted by fpp, the focal length of the aspheric plastic lens having negative refractive power of the second lens group G2 is denoted by fpn, and the focal length of the entire system at the wide angle end is denoted by fw. When configured, the following conditional expressions (1A) and (2A) are satisfied.
3.5 <fpp / fw <5.0 (1A)
−11.0 <fpn / fw <−3.0 (2A)

  The first lens group G1 in the example shown in FIGS. 1A and 1B includes a lens L11, a lens L12, and a lens L13 arranged in order from the object side, and the second lens group G2 includes: A lens L21, a lens L22, a lens L23, and a lens L24 are arranged in this order from the object side.

  1A and 1B, the left side is the object side, the right side is the image side, and FIG. 1A shows the lens arrangement at the infinite focus at the wide angle end. Reference numeral 1 (B) shows the lens arrangement at the infinite focus at the telephoto end. The arrows between FIG. 1 (A) and FIG. 1 (B) schematically show the movement locus of each lens group when zooming from the wide-angle end to the telephoto end. Further, the aperture stop St shown in FIGS. 1 (A) and 1 (B) does not necessarily indicate the size or shape, but indicates the position on the optical axis Z.

  When applying the variable magnification optical system to the image pickup apparatus, a cover glass, a prism, an infrared cut filter, a low-pass filter, etc. are provided between the optical system and the image plane Sim depending on the configuration of the camera side where the lens is mounted. Various filters and the like are preferably arranged. In FIGS. 1A and 1B, a parallel plate-like optical member PP that assumes these is arranged between the second lens group G2 and the image plane Sim. An example is shown.

  The operational effects of the variable magnification optical system according to the first embodiment of the present invention will be described below. A retrofocus type configuration in which a negative lens group and a positive lens group are arranged in order from the object side has characteristics that it is suitable for widening the angle and that it is relatively easy to secure a back focus. By making the most object side lens of the first lens group G1 a negative meniscus lens, it becomes easy to widen the angle. By using the two lenses on the object side of the first lens group G1 as negative lenses, the negative power necessary for the first lens group G1 can be dispersed to these two lenses, and aberration correction becomes easy.

  The second lens group G2 is configured to include one aspheric plastic lens having a positive refractive power and one aspheric plastic lens having a negative refractive power. Plastic lenses are cheaper and lighter than glass lenses and have a high degree of freedom in shape, but performance changes due to temperature changes are larger than glass lenses. Since the change in refractive power is remarkable among the performance changes due to temperature changes, this variable magnification optical system is configured to include a plastic lens having both positive and negative refractive powers, so that positive and negative during temperature changes Counteract the effects of plastic lenses.

  In addition, by arranging both positive and negative plastic lenses in the second lens group G2, it is easy to ensure performance in units of lens groups when the temperature changes. The second lens group G2 is an image side lens group, and is positioned relatively far from the most object side lens in contact with the outside world. Therefore, the environment changes more than when a plastic lens is disposed in the first lens group G1. It is possible to suppress changes in optical performance due to.

  Further, by using these positive and negative plastic lenses as aspherical lenses, it is possible to correct aberrations satisfactorily, and at the same time, it is possible to configure the glass lens at a lower cost than when aspherical lenses are used.

  By arranging the positive aspheric plastic lens on the most object side of the second lens group G2, spherical aberration when the divergent light beam after passing through the negative first lens group G1 is converged by the second lens group G2 is reduced. Correction is facilitated and a large aperture ratio is easily realized.

  In the configuration of the first embodiment, an aspheric plastic lens is disposed closest to the object side of the second lens group G2, and the positive and negative aspheric plastic lenses of the second lens group G2 are further defined by the conditional expression (1A) and the condition. By configuring so as to satisfy Expression (2A), it is possible to satisfactorily correct spherical aberration, curvature of field, and lateral chromatic aberration.

  Specifically, if the lower limit of conditional expression (1A) is not reached, spherical aberration and field curvature will be overcorrected at the wide angle end, and spherical aberration and field curvature will be undercorrected at the telephoto end. If the upper limit of conditional expression (1A) is exceeded, high-order spherical aberration will occur and correction will not be possible. If the lower limit of conditional expression (2A) is not reached, spherical aberration will be overcorrected at the telephoto end. If the upper limit of conditional expression (2A) is exceeded, spherical aberration and curvature of field become insufficiently corrected at the telephoto end.

More preferably, one or both of the following conditional expressions (1A-1) and (2A-1) are satisfied.
3.6 <fpp / fw <4.7 (1A-1)
−10.9 <fpn / fw <−3.3 (2A-1)

  Here, the position of the aspheric plastic lens having negative refractive power in the second lens group G2 is not defined, but when the second lens group G2 has four lenses, the object of the second lens group G2 It is preferable to arrange an aspheric plastic lens having a negative refractive power second or third from the side.

  When an aspheric plastic lens having a negative refractive power is disposed second from the object side of the second lens group G2, the second lens group G2 is arranged in order from the object side, a positive lens, a negative lens, a positive lens, and a negative lens. A configuration in which lenses are arranged is preferable. In this case, for example, the second lens group G2 includes, in order from the object side, four lenses in which biconvex lenses, a negative meniscus lens having a convex surface facing the image side, a biconvex lens, and a negative meniscus lens having a convex surface facing the object side are arranged. It can be configured.

  Alternatively, when an aspheric plastic lens having a negative refractive power is arranged third from the object side of the second lens group G2, the second lens group G2 includes, in order from the object side, a positive lens, a positive lens, and a negative lens. A configuration in which positive lenses are arranged is preferable. In this case, for example, the second lens group G2 may have a four-lens configuration in which a biconvex lens, a biconvex lens, a biconcave lens, and a biconvex lens are arranged in this order from the object side. The sign and shape of the refractive power of the lens described above are considered in the paraxial region for an aspheric lens.

  Next, a variable magnification optical system according to the second embodiment of the present invention will be described. The example shown in FIGS. 1A and 1B also has the configuration of the variable magnification optical system according to the second embodiment of the present invention described below.

  The variable magnification optical system according to the second embodiment of the present invention has a first lens group G1 having a negative refractive power, an aperture stop St, and a positive refractive power in order from the object side along the optical axis Z. The second lens group G2 is provided, and zooming is performed by changing the distance in the optical axis direction between the first lens group G1 and the second lens group G2, and the first lens group G1 is negative in order from the object side. The second lens group G2 includes a meniscus lens, a negative lens, and a positive lens, and includes a single aspheric plastic lens having a positive refractive power and a single aspheric plastic lens having a negative refractive power. It is configured.

Further, in the variable magnification optical system according to the second embodiment of the present invention, the focal length of the aspheric plastic lens having the positive refractive power of the second lens group G2 is set to fpp, and the negative of the second lens group G2 is set. When the focal length of the aspheric plastic lens having refractive power is fpn and the focal length of the entire system at the wide angle end is fw, the following conditional expressions (1B) and (2B) are satisfied.
3.8 <fpp / fw <5.0 (1B)
−11.0 <fpn / fw <−9.0 (2B)

  Of the configuration of the variable magnification optical system of the second embodiment described above, the same effect as that of the first embodiment can be obtained. Compared to the variable magnification optical system of the first embodiment, the variable magnification optical system of the second embodiment is not limited to defining the position of the positive aspheric plastic lens of the second lens group G2, but the second lens group. By setting the refractive powers of the positive and negative aspherical plastic lenses of G2 more suitably, good optical performance can be obtained.

  Below the lower limit of conditional expression (1B), spherical aberration and field curvature are overcorrected at the wide-angle end, and spherical aberration and field curvature are undercorrected at the telephoto end. If the upper limit of conditional expression (1B) is exceeded, high-order spherical aberration will occur and correction will not be possible. If the lower limit of conditional expression (2B) is not reached, spherical aberration will be overcorrected at the telephoto end. If the upper limit of conditional expression (2B) is exceeded, spherical aberration and curvature of field become insufficiently corrected at the telephoto end.

More preferably, it satisfies either or both of the following conditional expressions (1B-1) and (2B-1).
3.98 <fpp / fw <4.7 (1B-1)
−10.9 <fpn / fw <−9.0 (2B-1)

  In the variable power optical system of the second embodiment, the aspheric plastic lens having the positive refractive power of the second lens group G2 may be disposed closest to the object side of the second lens group G2. In such an arrangement, it becomes easy to correct spherical aberration, and a large aperture ratio can be easily realized.

  The aspheric plastic lens having negative refractive power in the second lens group G2 may be arranged second from the object side of the second lens group G2 when the second lens group G2 has four lenses. .

  When an aspheric plastic lens having a negative refractive power is disposed second from the object side of the second lens group G2, the second lens group G2 is arranged in order from the object side, a positive lens, a negative lens, a positive lens, and a negative lens. A configuration in which lenses are arranged is preferable. In this case, for example, the second lens group G2 includes, in order from the object side, four lenses in which biconvex lenses, a negative meniscus lens having a convex surface facing the image side, a biconvex lens, and a negative meniscus lens having a convex surface facing the object side are arranged. It can be configured.

  Furthermore, it is preferable that the variable magnification optical system of the first and second embodiments has the configuration described below. In addition, as a preferable aspect, you may have any one of the following structures, or you may have arbitrary combinations.

When the Abbe number at the d-line of the aspheric plastic lens having positive refractive power of the second lens group G2 is νdpp, the following conditional expression (3) is preferably satisfied. In particular, it is more effective when an aspheric plastic lens having a positive refractive power is disposed closest to the object side of the second lens group G2, and the aspheric plastic lens satisfies the following conditional expression (3). If the lower limit of conditional expression (3) is not reached, axial chromatic aberration and lateral chromatic aberration will deteriorate at the wide-angle end, making it difficult to correct well.
νdpp> 54.0 (3)

More preferably, the following conditional expression (3-1) is satisfied. If the upper limit of conditional expression (3-1) is exceeded, axial chromatic aberration will deteriorate at the telephoto end.
55.0 <νdpp <58.0 (3-1)

When the Abbe number in the d-line of the aspheric plastic lens having negative refractive power of the second lens group G2 is νdpn, it is preferable that the following conditional expression (4) is satisfied. If the upper limit of conditional expression (4) is exceeded, axial chromatic aberration and lateral chromatic aberration will deteriorate at the wide-angle end, and axial chromatic aberration will deteriorate at the telephoto end, making it difficult to correct well.
νdpn <29.0 (4)

More preferably, the following conditional expression (4-1) is satisfied. If the lower limit of conditional expression (4-1) is not reached, axial chromatic aberration and lateral chromatic aberration will deteriorate at each of the wide-angle end and the telephoto end.
22.0 <νdpn <27.0 (4-1)

  The object side surface of the aspherical plastic lens having negative refractive power of the second lens group G2 is preferably a concave surface in the vicinity of the optical axis and has an inflection point within the effective diameter. By adopting such a shape, it becomes easy to correct curvature of field.

  It is preferable that the first lens group G1 is composed of three lenses and the second lens group G2 is composed of four lenses. As a result, it is possible to achieve a small size and low cost while ensuring good optical performance.

  In zooming from the wide-angle end to the telephoto end, as indicated by an arrow between FIGS. 1A and 1B, the first lens group G1 moves from the object side to the image side, and then the image side. It is preferable to be configured to move from the object side to the object side. Thereby, the length in the optical axis direction of the entire system in consideration of zooming can be shortened, and the whole can be made compact.

  When the zoom lens system is required to have environment resistance, it is preferable that the most object side lens and the most image side lens of the entire system are made of a material other than plastic. Since the lens arranged closest to the object side is always exposed to sunlight when used outdoors for surveillance camera applications or the like, if it is composed of a plastic lens, there is a concern about deterioration or deterioration. In addition, since plastics are more likely to adhere dust due to static electricity or the like than glass, if the lens arranged closest to the image side is a plastic lens, there is a concern about image deterioration due to dust adhesion.

  When this variable magnification optical system is used in severe environments such as surveillance camera applications and it is desired to reduce performance changes due to temperature changes as much as possible, the plastic lens included in this variable magnification optical system is a positive lens of the second lens group G2. Only two negative aspheric plastic lenses may be used. Alternatively, when importance is attached to cost reduction and weight reduction, the plastic lens included in the zoom optical system may be three or more.

  When the variable magnification optical system is used in a harsh environment, a protective multilayer coating is preferably applied. Further, in addition to the protective coat, an antireflection coat for reducing ghost light during use may be applied.

  In the example shown in FIG. 1A and FIG. 1B, an example in which the optical member PP is disposed between the lens system and the image plane is shown. However, a low-pass filter or a specific wavelength region is cut. Instead of arranging various filters or the like, these various filters may be arranged between the lenses, or the lens surface of any lens may be coated with the same action as the various filters. .

Next, numerical examples of the variable magnification optical system of the present invention will be described. Of Examples 1 to 4 described below, Example 3 is shown as a reference example of the present invention. The lens cross-sectional views of the variable magnification optical system of Example 1 are those shown in FIGS. 1 (A) and 1 (B). 2A and 2B are sectional views of the lens of the second embodiment, FIG. 3A and FIG. 3B are sectional views of the lens of the third embodiment, and FIG. 3B is a sectional view of the lens of the fourth embodiment. This is shown in FIGS. 4 (A) and 4 (B). The method for illustrating the lens cross-sectional views of Examples 2 to 4 is the same as that of the lens cross-sectional view of Example 1 described above.

  Table 1 shows lens data of the variable magnification optical system of Example 1, Table 2 shows data relating to zooming, and Table 3 shows aspherical data. Similarly, lens data, zoom data, and aspherical data of the variable magnification optical systems of Examples 2 to 4 are shown in Tables 4 to 12, respectively. In the following, the meaning of the symbols in the table will be described using the example 1 as an example, but the same applies to the examples 2 to 4.

  In the lens data of Table 1, the column of Si indicates the i-th (i = 1, 2, 3,...) Surface number that sequentially increases toward the image side with the surface of the component closest to the object side as the first. The Ri column shows the radius of curvature of the i-th surface, and the Di column shows the surface spacing on the optical axis Z between the i-th surface and the i + 1-th surface. In the column of Ndj, the d-line (wavelength: 587.6 nm) of the j-th (j = 1, 2, 3,...) Optical element that sequentially increases toward the image side with the most optical element on the object side as the first. The refractive index with respect to is shown, and the νdj column shows the Abbe number of the j-th optical element with respect to the d-line. The sign of the radius of curvature is positive when convex on the object side and negative when convex on the image side. The lens data includes the aperture stop St, the optical member PP, and the image plane. In the surface number column of the surface corresponding to the aperture stop St and the image surface Sim, the words (aperture stop) and (image surface) are described together with the surface number.

  In the lens data of Table 1, the variable distance, variable 2, and variable 3 are described in the column of the surface interval in which the interval changes at the time of zooming. Variable 1 is the distance between the first lens group G1 and the aperture stop St, Variable 2 is the distance between the aperture stop St and the second lens group G2, and Variable 3 is the distance between the second lens group G2 and the optical member PP. It is an interval.

  The zoom-related data in Table 2 shows the focal length, F value, total angle of view, variable 1, variable 2, and variable 3 of the entire system for the d-line at the wide angle end and the telephoto end. In the lens data and zoom data, degrees are used as the unit of angle and mm is used as the unit of length. However, since the optical system can be used in proportional expansion or proportional reduction, Appropriate units can also be used.

In the lens data in Table 1, the surface number of the aspheric surface is marked with *, and the paraxial curvature radius is shown as the curvature radius of the aspheric surface. The aspheric data in Table 3 shows the aspheric coefficients related to these aspheric surfaces. The numerical value “E−n” (n: integer) of the aspheric data in Table 3 means “× 10 ⁻ⁿ ”, and “E + n” means “× 10 ⁿ ”. The aspheric coefficient is a value of each coefficient K, Am (m = 2, 3, 4,... 20) in the aspheric expression expressed by the following expression.
Zd = C · h ² / {1+ (1−K · C ² · h ² ) ^1/2 } + ΣAm · h ^m
However,
Zd: Depth of aspheric surface (length of a perpendicular line drawn from a point on the aspherical surface at height h to a plane perpendicular to the optical axis where the aspherical vertex contacts)
h: Height (distance from the optical axis to the lens surface)
C: Paraxial curvature K, Am: Aspheric coefficient (m = 2, 3, 4,... 20)

  The schematic configuration of the variable magnification optical system of Example 1 is as follows. In the variable magnification optical system of Example 1, in order from the object side, the first lens group G1 has a negative meniscus lens L11 having a convex surface facing the object side, a biconcave lens L12, and a convex surface facing the object side. It has a three-lens configuration including a positive meniscus lens L13. The second lens group G2 includes a biconvex lens L21 in the paraxial region, a negative meniscus lens L22 having a convex surface facing the image side in the paraxial region, and both The four-lens configuration includes a convex lens L23 and a negative meniscus lens L24 having a convex surface facing the object side. The aspheric surfaces are provided on both surfaces of the lens L21 and both surfaces of the lens L22. The lens L21 and the lens L22 are plastic lenses. The aperture stop St is fixed at the time of zooming.

  The schematic configuration of the variable magnification optical system of Examples 2 and 4 is the same as that of Example 1 described above. The schematic configuration of the variable magnification optical system of Example 3 is the same as that of Example 1 for the first lens group G1, but the second lens group G2 includes a biconvex lens L21, both lenses in the paraxial region. The lens is composed of a convex lens L22, a biconcave lens L23 in the paraxial region, and a biconvex lens L24. The aspheric surfaces are applied to both surfaces of the lens L21 and both surfaces of the lens L23. The lens L21 and the lens L23 are plastic lenses. The aperture stop St of the zoom optical system of Example 3 is fixed at the time of zooming.

  The spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration (chromatic aberration of magnification) at the wide angle end of the variable magnification optical system of Example 1 are shown in FIGS. 5 (A) to 5 (D), respectively. 5E to 5H show spherical aberration, astigmatism, distortion (distortion aberration), and chromatic aberration of magnification (chromatic aberration of magnification), respectively. Each aberration diagram is based on the d-line, but the spherical aberration diagram also shows aberrations related to the g-line (wavelength 435.8 nm) and the C-line (wavelength 656.3 nm), and the lateral chromatic aberration diagram shows the g-line and C-line. The aberration about is shown. In the astigmatism diagram, the sagittal direction is indicated by a solid line, and the tangential direction is indicated by a dotted line. Fno. Of spherical aberration diagram. Means F value, and ω in other aberration diagrams means half angle of view.

  Similarly, aberration diagrams at the wide-angle end and the telephoto end of the zoom optical system of Example 2 are shown in FIGS. 6A to 6H, and the wide-angle end and the telephoto end of the zoom optical system of Example 3 are shown. FIGS. 7A to 7H show aberration diagrams at the zoom lens, and FIGS. 8A to 8H show aberration diagrams at the wide angle end and the telephoto end of the variable magnification optical system of Example 4. FIGS. Show.

  Table 13 shows values corresponding to the conditional expressions (1A), (1B), (2A), (2B), (3), and (4) of the variable magnification optical systems of Examples 1 to 4. The values in Table 13 are for the d line.

  The variable power optical systems of Examples 1 to 4 are all 7-lens lens systems including two plastic lenses, and can be made small and inexpensive. In addition, the variable magnification optical systems of Examples 1 to 4 have a wide angle of about 130 ° at the wide angle end while maintaining a large aperture ratio where the F value at the wide angle end is about 1.3. Each aberration is corrected well and has high optical performance.

  FIG. 9 shows a schematic configuration diagram of an imaging apparatus using the variable magnification optical system of the embodiment of the present invention as an example of the imaging apparatus of the embodiment of the present invention. Examples of the imaging device include a surveillance camera, a video camera, and an electronic still camera.

  An imaging apparatus 10 shown in FIG. 9 includes a variable magnification optical system 1, a filter 2 disposed on the image side of the variable magnification optical system 1, and an imaging element 3 that captures an image of a subject formed by the variable magnification optical system. And a signal processing unit 4 that performs arithmetic processing on an output signal from the image sensor 3. The variable magnification optical system 1 has a negative first lens group G1, an aperture stop St, and a positive second lens group G2. FIG. 9 conceptually shows each lens group. The imaging device 3 converts an optical image formed by the variable magnification optical system 1 into an electric signal, and the imaging surface thereof is arranged so as to coincide with the image plane of the variable magnification optical system. For example, a CCD or a CMOS can be used as the imaging element 3.

  In addition, the imaging apparatus 10 includes a zoom control unit 5 for performing zooming of the zooming optical system 1, a focus control unit 6 for adjusting the focus of the zooming optical system 1, and the aperture diameter of the aperture stop St. A diaphragm control unit 7 for changing is provided. Although FIG. 9 shows a configuration in which the focus adjustment is performed by moving the first lens group G1, the focus adjustment method of the present invention is not necessarily limited to this example.

  The present invention has been described with reference to the embodiments and examples. However, the present invention is not limited to the above embodiments and examples, and various modifications can be made. For example, the values of the radius of curvature, the surface interval, the refractive index, the Abbe number, the aspherical coefficient, etc. of each lens component are not limited to the values shown in the above numerical examples, and can take other values.

DESCRIPTION OF SYMBOLS 1 Variable magnification optical system 2 Filter 3 Image pick-up element 4 Signal processing part 5 Zoom control part 6 Focus control part 7 Aperture control part 10 Imaging device G1 1st lens group G2 2nd lens group L11, L12, L13, L21, L22, L23 , L24 Lens PP Optical member Sim Image surface St Aperture stop Z Optical axis

Claims (5)

In order from the object side, the lens unit includes a first lens group having a negative refractive power, a stop, and a second lens group having a positive refractive power. The first lens group and the second lens group are arranged in the optical axis direction. A zooming optical system that zooms by changing the interval,
The first lens group includes, in order from the object side, a negative meniscus lens, a negative lens, and a positive lens.
The second lens group includes four lenses, an aspheric plastic lens having a positive refractive power is disposed closest to the object side of the second lens group, and the aspheric plastic lens having a negative refractive power is is arranged from the object side in the second lens unit to the second,
When the focal length of the aspheric plastic lens having the positive refractive power is fpp, the focal length of the aspheric plastic lens having the negative refractive power is fpn, and the focal length of the entire system at the wide angle end is fw, A variable magnification optical system characterized by satisfying the following conditional expressions (1 B ) and (2 B ).
3.8 <fpp / fw <5.0 (1 B )
−11.0 <fpn / fw < −9.0 (2 B ) When the νdpp an Abbe number at the d-line of the aspherical plastic lens having a positive refractive power, according to claim 1 Symbol placement of the variable magnification optical system is characterized in that satisfies the following conditional expression (3).
νdpp> 54.0 (3) When the νdpn an Abbe number at the d-line of the aspherical plastic lens having negative refractive power, according to claim 1 or 2 variable magnification optical system according to and satisfies the following conditional expression (4).
νdpn <29.0 (4) Object-side surface of the aspherical plastic lens having negative refractive power, a concave near the optical axis, any one of claims 1 to 3, characterized in that it has an inflection point in the effective diameter The zoom optical system according to 1. Imaging apparatus characterized by comprising a variable magnification optical system according to any one of claims 1 to 4.

Images