JP2010066661A - Zoom lens and imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the diameter of a front lens in size and shorten the entire length of a lens system in the direction of an optical axis while retaining high optical performance in a zoom lens. <P>SOLUTION: The zoom lens includes four lens groups, which are the first lens group G1 to the fourth lens group G4. On zooming, the first lens group G1 and the third lens group G3 are fixed and the second lens group G2 is moved to zoom. The fourth lens group G4 is moved for correcting the position of an image plane and focusing accompanied by the zooming operation. The first lens group G1 includes three lenses which are negative, positive, and positive refractive powers. The second lens group G2 is composed of four lenses including a negative lens that has an aspherical face. The zoom lens satisfies conditional expression (1) 6.0<f1/fw<8.0 and (2) 0.10<H2f/fw<0.20, wherein f1 is the focal distance of the first lens group, fw is the focal distance of the entire system at a wide-angle end, and H2f is a distance on an optical axis from the object side face of a lens closest to the object in the second lens group to the object-side principal point of the second lens group. <P>COPYRIGHT: (C)2010,JPO&INPIT

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.

Conventionally, as a zoom lens used for a consumer video camera or the like, many 4-group type and 5-group type zoom lenses have been proposed. For example, in Patent Documents 1, 2, and 3, zooms having a high zoom ratio of about 10 times and an F-number of about 1.8 are configured with three second lens groups that are moving groups at the time of zooming. A lens is disclosed. Patent Document 4 discloses a five-group type zoom lens having four second lens groups and a total of 15 lenses. Patent Document 5 discloses a zoom lens using a composite aspherical lens in which an aspherical resin is formed on a glass spherical surface.
JP 2004-279726 A JP 2006-220737 A JP 2006-113257 A Japanese Patent Laid-Open No. 2003-322795 JP 2006-276844 A

  By the way, in recent years, zoom lenses used in consumer video cameras and the like have also become high-definition, and the demand for high-performance zoom lenses is increasing. In particular, 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 the respective colors are superposed. There is an increasing demand for a zoom lens that can support the 3CCD system for obtaining image quality. Furthermore, zoom lenses in this field are also required to be small in size while having high performance.

  Therefore, there is a demand for further miniaturization while maintaining high performance without increasing the number of lenses, compared to the conventionally proposed lenses. As the downsizing, it is conceivable to shorten the total length of the lens system in the optical axis direction and to reduce the front lens diameter (the diameter of the lens located closest to the object side). In particular, in a surveillance camera, there is a demand for reducing the lens portion exposed to the outside world, and not only the total length of the lens system in the optical axis direction but also the reduction of the front lens diameter is important.

  The thing of patent document 4 has a 15 lens structure in the whole system, and is insufficient in the point of size reduction of the lens system full length of an optical axis direction. The thing of patent document 5 uses the compound type aspherical lens for the 2nd lens group, and since the 2nd lens group cannot give so much negative power, the movement amount of the 2nd lens group is large, There is a risk that the lens system in the direction of the optical axis will increase in size.

  In addition, the lens described in Patent Document 5 has a configuration in which the front lens diameter tends to increase because the object side principal point of the second lens group cannot be brought too close to the object side. In order to reduce the diameter of the front lens, it is necessary to devise the configuration of the second lens group. In this respect, the devices described in Patent Documents 1 to 3 have room for improvement.

  The present invention has been made in view of the above circumstances, and a zoom lens that realizes both a reduction in the front lens diameter and a reduction in the total length of the lens system in the optical axis direction while maintaining high optical performance, and the zoom An object of the present invention is to provide an imaging apparatus including a lens.

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 A fourth lens group that performs correction and focusing, and the first lens group includes, in order from the object side, a cemented lens including a negative meniscus lens and a positive lens having a convex surface facing the object side, and a positive meniscus lens. The second lens group is composed of four lenses including a negative lens having an aspherical surface, the focal length of the first lens group is f1, and the focal length of the entire system at the wide angle end is fw. From the object side surface of the lens closest to the object side of the second lens group, When the distance on the optical axis to the side principal point was H2F, the following conditional expressions (1) and is characterized by satisfying the (2).
6.0 <f1 / fw <8.0 (1)
0.10 <H2f / fw <0.20 (2)

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

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

  It should be noted that the “negative lens having an aspheric surface” in the second lens group of the present invention is sufficient if at least one surface is an aspheric surface.

  The zoom lens according to the present invention has a four-lens configuration including a negative lens having an aspheric surface in the second lens group, so that it is possible to easily correct distortion and to suppress image plane fluctuation for each angle of view and zoom magnification. Thus, it is advantageous to obtain high optical performance while achieving miniaturization. Further, by satisfying conditional expression (1), it is possible to correct aberrations satisfactorily without significantly increasing the number of lenses, and it is possible to shorten the entire lens system and reduce the front lens diameter. Furthermore, by satisfying conditional expression (2), it is possible to reduce the front lens diameter while suppressing aberration fluctuation during zooming and increasing the tolerance of manufacturing errors and assembly errors.

Furthermore, in the zoom lens according to the present invention, when the radius of curvature of the object side surface of the positive lens constituting the cemented lens of the first lens group is R12 and the refractive index of the positive lens is N12, the following conditional expression ( It is preferable to satisfy 3).
3.0 <R12 / (N12 · fw) <4.0 (3)

In the zoom lens of the present invention, the two lenses from the object side of the second lens group are both negative lenses, and the combined focal length of the two negative lenses is f2a, and the focal length of the second lens group. Is preferably f2, the following conditional expression (4) is preferably satisfied.
0.60 <f2a / f2 <0.80 (4)

  In the zoom lens according to the present invention, the third lens group may be composed of a single lens. In that case, it is preferable that at least one of the lenses of the third lens group is an aspherical surface. Further, the material of one lens constituting the third lens group may be plastic.

In the zoom lens of the present invention, it is preferable that the following conditional expression (5) is satisfied, where f3 is the focal length of the third lens group.
10 <f3 / fw <23 (5)

In the zoom lens of the present invention, when the third lens group is composed of one lens, both the lens in the third lens group and the lens closest to the object side in the fourth lens group may be positive lenses. In this case, when the Abbe number of the lens in the third lens group is ν31 and the Abbe number of the lens on the most object side in the fourth lens group is ν41, it is preferable that the following conditional expression (6) is satisfied.
41 <ν41−ν31 <58 (6)

Furthermore, in the zoom lens according to the present invention, it is preferable that the following conditional expression (7) is satisfied when the focal length of the fourth lens group is f4.
3.3 <f4 / fw <3.9 (7)

  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 fourth lens group is moved, the first lens group has the above-described three-lens configuration, the second lens group has the above-described four-lens configuration, and satisfies the conditional expressions (1) and (2). Therefore, while maintaining high optical performance, a zoom lens that realizes both a reduction in the front lens diameter and a reduction in the total length of the lens system in the optical axis direction, and an image pickup apparatus including the zoom lens Can be provided.

  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.

  This zoom lens 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, an aperture stop St, a third lens group G3 that has positive refractive power and is fixed at the time of zooming, and has positive refractive power. And a fourth lens group G4 for correcting and focusing the 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, during 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, and the second lens group G2 is imaged along the optical axis. The zoom lens is configured to perform zooming by moving to the side, and to correct and focus the image plane position accompanying the zooming by moving the fourth lens group G4 along the optical axis. 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 first lens group G1 includes, in order from the object side, a cemented lens including a meniscus negative lens L11 and a positive lens L12 having a convex surface facing the object side, and a meniscus positive lens L13.

  The second lens group G2 has a four-lens configuration, one of which is a negative lens having at least one aspheric surface. This aspherical negative lens is preferably a lens arranged second most from the object side or the object side of the second lens group G2. As will be described later in the description of conditional expression (3), there is a possibility that distortion may be deteriorated when the front lens diameter is reduced. However, by using an aspherical lens for the second lens group G2, the distortion is improved. It becomes possible to correct. In addition, since the second lens group G2 includes at least one aspherical negative lens, it is possible to suppress image plane fluctuation for each angle of view and each zoom magnification.

  As a specific configuration of the zoom lens of the present embodiment, for example, as in the example illustrated in FIG. 1, the second lens group G2 includes, in order from the object side, a negative lens L21, a negative lens L22, and a positive lens L23. The third lens group G3 can be composed of one positive lens L31, and the fourth lens group G4 is composed of a positive lens L41 and a negative lens in order from the object side. It can comprise from the cemented lens of L42 and the positive lens L43. As described above, the zoom lens according to the present embodiment includes a first lens group G1 having a three-lens structure, a second lens group G2 having a four-element structure, and a third lens having a one-piece structure, as in the example illustrated in FIG. A total of eleven lens configurations comprising the group G3 and the fourth lens group G4 having a three-lens configuration can be employed.

  The zoom lens shown in FIG. 1 has two negative lenses L21 and L22 on the object side of the second lens group G2. In this case, the above-described effect due to the aspheric surface can be obtained in any of the two sheets. However, the second negative lens L22 from the object side is advantageous in terms of cost because the ray height of the light beam passing through the object-side negative lens L21 is smaller and can be made smaller. Further, when the negative lens L21 is an aspheric lens, the allowable amount of manufacturing error and assembly error is likely to be smaller than that of the negative lens L22. Therefore, if either one of the negative lenses L21 and L22 is an aspheric lens, it is more preferable that the negative lens L22 is an aspheric lens.

  In the configuration having two negative lenses on the object side of the second lens group G2 such as the zoom lens shown in FIG. 1, a large amount of negative power is distributed on the object side, and the object side principal point of the second lens group. The front lens diameter is reduced by moving the position of the lens toward the object side, shortening the distance between the principal points of the first lens group and the second lens group, and reducing the height of the off-axis light beam passing through the first lens group. Can be achieved.

  One feature of the zoom lens according to the present embodiment is that the second lens group G2 includes four lenses. Conventionally proposed 4-group type and 5-group type zoom lenses are composed of three negative, negative, and positive lenses in the second lens group, as described in Patent Documents 1, 2, and 3. There are many examples, and there are very few examples in which the second lens unit has four lenses in a 10 × 3 CCD zoom lens. As a few examples in which the second lens group is composed of four lenses, there is a zoom lens described in Patent Document 4. This zoom lens uses a large number of lenses after the third lens group, and 15 lenses in the entire system. This lens is used. That is, in the case where the conventional second lens group has four lenses, the performance is given the highest priority, and there are many cases where restrictions on the number of lenses and size reduction are not severe. On the other hand, the zoom lens according to the present embodiment is different in that, as in the example shown in FIG. 1, an eleven lens configuration is possible in the entire system, and high performance and miniaturization are realized.

  In the zoom lens shown in FIG. 1, the negative power necessary for the second lens group G2 is dispersed by adding one negative lens to the second lens group in addition to negative, negative, and positive. In order to shorten the overall length of the lens system, 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. By adding one negative lens to the second lens group, the negative power necessary for the second lens group G2 can be dispersed even when the second lens group G2 has a strong negative power. Therefore, the total length of the lens system can be reduced without the intensity of power carried by each lens becoming too strong and aberration fluctuations during zooming becoming large.

  As described above, if the number of lenses in the second lens group G2 is increased by one from the conventional example, it may be disadvantageous in terms of cost and miniaturization. By reducing the number of lenses, the increase in the total number of lenses is suppressed.

  Regarding the third lens group, the conventionally proposed four-group type and five-group type zoom lenses are composed of two or three lenses as in the patent documents 1 and 2. However, there are few examples in which the third lens group is composed of one lens. As a few examples in which the third lens group is composed of one lens, there is a zoom lens described in Patent Document 3. This zoom lens is a five-group type, and the number of lenses after the fourth lens group is increased. Yes.

  On the other hand, in the zoom lens of the present embodiment, the third lens group G3 is composed of one lens in order to reduce the size after the third lens group G3, and the total number of lenses is as small as about 11 lenses. The 4 group type is realized. By configuring the third lens group G3 with one lens, an increase in the number of lenses in the second lens group G2 can be canceled, and the lens system can be downsized.

  When the third lens group G3 is composed of one lens, it is preferably composed of an aspheric lens having at least one aspheric surface, and the material of this lens is preferably plastic. The use of an aspheric lens is advantageous in terms of aberration correction, and it is easy to configure the third lens group G3 as a single lens. By using plastic as the material, it is possible to reduce the cost and weight.

  Note that plastic is generally more affected by temperature changes than glass. However, in the zoom lens according to the present embodiment, as described later in the description of conditional expression (5), the third is used to obtain a long back focus. Since the lens group G3 is configured to have a weak power, even if plastic is used as the material of the lens, the influence on the entire system due to a temperature change is small.

  Further, when the material of one lens constituting the third lens group G3 is plastic and the material of the lens barrel of the applied image pickup apparatus is plastic, the expansion and contraction accompanying the temperature change of the lens barrel, the plastic lens As a result of the movement of the image position accompanying the temperature change, it is possible to obtain an effect that the apparent image position hardly changes even if the temperature changes. In this respect, it can be said that the plastic lens is more advantageous for temperature change than the glass lens.

  The fourth lens group G4 is arranged in the order of positive, negative, and positive in the zoom lens of the present embodiment as shown in FIG. In this way, when the positive lens is arranged on the most object side of the fourth lens group G4, the ray height of the light ray incident on the central negative lens can be reduced, which is advantageous for Petzval sum and good aberration correction. This can be performed, and an increase in the size of the fourth lens group G4 can be prevented. Considering the aberration correction with the fourth lens group G4 alone, the positive and negative as in the present embodiment rather than the negative, positive and positive power arrangement as described in Patent Document 5 is used for the fourth lens group G4. The power arrangement in the positive order is advantageous, and further, there is an advantage that the moving amount at the time of zooming or focusing can be easily reduced.

  Furthermore, it is preferable that at least one surface of the fourth lens group G4 closest to the object side is an aspherical surface. By providing an aspheric lens in the fourth lens group G4, it is possible to satisfactorily correct spherical aberration generated by the fourth lens group G4 alone, and to reduce aberration fluctuation during zooming or focusing. it can. In particular, when an aspheric lens is provided in the fourth lens group G4, a higher effect can be obtained if the lens closest to the object side in which the air interval changes during zooming is an aspheric surface.

The zoom lens of the present embodiment is configured to satisfy the following conditional expression (1), where f1 is the focal length of the first lens group G1 and fw is the focal length of the entire system at the wide angle end.
6.0 <f1 / fw <8.0 (1)

  Conditional expression (1) defines the ratio of the focal length of the first lens group G1 to the focal length of the entire system at the wide-angle end. In other words, it defines the power ratio of the first lens group G1 to the entire system. It is. Conditional expression (1) is an expression for satisfactorily correcting aberrations while preventing an increase in the total length of the lens system in the optical axis direction. If the focal length of the first lens group G1 becomes shorter than the lower limit of conditional expression (1), it becomes difficult to correct aberrations satisfactorily, and in particular, it becomes difficult to correct spherical aberration and axial chromatic aberration at the telephoto end. If the focal length of the first lens group G1 becomes longer than the upper limit of the conditional expression (1), it is advantageous for aberration correction, but the total length of the lens system becomes longer and the front lens diameter becomes larger.

In the zoom lens according to the present embodiment, the distance on the optical axis from the object-side surface of the lens closest to the object side of the second lens group G2 to the object-side principal point of the second lens group G2 is H2f, and the wide-angle end. When the focal length of the entire system is fw, the following conditional expression (2) is satisfied.
0.10 <H2f / fw <0.20 (2)

  Conditional expression (2) defines the relationship between the object-side principal point position of the second lens group G2 and the focal length at the wide-angle end. If stronger power is given to the negative lens on the object side of the second lens group G2 as it falls below the lower limit of the conditional expression (2), the aberration fluctuation at the time of zooming increases, and the tolerance of manufacturing error and assembly error is increased. It becomes small and is not preferable. If the upper limit of conditional expression (2) is exceeded, it will be difficult to reduce the diameter of the front lens.

  In addition to the above-described configuration, the zoom lens of the present embodiment preferably further employs the following configuration, whereby further downsizing or good optical performance can be achieved.

When the radius of curvature of the object side surface of the positive lens L12 of the first lens group G1 is R12, the refractive index of the positive lens L12 is N12, and the focal length of the entire system at the wide angle end is fw, the following conditional expression ( It is preferable to satisfy 3).
3.0 <R12 / (N12 · fw) <4.0 (3)

  Conditional expression (3) indicates that the ratio of the radius of curvature of the object side surface of the positive lens L12 (the cemented surface of the cemented lens of the first lens group G1) and the refractive index of the positive lens L12 is the focal length of the entire system at the wide angle end. It is standardized by. When the refractive index of the positive lens L12 is increased, a lens having the same power can relax the curvature of the lens surface as compared with a lens having a small refractive index. Therefore, a lens for securing a necessary edge (lens edge). The center thickness is small and the lens can be thinned, which is advantageous for downsizing. However, a material having a high refractive index generally has a large dispersion, and there is a problem that chromatic aberration, mainly axial chromatic aberration at the telephoto end increases. Therefore, it is not preferable that the value falls below the lower limit of conditional expression (3).

  Conversely, if the material or shape exceeds the upper limit of conditional expression (3), it is advantageous for chromatic aberration correction, but the center thickness must be increased in the positive lens L12 in order to ensure the necessary edge thickness. I will have to. In this case, the height of the off-axis light beam incident on the negative lens L11 increases, and there is a problem that the front lens diameter increases. In order to reduce the size of the front lens while correcting chromatic aberration well, it is necessary to control the shape so that the curvature does not become too large even when a low dispersion material is used for the positive lens L12. However, in that case, it becomes difficult to correct the distortion. Therefore, in the zoom lens of the present invention, the second lens group G2 includes a negative lens having an aspherical surface, so that distortion can be corrected well even when the front lens diameter is reduced. .

When the two lenses from the object side of the second lens group G2 are negative lenses, when the combined focal length of the two negative lenses is f2a and the focal length of the second lens group G2 is f2, It is preferable to satisfy conditional expression (4).
0.60 <f2a / f2 <0.80 (4)

  Conditional expression (4) is the ratio of the combined focal length of two negative lenses (negative lens L21 and negative lens L22 in the example of FIG. 1) from the object side of the second lens group G2 to the focal length of the second lens group G2. Is stipulated. Conditional expression (4) prescribes the power strength of the two negative lenses from the object side of the second lens group G2. If the lower limit of conditional expression (4) is not reached, the position of the object-side principal point of the second lens group G2 can be made closer to the object side, and the front lens diameter can be reduced, but aberration fluctuations during zooming increase. If the upper limit of conditional expression (4) is exceeded, not only will the front lens diameter increase, but it will also be difficult to cancel the longitudinal chromatic aberration generated in the first lens group G1 at the telephoto end.

When the third lens group G3 is composed of one lens (positive lens L31 in the example of FIG. 1), when the focal length of the third lens group G3 is f3 and the focal length of the entire system at the wide angle end is fw, It is preferable to satisfy the following conditional expression (5).
10 <f3 / fw <23 (5)

  Conditional expression (5) defines the ratio of the focal length of the positive lens L31 (third lens group G3) to the focal length of the entire system at the wide-angle end, and is used to form the third lens group G3 as a single lens. It can also be called a condition. For example, in order to obtain a long back focus enough to insert a color separation prism or the like between the lens closest to the image side and the image plane, the power of the third lens group G3 is weak. If the lower limit of conditional expression (5) is not reached, the power of the third lens group G3 becomes strong, and the necessary back focus cannot be obtained. On the contrary, if the power of the positive lens L31 is weakened so as to exceed the upper limit of the conditional expression (5), the curvature of the lens becomes small, and the aberration cannot be corrected. In such a long back focus and aberration correction trade-off relationship, by satisfying the conditional expression (5), both can be satisfied, and the third lens group G3 can be formed by a single lens. Thus, it is possible to obtain desired performance, and even when the second lens group G2 has a four-lens configuration, it is possible to maintain the performance with a small number of lenses of about 11 as the entire system.

When the third lens group G3 is composed of one lens (positive lens L31 in the example of FIG. 1), the lens of the third lens group G3 and the lens closest to the object side of the fourth lens group G4 (in the example of FIG. 1) When both positive lenses L41) are positive lenses, the Abbe number of the positive lens in the third lens group G3 is ν31, and the Abbe number of the positive lens closest to the object side in the fourth lens group G4 is ν41, the following conditional expression It is preferable to satisfy (6).
41 <ν41−ν31 <58 (6)

  Conditional expression (6) defines the difference between the Abbe numbers of the two positive lenses. 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 G4 is used for the positive lens L31 of the third lens group G3. If the lower limit of conditional expression (6) is not reached, chromatic aberration, particularly chromatic aberration from the middle range of the zoom to the telephoto end cannot be corrected sufficiently. If the upper limit of conditional expression (6) is exceeded, it is advantageous for correcting chromatic aberration. In this case, an ultra-low dispersion material is used for the positive lens L41 or a high dispersion material is used for the positive lens L31. In that case, it becomes difficult to select a material suitable for it. Further, the correction of chromatic aberration is devoted to correction of a specific color, and good chromatic aberration correction cannot be performed.

The following conditional expression (7) is preferably satisfied, where f4 is the focal length of the fourth lens group G4 and fw is the focal length of the entire system at the wide angle end.
3.3 <f4 / fw <3.9 (7)

  Conditional expression (7) defines the ratio between the focal length of the fourth lens group G4 and the focal length of the entire system at the wide angle end. If the lower limit of conditional expression (7) is not reached, coma will occur greatly. If the upper limit of conditional expression (7) is exceeded, the amount of movement of the fourth lens group G4 at the time of focusing increases, the overall length becomes longer, and the image plane is not canceled without canceling out the Petzval sum generated in the second lens group G2. Increases curvature.

When the focal length of the second lens group G2 is f2, and the focal length of the entire system at the wide angle end is fw, it is preferable that the following conditional expression (8) is satisfied.
1.25 <| f2 | / fw <1.50 (8)

  Conditional expression (8) defines the ratio between the focal length of the second lens group G2 and the focal length of the entire system at the wide-angle end. In order to shorten the overall length of the lens system, it is necessary to give a strong negative power to the second lens group G2 and reduce the amount of movement during zooming. However, the lower the lower limit of conditional expression (8), the second If strong power is given to the lens group G2, the power of each lens constituting the second lens group becomes too large, and the aberration fluctuation at the time of zooming becomes large. Furthermore, there is a problem that the tolerance of lens manufacturing errors and assembly errors is reduced. If the upper limit of conditional expression (8) is exceeded, the total length of the lens system becomes long, which is not preferable.

When the second lens group G2 has a positive lens and the refractive index of this positive lens is N23, it is preferable that the following conditional expression (9) is satisfied.
N23> 1.80 (9)

  Conditional expression (9) defines the refractive index of the positive lens included in the second lens group G2. As described above, it is preferable to give a strong negative power to the negative lens in the second lens group in order to reduce the front lens diameter and the total length of the lens system. In order to achieve this, it is necessary to give the positive lens included in the second lens group G2 also a strong positive power. At that time, if the refractive index of the material composing the positive lens is lower than the lower limit of the conditional expression (9), the curvature of the lens increases, and it is necessary to increase the center thickness in order to secure the necessary edge. Yes, the second lens group G2 becomes large. Alternatively, if the lower limit of the conditional expression (9) is set, the material is made of a material having a lower refractive index, and if it is intended to secure a necessary edge with a small center thickness, the image surfaces for each magnification and angle of view are not aligned. In addition, it becomes difficult to correct chromatic aberration well.

When the maximum image height is IH and the focal length of the entire system at the wide angle end is fw, it is preferable that the following conditional expression (10) is satisfied.
1.6 <fw / IH <2.0 (10)
Note that IH can be determined by, for example, the specifications of the zoom lens and the specifications of the imaging device to be applied.

  Conditional expression (10) defines the ratio between the focal length of the entire system at the wide-angle end and the maximum image height. In the zoom lens having such a configuration, when attempting to widen the angle so as to fall below the lower limit of the conditional expression (10), the height of the off-axis light beam incident on the first lens group G1 at the intermediate zoom position from the wide-angle end is increased. As a result, the size of the first lens unit becomes larger. If the upper limit of conditional expression (10) is exceeded, the angle of view becomes insufficient for application to a video camera or the like.

The zoom lens according to the present embodiment replaces the conditional expressions (1), (2), (4), (5), and (7) with the following conditional expressions (1-1), (2-1), and (4). -1), (5-1), and (7-1) are preferably satisfied, whereby the effects obtained by satisfying the above conditional expressions can be further enhanced.
6.4 <f1 / fw <7.8 (1-1)
0.12 <H2f / fw <0.20 (2-1)
0.62 <f2a / f2 <0.78 (4-1)
12 <f3 / fw <20 (5-1)
3.4 <f4 / fw <3.8 (7-1)

  The values of the above conditional expressions 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 refractive index described in the above conditional expressions. And the Abbe number is at the d-line.

  When the zoom lens of the present invention is used in a harsh environment such as outdoors, the lens arranged closest to the object is resistant to surface deterioration due to wind and rain, temperature change due to direct sunlight, and further oils, detergents, etc. 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 arranged 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, by appropriately adopting the above-described preferable configuration according to required specifications and the like, aberration correction can be performed satisfactorily without significantly increasing the number of lenses. While performing, both the reduction of the front lens diameter and the shortening of the total length of the lens system in the optical axis direction can be achieved, and a compact configuration can be achieved.

  Next, specific numerical examples of the zoom lens according to the present invention will be described. A lens cross-sectional view of the zoom lens of Example 1 is shown in FIG. Cross-sectional views of the zoom lenses of Examples 2 to 8 are shown in FIGS. The method shown in FIGS. 2 to 8 is the same as that shown in FIG.

  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, Tables 4 to 24 show basic data, zoom-related data, and aspherical data of the zoom lenses according to Examples 2 to 8, respectively. Hereinafter, the meaning of the symbols in the table will be described by taking Example 1 as an example, but the same applies to Examples 2 to 8.

  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 (corresponding to D22) indicates the surface interval between the 22nd surface and the image surface. 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 thirteenth 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. And D4 (variable), D12 (variable), D15 (variable), and D20 (variable) respectively 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. 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 angle of view 2ω, and the values of the surface spacings D5, D12, D15, and D20 that change with zooming. 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 D20 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 codes (L22, L31, L41) of lenses that are aspherical lenses, surface numbers of aspherical surfaces, and 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 25 shows values corresponding to the conditional expressions (1) to (10) in Examples 1 to 8. As can be seen from Table 25, all of Examples 1 to 8 satisfy the conditional expressions (1) to (10).

  FIG. 9A to FIG. 9H 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. 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, FIGS. 10 (A) to 10 (H), FIGS. 11 (A) to 11 (H), FIGS. 12 (A) to 12 (H), and FIGS. 13 (A) to 13 (H). 14 (A) to 14 (H), 15 (A) to 15 (H), and 16 (A) to 16 (H), the wide-angle end of the zoom lenses of Examples 2 to 8 and Each aberration diagram of spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration at the telephoto end is shown.

  From the above data, the zoom lenses of Examples 1 to 8 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. 17 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. 17, 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. 17 is a so-called 3CCD type image pickup device having three image pickup devices, but the image pickup device of the present invention is not limited to this and may be one that picks up the entire wavelength band with one image pickup device. 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 then separated into red, green, and blue color light by the color separation prisms 3R, 3G, and 3B, and 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 reduce the lens diameter exposed to the outside and can reduce the length in the optical axis direction and can be downsized. It can be configured and high-quality video can be obtained.

  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. Sectional drawing which shows the lens structure of the zoom lens concerning Example 7 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 8 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 Each aberration diagram of the zoom lens according to Example 7 of the present invention Each aberration diagram of the zoom lens according to Example 8 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 (9)

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. With 4 lens groups,
The first lens group, in order from the object side, includes a cemented lens composed of a negative meniscus lens and a positive lens having a convex surface facing the object side, and a positive meniscus lens,
The second lens group includes four lenses including a negative lens having an aspheric surface,
The focal length of the first lens group is f1, the focal length of the entire system at the wide-angle end is fw, and the object side main surface of the second lens group from the object side surface of the lens closest to the object side of the second lens group. A zoom lens satisfying the following conditional expressions (1) and (2), where H2f is a distance on the optical axis to a point.
6.0 <f1 / fw <8.0 (1)
0.10 <H2f / fw <0.20 (2) When the radius of curvature of the object side surface of the positive lens constituting the cemented lens of the first lens group is R12 and the refractive index of the positive lens is N12, the following conditional expression (3) is satisfied. The zoom lens according to claim 1.
3.0 <R12 / (N12 · fw) <4.0 (3) When the two lenses from the object side of the second lens group are negative lenses, and the combined focal length of the two negative lenses is f2a and the focal length of the second lens group is f2, the following conditions are satisfied. The zoom lens according to claim 1, wherein the zoom lens satisfies Formula (4).
0.60 <f2a / f2 <0.80 (4)   The zoom lens according to any one of claims 1 to 3, wherein the third lens group includes one lens. 5. The zoom lens according to claim 4, wherein the following conditional expression (5) is satisfied, where f3 is a focal length of the third lens group.
10 <f3 / fw <23 (5) The lens of the third lens group and the lens closest to the object side of the fourth lens group are both positive lenses, the Abbe number of the lens of the third lens group is ν31, and the lens closest to the object side of the fourth lens group is The zoom lens according to claim 4 or 5, wherein the following conditional expression (6) is satisfied when the Abbe number of the lens is ν41.
41 <ν41−ν31 <58 (6)   The zoom lens according to claim 4, wherein a material of the lens of the third lens group is plastic. The zoom lens according to claim 1, wherein the following conditional expression (7) is satisfied when a focal length of the fourth lens group is f4.
3.3 <f4 / fw <3.9 (7)   An imaging apparatus comprising the zoom lens according to claim 1.

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