JP5335352B2 - Zoom lens and imaging device - Google Patents

Description

  The present invention relates to a zoom lens and an imaging apparatus, and more particularly to a zoom lens that can be suitably used for a video camera, an electronic still camera, a surveillance camera, and the like, and an imaging apparatus including the zoom lens.

In recent years, there has been an increasing demand for a compact zoom lens with high image quality in a lens for a video camera. For example, the photographed light is divided into R (red), G (green), and B (blue) colors by a color separation prism, and images captured by three CCDs (Charge Coupled Devices) corresponding to each color are superposed to increase the height. There is a need for a compact zoom lens that can support the 3CCD system to obtain image quality. As a zoom lens having a small zoom ratio and a compact zoom ratio having a high zoom ratio of about 10 times and an F-number of about 1.8, there are those described in Patent Documents 1 and 2 including four lens groups. Can be mentioned. The fourth lens group of the zoom lens described in Patent Documents 1 and 2 moves so as to correct the image plane position, and includes an aspheric lens having a positive power for good aberration correction. It consists of
JP 2004-279726 A JP 2006-343724 A

  Incidentally, in recent years, in the zoom lens in the above field, in addition to the demand for high image quality and miniaturization, the demand for cost reduction has also increased. As one method for reducing the cost, it is conceivable that the aspheric lens is made of plastic. However, in the zoom lens having the configuration described in Patent Documents 1 and 2, it is difficult to configure the aspherical lens of the fourth lens group from plastic. This is because plastic has a large change in refractive index and curvature due to temperature, and the positive lens in the fourth lens group having the above configuration has a strong power. This is because the influence of the process becomes very large. Therefore, this aspherical lens generally uses a lens (glass molded lens) in which an aspherical shape is formed by molding using glass as a material, which is very expensive.

  In addition, since the fourth lens group as a whole has a strong positive power, the power carried by each lens included in the fourth lens group also increases, and the curvature thereof tends to increase. As the curvature increases, it is necessary to increase the lens thickness in order to secure the necessary edge (border), so it is difficult to correct aberrations while making the fourth lens group compact. .

  The present invention has been made in view of the above circumstances, and includes a zoom lens capable of supporting high image quality while maintaining good optical performance, and a low-cost and miniaturized zoom lens, and the zoom lens. It is an object of the present invention to provide an image pickup apparatus.

The zoom lens of the present invention has, in order from the object side, a first lens group having a positive refractive power and fixed at the time of zooming, a negative refractive power, and moving along the optical axis. A second lens group that performs zooming, a stop, a third lens group that has positive refractive power and is fixed at the time of zooming, and has a positive refractive power, and the position of the image plane associated with zooming consists of a fourth lens group for correcting and focusing, the fourth lens unit, in order from the object side, the material at least one surface aspherical surface and the 41 lens plastic, a positive second lens, a negative When the focal length of the forty-first lens is f41, the focal length of the fourth lens group is f4, and the focal length of the third lens group is f3. It is characterized by satisfying the expressions (1) and (2).
| F41 | / f4> 4.2 (1)
2.2 <f3 / f4 <4.2 (2)

  In the present invention, each “lens group” includes not only a plurality of lenses but also a lens group.

  In the present invention, the sign of the power in the paraxial region of the 41st lens may be either positive or negative.

  In the zoom lens according to the present invention, a plastic lens having an aspheric surface is arranged on the most object side of the fourth lens group, thereby achieving good aberration correction and cost reduction. The power of the plastic lens is set to a weak power as defined by the conditional expression (1), so that the performance deterioration due to the temperature change is suppressed and the good performance is maintained. In addition, in order to achieve high image quality, a back focus that is long enough to insert a color separation prism used in the 3CCD system is necessary. In the zoom lens according to the present invention, the conditional expression (2) is satisfied to achieve compactness. However, long back focus is ensured.

  Here, in the zoom lens of the present invention, the 43rd lens and the 44th lens may be bonded together to constitute a cemented lens.

Furthermore, in the zoom lens according to the present invention, it is preferable that the following conditional expression (3) is satisfied when the Abbe number of the 44th lens is ν44.
61 <ν44 <83 (3)

In the zoom lens according to the present invention, it is preferable that the following conditional expression (4) is satisfied when the refractive index of the forty-second lens is N42.
1.55 <N42 <1.75 (4)

  In the zoom lens according to the present invention, it is preferable that the third lens group includes one positive lens having at least one aspheric surface. In this case, it is preferable that the material of the positive lens of the third lens group is plastic.

  In the present invention, a compound lens in which an aspheric resin is formed on a glass spherical surface is not regarded as “one lens”.

In the zoom lens of the present invention, when the average Abbe number of the 42nd lens and the 44th lens is ν4p and the Abbe number of the positive lens of the third lens group is ν31, the following conditional expression (5) is satisfied. It is preferable.
28 <ν4p−ν31 <50 (5)

  The values of the conditional expressions in this specification are those at the reference wavelength of the zoom lens. For example, when the reference wavelength of the zoom lens is d-line (wavelength 587.6 nm), the values are described in the above conditional expressions. The refractive index and Abbe number are those at the d-line.

  An image pickup apparatus according to the present invention includes the zoom lens according to the present invention described above.

  According to the present invention, the first lens group and the third lens group are fixed groups, and the second lens group is moved along the optical axis to perform zooming, thereby correcting and focusing the image plane position. In the zoom lens of the type in which the lens is moved by moving the fourth lens group, the configuration of the fourth lens group is preferably set so as to satisfy the conditional expressions (1) and (2). Therefore, it is possible to provide a zoom lens that can cope with high image quality while maintaining low cost and reduced size, and an imaging device including the zoom lens.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a cross-sectional view showing a configuration of a zoom lens according to an embodiment of the present invention, and corresponds to a zoom lens of Example 1 described later. FIGS. 2 to 6 are cross-sectional views each showing a configuration of a zoom lens of Examples 2 to 6 described later. Since the basic configuration of the zoom lens shown in FIGS. 1 to 6 is the same and the method of illustration is also the same, the following description will be mainly made with reference to FIG. 1 as an example.

  The zoom lens according to the embodiment of the present invention has, in order from the object side along the optical axis Z, a positive refractive power, a first lens group G1 fixed at the time of zooming, and a negative refractive power. A second lens group G2 that performs zooming by moving along the optical axis Z, an aperture stop St, and a third lens group G3 that has positive refractive power and is fixed during zooming. And a fourth lens group G4 that has a positive refractive power and corrects and focuses an image plane position accompanying zooming.

  Note that the aperture stop St shown in FIG. 1 does not necessarily indicate the size or shape, but indicates the position on the optical axis Z. In FIG. 1, the left side is the object side, and the right side is the image side. In FIG. 1, the upper lens arrangement is shown when focusing on infinity at the wide-angle end, the lower lens arrangement is shown when focusing on infinity at the telephoto end, and each lens group for zooming from the wide-angle end to the telephoto end. The general movement trajectory is indicated by arrows.

  In FIG. 1, the imaging position of the axial light beam from an infinite object is shown as Pim. For example, when this zoom lens is applied to an image pickup apparatus, the zoom lens is arranged so that the image pickup surface of the image pickup element is positioned at the image formation position Pim.

  When applying a zoom lens to an imaging device, depending on the configuration of the camera side on which the lens is mounted, a cover glass, prism, infrared cut filter, between the lens on the most image side and the imaging surface (imaging surface) It is preferable to arrange various filters such as a low-pass filter. In the example shown in FIG. 1, a parallel plate-shaped optical member PP that assumes these is arranged between the fourth lens group G4 and the imaging position Pim. Yes.

  In this zoom lens, when zooming from the wide-angle end to the telephoto end, the first lens group G1 and the third lens group G3 are fixed on the optical axis Z, and the second lens group G2 is along the optical axis Z. The zooming is performed by moving the zoom lens to the image side, and the correction and focusing of the image plane position accompanying the zooming is performed by moving the fourth lens group G4 along the optical axis Z. Yes. That is, the second lens group G2 functions as a variator group, and the fourth lens group G4 functions as a compensator group and a focus group.

  The zoom lens according to the present embodiment is mainly characterized by the fourth lens group G4. The fourth lens group G4 includes, in order from the object side, at least one aspherical surface and a plastic material L41, and a positive lens. L42, the negative lens L43, and the positive lens L44 are comprised.

  The lens L41, which is a plastic lens, is set so that its power is weak, as represented by conditional expression (1) described later. This lens L41 hardly influences the power distribution, and plays a role as a so-called “aberration correction lens” for correcting aberrations satisfactorily. Providing this lens L41 makes it possible to correct aberrations satisfactorily without providing an aspherical surface on lenses other than the lens L41 in the fourth lens group G4.

  If the lens L41 of the “aberration correction lens” is not provided, the fourth lens group G4 has a three-lens configuration including a positive lens L42, a negative lens L43, and a positive lens L44. It becomes the adopted composition. In this conventional configuration, when an aspheric surface is provided in the fourth lens group, it is normally provided in a positive lens. However, since the positive lens in the fourth lens group in the conventional configuration has a strong power, if plastic is used as a material, the influence of the refractive power change due to temperature becomes very large, and the performance deterioration at the time of temperature change cannot be ignored. It becomes. For this reason, conventionally, a glass mold lens is used as the aspherical lens of the fourth lens group, which is expensive.

  Moreover, since the fourth lens group has a strong positive power as a whole, in the conventional three-lens configuration, the power of each lens included in the fourth lens group increases, and the curvature tends to increase. In particular, when a low dispersion material is used to correct chromatic aberration satisfactorily, the low dispersion material generally has a low refractive index, and thus the lens curvature tends to increase. As the curvature increases, it is necessary to increase the lens thickness in order to secure the necessary edge (edge), which is contrary to miniaturization.

  With respect to the above problem, the zoom lens according to the present embodiment can reduce the influence of temperature change even when plastic is used as the lens L41, which is an aspherical lens in the fourth lens group. L41 can be made of plastic. Further, since the power of the lens L41 is weak, the lens thickness of the lens L41 can be reduced.

  The zoom lens according to the present embodiment has one more lens as a result compared to the conventional lens in which the fourth lens group has a three-lens structure. In many cases, spherical lenses can be constructed at low cost. Further, since the lens L41 is made of plastic, it can be reduced in weight, which is advantageous for the fourth lens group G4 that is a moving group.

  Furthermore, instead of providing an aspherical surface to the conventional three-lens positive lens, a positive lens other than the lens L41 of the fourth lens group G4 is formed by a glass mold by providing a lens L41 having an aspherical surface as in this embodiment. This eliminates the need for a lens, increases the degree of freedom in selecting materials used for positive lenses other than the lens L41 of the fourth lens group G4, and is advantageous for correcting chromatic aberration.

  That is, the fourth lens group G4 is made of glass by disposing a lens L41, which is a thin lens having a good aberration correction capability and being less affected by temperature change while being made of an inexpensive and lightweight plastic. It is possible to obtain a zoom lens in which the number of aspherical lenses is reduced, the cost is reduced, and aberrations are corrected favorably.

  As an example, the zoom lens according to the present embodiment may have a configuration as shown in FIG. That is, in each lens group, in order from the object side, the first lens group G1 includes a cemented lens obtained by bonding a meniscus negative lens L11 having a convex surface facing the object side and a positive lens L12, and a meniscus positive lens L13. The second lens group G2 has a four-lens configuration including a negative lens L21, a negative lens L22, a cemented lens formed by bonding the positive lens L23 and the negative lens L24, and the third lens group G3 is a positive lens. The fourth lens group G4 can have a four-lens configuration including a lens L41, a positive lens L42, and a cemented lens formed by bonding the negative lens L43 and the positive lens L44.

  As in the example shown in FIG. 1, the negative lens L43 and the positive lens L44 of the fourth lens group G4 are preferably bonded together to form a cemented lens, which is advantageous in correcting chromatic aberration.

  As in the example shown in FIG. 1, the third lens group G3 preferably has a single positive lens L31 configuration, which can reduce the number of lenses and reduce the size. When the third lens group G3 has a single lens configuration, the positive lens L31 preferably has at least one aspheric surface. By using an aspheric lens as the positive lens L31, it is advantageous in correcting aberrations, and it is easy to configure the third lens group G3 as a single lens.

  When the positive lens L31 is an aspheric lens, it is preferably a plastic lens. By using plastic as the material, it is possible to reduce the cost and weight. In addition, when the lens barrel of the applied imaging device is made of a plastic material, the expansion / contraction due to the temperature change of the lens barrel and the movement of the image position due to the temperature change of the positive lens made of the plastic material cancel each other. The change in the apparent image position due to the temperature change can be suppressed. In this respect, it can be said that the plastic lens is more advantageous for temperature change than the glass lens.

  Since the zoom lens of the present embodiment shown in FIG. 1 has a single third lens group G3, the third lens group G3 has a third lens group G3 as compared with the conventional example in which the third lens group G3 has two lenses. The number of lenses in the lens group G3 can be reduced. Therefore, when the third lens group G3 has a single lens structure, unlike the conventional example in which the second lens group G2 has a three-lens structure, four second lens groups G2 are used as in this embodiment. It is conceivable to have a configuration.

  The meaning of the second lens group G2 having four lenses will be described below. In order to shorten the overall length of the lens system and reduce the size, it is necessary to reduce the amount of movement during zooming. For this purpose, it is preferable to give the second lens group G2 a strong negative power. However, if the power of the lens becomes too strong, the second lens group G2 is a moving group, so that aberration fluctuations during zooming increase.

  In the lens of Patent Document 1, the second lens group has three negative, negative, and positive lenses. However, in the zoom lens of the present embodiment shown in FIG. 1, four negative lenses are further added. With this configuration, even when the second lens group G2 has a strong negative power, the negative power necessary for the second lens group G2 is dispersed. As a result, it is possible to reduce the overall length of the lens system without the intensity of power carried by each negative lens of the second lens group G2 becoming too strong and aberration fluctuations during zooming becoming large.

  In the zoom lens according to the present embodiment, the second lens group G2 has two negative lenses on the object side. In this way, by distributing a large amount of negative power to the object side, the position of the object side principal point of the second lens group is brought closer to the object side, so that the principal point interval between the first lens group G1 and the second lens group G2 is increased. Can be shortened. Accordingly, the height of the off-axis light beam passing through the first lens group G1 can be reduced, and the front lens diameter (the diameter of the lens closest to the object side) can be reduced.

Furthermore, in the zoom lens according to the present embodiment, when the focal length of the lens L41 closest to the object side in the fourth lens group G4 is f41 and the focal length of the fourth lens group G4 is f4, the following conditional expression (1) is satisfied. It is configured to meet.
| F41 | / f4> 4.2 (1)

  Conditional expression (1) defines the ratio of the focal length of the lens L41 and the focal length of the fourth lens group G4. In other words, it defines the ratio of the power of the lens L41 to the fourth lens group G4. If the lower limit of conditional expression (1) is not reached, the power of the lens L41 becomes strong, and as described above, the influence of temperature change becomes large, which is not preferable.

Furthermore, it is more preferable to satisfy the following conditional expression (1-1). By satisfying the lower limit of conditional expression (1-1), it is possible to further reduce the influence due to temperature change.
5.0 <| f41 | / f4 <80 (1-1)

  When the upper limit of the conditional expression (1-1) is exceeded, the object side and the image side surfaces have a meniscus shape having a close curvature and a first lens shape having an effective magnitude, It can be considered that the image side surface has a shape close to a flat surface and a second lens shape having almost no power. As described above, in the present invention, a so-called “aberration correction lens” that hardly affects power distribution is assumed as the lens L41, and thus the second lens shape will be considered. In the second lens shape, when the power is weakened so as to exceed the upper limit of the conditional expression (1-1), the central portion of the lens is close to a flat surface, and the curvature tends to increase rapidly in the peripheral portion. In this case, the influence of the eccentricity of the lens and the axis deviation becomes large, the performance deteriorates due to a minute error, and good optical performance cannot be maintained. In addition, manufacturability is also deteriorated. Alternatively, when the power is weakened so as to exceed the upper limit of the conditional expression (1-1), if the lens is not in a shape that is close to a plane at the center and suddenly increases in curvature at the periphery, the aberration correction capability And the optical performance cannot be maintained.

The zoom lens according to the present embodiment is configured to satisfy the following conditional expression (2) when the focal length of the third lens group G3 is f3 and the focal length of the fourth lens group G4 is f4. .
2.2 <f3 / f4 <4.2 (2)

  Conditional expression (2) defines the ratio of the focal lengths of the third lens group G3 and the fourth lens group G4. Conditional expression (2) achieves good optical performance, reduces the size of the lens system after the third lens group G3, and ensures a long back focus in which a color separation prism or the like used in the 3CCD system can be inserted. Therefore, a suitable range of power distribution between the third lens group G3 and the fourth lens group G4 is defined.

  If the power of the third lens group G3 is increased so as to fall below the lower limit of the conditional expression (2), the variation in spherical aberration at the time of zooming or focusing becomes large. In addition, it is very difficult to ensure the necessary back focus. On the other hand, if the upper limit of conditional expression (2) is exceeded, the total length of the lens system becomes longer, the height of light incident on the fourth lens group G4 becomes larger, and the fourth lens group G4 becomes larger.

Furthermore, it is more preferable to satisfy the following conditional expression (2-1). By satisfying conditional expression (2-1), the effect obtained by satisfying conditional expression (2) can be further enhanced.
2.4 <f3 / f4 <4.0 (2-1)

  In addition to the above-described configuration, the zoom lens according to the present embodiment is preferably configured to satisfy the following conditional expression, whereby even better characteristics can be obtained.

When the Abbe number of the positive lens L44 is ν44, it is preferable that the following conditional expression (3) is satisfied.
61 <ν44 <83 (3)

  Conditional expression (3) defines the Abbe number of the positive lens L44. By using a low dispersion material defined by the conditional expression (3) in the fourth lens group G4, it is possible to suppress chromatic aberration variation at the time of zooming and focusing, and chromatic aberration generated in the fourth lens group G4. Can do.

  When the conditional expression (3) is not satisfied, chromatic aberration cannot be corrected satisfactorily. On the other hand, using a low dispersion material that exceeds conditional expression (3) is advantageous for correcting chromatic aberration, but a low refractive index material is used, so that a lens is used to secure the necessary edge. It will be thick.

When the refractive index of the positive lens L42 is N42, the following conditional expression (4) is preferably satisfied.
1.55 <N42 <1.75 (4)

  Conditional expression (4) defines the refractive index of the positive lens L42. The fourth lens group G4 has a strong positive power, and the power carried by each lens is large. For this reason, when the positive lens L42 is made of a material that is less than the conditional expression (4), the curvature of the lens increases, aberration fluctuations during zooming or focusing increase, and aberration correction becomes difficult. . In addition, the lens thickness necessary for securing the necessary edge increases, and the fourth lens group G4 increases in size. If the conditional expression (4) is exceeded, it is advantageous for making the lens thin, but a high refractive index material is used, and the high refractive index material generally has a large dispersion, so that it is difficult to perform good chromatic aberration correction. become.

When the third lens group G3 includes one positive lens L31 having at least one aspheric surface, the average Abbe number of the positive lens L42 and the positive lens L44 is ν4p, and the Abbe number of the positive lens L31 is ν31. It is preferable that the following conditional expression (5) is satisfied.
28 <ν4p−ν31 <50 (5)

  Conditional expression (5) defines the difference between the average Abbe number of the positive lens L42 and the positive lens L44 and the Abbe number of the positive lens L31 of the third lens group G3. It is desirable to perform achromatization in the zoom lens in each lens group, but when the third lens group G3 is composed of one lens, it is necessary to perform achromatization together with the fourth lens group G4. In the fourth lens group G4, a low-dispersion material is used for the positive lens in order to suppress variation in chromatic aberration associated with zooming and focusing. Therefore, a high dispersion material that cancels chromatic aberration generated in the fourth lens group is used for the positive lens L31 of the third lens group G3.

  If the conditional expression (5) is not satisfied, chromatic aberration, in particular, chromatic aberration from the middle range of the zoom to the telephoto end cannot be sufficiently corrected. If the conditional expression (5) is exceeded, it is advantageous for correction of chromatic aberration. In this case, an ultra-low dispersion material is used for the lens L41 or a high dispersion material is used for the positive lens L31. It becomes difficult to select a material suitable for it. In addition, good chromatic aberration correction cannot be performed.

Furthermore, it is more preferable to satisfy the following conditional expression (5-1). By satisfying conditional expression (5-1), the effect obtained by satisfying conditional expression (5) can be further enhanced.
30 <ν4p−ν31 <48 (5-1)

  In addition, when this zoom lens is used in harsh environments such as outdoors, the lens placed closest to the object is resistant to surface deterioration due to wind and rain, temperature changes due to direct sunlight, and oils and detergents. It is preferable to use a material resistant to chemicals, that is, a material having high water resistance, weather resistance, acid resistance, chemical resistance, and the like, and further, a material that is hard and difficult to break. From the above, as the material disposed closest to the object side, specifically, glass is preferably used, or transparent ceramics may be used.

  It is preferable to use plastic as the material of the lens on which the aspherical shape is formed. In this case, the aspherical shape can be manufactured with high accuracy, and the weight and cost can be reduced. Become.

  When the zoom lens is required to be usable in a wide temperature range, it is preferable to use a material having a small linear expansion coefficient as the material of each lens. In addition, when the zoom lens is used in a harsh environment, a protective multilayer coating is preferably applied. In addition to the protective coat, an antireflection coating film for reducing ghost light during use may be applied.

  In the example shown in FIG. 1, an example in which the optical member PP is disposed between the lens system and the imaging surface is shown, but instead of disposing a low-pass filter, various filters that cut a specific wavelength range, or the like, These various filters may be disposed between the lenses, or a coating having the same action as the various filters may be applied to the lens surface of any lens.

  As described above, according to the zoom lens of the present embodiment, according to the required specifications and the like, by appropriately adopting the preferred configuration described above, it can be configured compactly without significantly increasing the number of lenses, It is possible to achieve both good aberration correction and cost reduction.

  Next, numerical examples of the zoom lens according to the present invention will be described. The lens cross-sectional views of the zoom lenses of Examples 1 to 6 are those shown in FIGS.

  Table 1 shows basic lens data of the zoom lens according to Example 1, Table 2 shows data relating to zooming (magnification), and Table 3 shows aspherical data. Similarly, basic data, zoom-related data, and aspherical data of the zoom lenses according to Examples 2 to 6 are shown in Tables 4 to 18. In the following, the meaning of the symbols in the table will be described using Example 1 as an example, but the same applies to Examples 2 to 6.

  In the basic lens data of Table 1, Si indicates the i-th (i = 1, 2, 3,...) Surface number that increases sequentially toward the image side with the most object-side component surface being first. Indicates the radius of curvature of the i-th surface, and Di indicates the surface interval on the optical axis Z between the i-th surface and the i + 1-th surface. The bottom column of the surface interval indicates the surface interval between the final surface and the imaging surface in the table. In the basic lens data, Ndj is 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 object-side lens as the first lens. ), And νdj represents the Abbe number of the j-th optical element with respect to the d-line. The basic lens data includes the aperture stop St and the optical member PP. In the column of the radius of curvature of the surface corresponding to the aperture stop St, (aperture stop) is described. The radius of curvature of the basic lens data is positive when convex on the object side and negative when convex on the image side.

  In the basic lens data in Table 1, the interval changes for zooming, the interval between the first lens group G1 and the second lens group G2, the interval between the second lens group G2 and the aperture stop St, the third lens group G3. D5 (variable), D12 (variable), D15 (variable), and D22 (variable) in the columns of the distance between the fourth lens group G4 and the surface distance corresponding to the distance between the fourth lens group G4 and the optical member PP, respectively. It is described.

  The zoom-related data in Table 2 includes the focal length f of the entire system at the wide-angle end and the telephoto end, the F number Fno. , The total field angle 2ω, and the values of the surface spacings D5, D12, D15, and D22 that change with zooming are shown. The unit of the total angle of view 2ω is degrees.

  As the unit of Ri and Di in Table 1 and the unit of f, D5, D12, D15, and D22 in Table 2, “mm” can be used. Since the performance can be obtained, the unit is not limited to “mm”, and other appropriate units can be used.

In the basic lens data in Table 1, the surface number of the aspheric surface is marked with *, and the paraxial radius of curvature is shown as the radius of curvature of the aspheric surface. The aspherical data in Table 3 shows the sign of a lens that is an aspherical lens, the surface number of the aspherical surface, and the aspherical coefficients related to these aspherical surfaces. The aspheric coefficient is a value of each coefficient KA, RA _m (m = 3, 4, 5,... 10) in the aspheric expression expressed by the following expression (A).

Zd = C · h ² / {1+ (1−KA · C ² · h ² ) ^1/2 } + ΣRA _m · h ^m (A)
However,
Zd: Depth of aspheric surface (length of perpendicular drawn from a point on the aspherical surface of 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: Reciprocal number of paraxial radius of curvature KA, RA _m : aspheric coefficient (m = 3, 4, 5,... 10)
In addition, when mm is used as the unit of Ri and Di in Table 1, the unit of Zd and h is also mm.

  Table 19 shows values corresponding to the conditional expressions (1) to (5) in Examples 1 to 6. As can be seen from Table 19, all of Examples 1 to 6 satisfy the conditional expressions (1) to (5).

  7A to 7H show respective aberration diagrams of spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration at the wide-angle end and the telephoto end of the zoom lens of Example 1. FIG. Each aberration diagram shows the aberration with the d-line (wavelength 587.6 nm) as the reference wavelength, while the spherical aberration diagram and the magnification chromatic aberration diagram also show the aberrations for the wavelength 460.0 nm and the wavelength 615.0 nm. Fno. Of spherical aberration diagram. Means F number, and ω in other aberration diagrams means half angle of view.

  Similarly, FIG. 8 (A) to FIG. 8 (H), FIG. 9 (A) to FIG. 9 (H), FIG. 10 (A) to FIG. 10 (H), FIG. 11 (A) to FIG. FIGS. 12A to 12H are graphs showing spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration at the wide-angle end and the telephoto end of the zoom lenses of Examples 2 to 6, respectively. Show.

  From the above data, the zoom lenses of Examples 1 to 6 have a magnification of about 10 times, and the F number at the wide-angle end is as small as about 1.8, and each aberration is well corrected while achieving downsizing. It can be seen that both the wide-angle end and the telephoto end have high optical performance in the visible range. These zoom lenses can be suitably used for imaging devices such as surveillance cameras, video cameras, and electronic still cameras.

  FIG. 13 shows a configuration diagram of a video camera 10 configured using the zoom lens 1 according to the embodiment of the present invention as an example of the imaging apparatus of the embodiment of the present invention. In FIG. 13, the positive first lens group G1, the negative second lens group G2, the aperture stop St, the positive third lens group G3, and the positive fourth lens group G4 included in the zoom lens 1 are schematically illustrated. Show.

  The video camera 10 shown in FIG. 13 is a so-called 3CCD image pickup device having three image pickup devices, but the image pickup device of the present invention is not limited to this, and the image pickup device of the present invention may pick up the entire wavelength band. Good. The video camera 10 includes a zoom lens 1, a filter 2 having functions such as a low-pass filter and an infrared cut filter disposed on the image side of the zoom lens 1, and color separation prisms 3R and 3G disposed on the image side of the filter 2. 3B, imaging elements 4R, 4G, and 4B provided on the end faces of the color separation prisms, and a signal processing circuit 5. The image sensors 4R, 4G, and 4B convert an optical image formed by the zoom lens 1 into an electric signal, and for example, a CCD (Charge Coupled Device) can be used. The imaging elements 4R, 4G, and 4B are arranged so that their imaging surfaces coincide with the imaging surface of the zoom lens 1.

  Unnecessary light components are removed from the light transmitted through the zoom lens 1 by the filter 2 and separated into red, green, and blue light by the color separation prisms 3R, 3G, and 3B, and then imaged by the image sensors 4R, 4G, and 4B. The image is formed on the surface. Output signals from the image sensors 4R, 4G, and 4B corresponding to red, green, and blue color lights are processed by the signal processing circuit 5 to form a color image and displayed on the display device 6.

  Since the zoom lens according to the embodiment of the present invention has the above-described advantages, the imaging apparatus of the present embodiment can be configured in a small size and at a low cost, and can obtain a high-quality image.

  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 spacing, the refractive index, the Abbe number, etc. of each lens component are not limited to the values shown in the above numerical examples, but can take other values.

Sectional drawing which shows the lens structure of the zoom lens concerning Example 1 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 2 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 3 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 4 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 5 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 6 of this invention. Each aberration diagram of the zoom lens according to Example 1 of the present invention Each aberration diagram of the zoom lens according to Example 2 of the present invention Each aberration diagram of the zoom lens according to Example 3 of the present invention Each aberration diagram of the zoom lens according to Example 4 of the present invention Each aberration diagram of the zoom lens according to Example 5 of the present invention Each aberration diagram of the zoom lens according to Example 6 of the present invention 1 is a schematic configuration diagram of an imaging apparatus according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Zoom lens 2 Filter 3B, 3G, 3R Color separation prism 4B, 4G, 4R Image pick-up element 5 Signal processing circuit 6 Display apparatus 10 Video camera G1 1st lens group G2 2nd lens group G3 3rd lens group G4 4th lens group PP Optical member St Aperture stop Z Optical axis

Claims (8)

In order from the object side, a first lens group having positive refractive power and fixed at the time of zooming, and a second lens having negative refractive power and zooming by moving along the optical axis A third lens group having a positive refracting power and fixed at the time of zooming, a first lens having a positive refracting power, and correcting and focusing the image plane position accompanying the zooming. It consists of a fourth lens group,
The fourth lens group includes, in order from the object side, at least one aspherical surface made of plastic and a forty-first lens, a positive forty-second lens, a negative forty-third lens, and a positive forty-fourth lens. ,
When the focal length of the 41st lens is f41, the focal length of the fourth lens group is f4, and the focal length of the third lens group is f3, the following conditional expressions (1) and (2) are satisfied. Zoom lens characterized by.
| F41 | / f4> 4.2 (1)
2.2 <f3 / f4 <4.2 (2)   The zoom lens according to claim 1, wherein the 43rd lens and the 44th lens are bonded to form a cemented lens. The zoom lens according to claim 1, wherein the following conditional expression (3) is satisfied when the Abbe number of the 44th lens is ν44.
61 <ν44 <83 (3) 4. The zoom lens according to claim 1, wherein the following conditional expression (4) is satisfied when a refractive index of the forty-second lens is N42. 5.
1.55 <N42 <1.75 (4)   5. The zoom lens according to claim 1, wherein the third lens group includes one positive lens having at least one aspheric surface. 6.   The zoom lens according to claim 5, wherein a material of the positive lens of the third lens group is plastic. The following conditional expression (5) is satisfied, where an average Abbe number of the forty-second lens and the forty-fourth lens is ν4p and an Abbe number of the positive lens of the third lens group is ν31. Item 7. The zoom lens according to Item 5 or 6.
28 <ν4p−ν31 <50 (5)   An imaging apparatus comprising the zoom lens according to claim 1.

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