JP2014178436A - Optical system and image capturing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical system capable of acquiring bright images with high resolution using a simple configuration and to provide an image capturing device having the same.SOLUTION: An optical system comprises, in order from the object side, a positive first lens group L1, an aperture stop, a positive second lens group L2, and a negative third lens group L3. The first lens group comprises, in order from the object side, a positive 1a lens group and a negative 1b lens group, and the second lens group comprises, in order from the object side, a 2a lens group having at least two positive lenses and a 2b lens group having at least one negative lens. The optical system satisfies the following conditional expressions.

Description

  The present invention relates to an optical system and an imaging apparatus, and more particularly to an optical system and an imaging apparatus for an imaging apparatus that can obtain a bright image with a simple configuration and high resolution.

  2. Description of the Related Art Conventionally, in a surveillance camera, an industrial camera, and the like (hereinafter referred to as a surveillance camera), a captured image is subjected to image processing, and thus a telephoto optical system used for these has been required to have high resolution. In recent years, the increase in the number of pixels of an image sensor has been remarkable, and for example, a high-pixel image sensor having 5 million pixels or more has been used for a surveillance camera or the like. For this reason, even in this type of telephoto optical system, higher resolution that can be applied to a high-pixel imaging device has been demanded. In addition, the telephoto optical system is also required to have a mechanism that does not focus even when vibration or the like occurs and a mechanism that can focus with high accuracy regardless of the position of the subject. Furthermore, surveillance cameras and the like are also used at night and in dim places. For this reason, the telephoto optical system also has a demand for a large-aperture lens system that can acquire a bright image even when the amount of light is insufficient.

  As this type of telephoto optical system, for example, there is one that employs a floating system in which a plurality of lens groups are independently moved when performing focusing (see, for example, “Patent Document 1”). According to the floating method, it is possible to suppress a change in aberration from an infinite distance to a close distance by adjusting the distance between the lens groups. For this reason, an image with a high resolution can be obtained regardless of the position of the subject, and it is easy to correspond to the high pixel imaging device. Further, the telephoto optical system described in Patent Document 1 achieves an F value of about 1.23, and a bright image can be obtained.

  As another focusing method, at the time of focusing, the final lens group disposed closest to the image plane side is a fixed lens group, and the lens group other than the final lens group is a focus lens group. An apparatus employing a moving method has also been proposed (see, for example, “Patent Document 2” and “Patent Document 3”).

JP 2009-251399 A JP 2004-302170 A JP 2008-257088 A

  However, the telephoto optical system described in Patent Document 1 can obtain a bright captured image with high resolution by adopting a floating method or the like. However, since it is necessary to move each lens group with a different amount of movement during focusing, a moving mechanism for moving the lens group becomes complicated. For this reason, there is a risk of focus blurring due to vibration or the like, and an increase in the size and cost of the lens barrel. Further, the complexity of the moving mechanism may lead to an increase in management effort such as maintenance.

  On the other hand, in the telephoto optical systems described in Patent Document 2 and Patent Document 3, since a system in which lens groups other than the final lens group are moved together is adopted, the lens group is compared with the case where the floating system is employed. The mechanism for moving is simple. For this reason, it becomes easy to solve various problems resulting from the complexity of the moving mechanism of the lens group. However, the telephoto optical system described in Patent Document 2 has an F value of about 4.08, which is insufficient in brightness. For this reason, the diffraction limit is lowered, and the resolution is insufficient as the telephoto optical system for the high-pixel imaging device. On the other hand, the F value of the telephoto optical system described in Patent Document 3 is about 2.88. As a telephoto optical system such as a surveillance camera, it has sufficient brightness for practical use. However, the axial chromatic aberration is large, and the resolution is still insufficient for use as a telephoto optical system for a high-pixel imaging device.

  An object of the present invention is to provide an optical system and an imaging apparatus that can obtain a bright image with a simple configuration and high resolution.

  As a result of intensive studies, the present inventors have achieved the above-mentioned problem by employing the following optical system.

  An optical system according to the present invention is an optical system including, in order from the object side, a positive first lens group, a stop, a positive second lens group, and a negative third lens group, and the first lens group Is composed of a positive first a lens group and a negative first b lens group in order from the object side, and the second lens group has at least two positive lenses in order from the object side. And a 2b lens group having at least one negative lens, and satisfying the following conditional expression.

      In the optical system according to the present invention, it is preferable that the following conditional expression is satisfied in the second lens group.

  In the optical system according to the present invention, it is preferable that the following conditional expressions are satisfied.

  In the optical system according to the present invention, in the 1b lens group, the image surface side surface of the lens disposed closest to the image surface side is a convex surface, and in the 2a lens group, the lens disposed closest to the object side. The object side surface is preferably a convex surface.

  In the optical system according to the present invention, it is preferable that the following conditional expressions are satisfied.

  In the optical system according to the present invention, it is preferable to move the first lens group and the second lens group together from the image plane side to the object side when focusing on an object at a short distance from infinity.

  An imaging apparatus according to the present invention includes the optical system and an imaging element, and the optical system forms an image of light incident from the object side on an imaging surface of the imaging element.

  According to the optical system of the present invention, the moving mechanism of the lens group for focusing can have a simple configuration, and a bright image with high resolution can be obtained. For this reason, it can be suitably used as an optical system for a high-pixel image sensor having a number of pixels exceeding 5 million, and in particular, can be suitably used as a telephoto optical system for a high-pixel image sensor.

2 is a cross-sectional view illustrating a configuration of an optical system according to Example 1. FIG. FIG. 4 is an aberration diagram (spherical aberration, astigmatism, distortion, lateral chromatic aberration; the same applies below) of the optical system according to Example 1 in an infinitely focused state. FIG. 3 is an aberration diagram for the optical system according to Example 1 in a close-up focus state. 5 is a cross-sectional view illustrating a configuration of an optical system according to Example 2. FIG. FIG. 6 is an aberration diagram for the optical system of Example 2 at an infinitely focused state. FIG. 5 is an aberration diagram for the optical system according to Example 2 in a close distance focusing state. 6 is a cross-sectional view illustrating a configuration of an optical system according to Example 3. FIG. FIG. 10 is an aberration diagram for the optical system according to Example 3 at an infinitely focused state. FIG. 10 is an aberration diagram for the optical system according to Example 3 in a close distance focusing state. 6 is a cross-sectional view showing a configuration of an optical system of Example 4. FIG. FIG. 10 is an aberration diagram for the optical system of Example 4 at an infinitely focused state. FIG. 10 is an aberration diagram for the optical system according to Example 4 in a close distance focusing state.

  Hereinafter, embodiments of the optical system according to the present invention will be described. In the following, first, a configuration example of an optical system according to the present invention will be described, and then conditions that the optical system should satisfy or conditions that should preferably be satisfied will be described in order.

1. Configuration of Optical System The optical system according to the present invention includes, for example, a positive first lens unit L1, a stop STOP, a positive second lens unit L2, and a negative third lens in order from the object side, as shown in FIG. A group L3 is provided. The optical system of the present embodiment will be described by taking an imaging optical system that forms an image of light incident from the object side on an imaging device arranged on the image plane IMG.

  The first lens unit L1 includes, in order from the object side, a positive 1a lens unit and a negative 1b lens unit. FIG. 1 shows a configuration example including a positive lens L11, a positive lens L12, and a negative lens L13 arranged in order from the object side, as a specific lens configuration of the 1a lens group. In addition, FIG. 1 includes a negative lens L14 and a positive lens L15 arranged in order from the object side as a specific lens configuration of the 1b lens group, and the negative lens L14 and the positive lens L15 are cemented lenses. The case was illustrated. However, the specific lens configurations of the 1a lens group and the 1b lens group are not limited to the configuration example shown in FIG. 1, and can be changed as appropriate.

  The second lens group L2 includes, in order from the object side, a second a lens group having at least two positive lenses L21 and L22 and a second b lens group having at least one negative lens L23. FIG. 1 shows the 2a lens group composed of two positive lenses L21 and L22. However, the 2a lens group may include other lenses in addition to the two positive lenses L21 and L22. Of course it is good. FIG. 1 shows the second b lens group including one negative lens L23, but the second b lens group may include other lenses in addition to the negative lens L23. It is. However, it is preferable to adopt the configuration example shown in FIG. 1 because the second lens unit L2 can be configured with a minimum number of lenses, and the optical system can be reduced in weight and cost.

  Here, as described above, the stop STOP is disposed between the first lens unit L1 having a positive refractive power and the second lens unit L2 having the same positive refractive power. In the present invention, it is preferable that the surfaces of the two lenses L15 and L21 adjacent to the stop STOP on the stop STOP side are convex surfaces. That is, the image surface side surface (surface number 9) of the lens L15 disposed closest to the image surface side in the 1b lens group is a convex surface, and the object of the lens L21 disposed closest to the object side in the 2a lens group. The side surface (surface number 11) is preferably a convex surface. By making convex surfaces (surface number 9 and surface number 11) on the stop STOP side of the lenses (L15 and L21) arranged before and after the stop STOP in the optical axis direction, the light flux entering and exiting the stop STOP is provided. , The optical system can be made compact, and a bright image can be obtained.

  As long as the third lens unit L3 has a negative refractive power as a whole, the specific lens configuration is not particularly limited. FIG. 1 illustrates a case where a positive lens L31 and a negative lens L32 arranged in order from the object side are provided as a specific lens configuration of the third lens unit L3.

  In the optical system according to the present invention, when focusing on an object at a short distance from infinity, it is preferable to move the first lens unit L1 and the second lens unit L2 integrally from the image plane IMG side to the object side. At this time, the third lens unit L3 is preferably a fixed lens unit. That is, by adopting a focusing method that moves the lens units (the first lens unit L1 and the second lens unit) other than the final unit (the third lens unit L3) at the time of focusing, the lens units move differently. The mechanism for moving the lens group can be simplified as compared with the case where the lens group is moved independently by the amount. In addition, since the moving mechanism can be simplified, it is possible to prevent focus blurring even when external vibration occurs.

2. Conditional expression 2-1. Conditional expression (1) to conditional expression (3)
The optical system according to the present invention has the above-described configuration and satisfies the following conditional expressions (1) to (3). An optical system with high resolution can be obtained by satisfying the following conditional expressions (1) to (3). For this reason, even when the optical system according to the present invention is used as an optical system for an image pickup apparatus equipped with a solid-state image pickup device (CCD, CMOS, etc.) having an ultra-high pixel exceeding 5 million pixels, for example, the resolution is bright. A clear image with high image quality can be obtained. In addition, according to the optical system according to the present invention, it is possible to obtain a high-quality image even when the ambient brightness is insufficient. Therefore, the optical system is particularly preferably used as an optical system for a monitoring camera or the like. it can.

1-1. Conditional expression (1)
First, conditional expression (1) will be described. The conditional expression (1) is an expression relating to the ratio of the combined focal length of the first lens unit L1 and the second lens unit L2 to the focal length of the entire optical system. That is, the conditional expression (1) is an expression relating to the ratio between the combined refractive power of the first lens unit L1 and the second lens unit L2 and the refractive power of the entire optical system. When the upper limit of conditional expression (1) is exceeded, the combined refractive power of the first lens unit L1 and the second lens unit L2 increases, and accordingly, the refractive power of the third lens unit L3 also increases. In this case, aberration (especially field curvature) is likely to occur in the third lens unit L3, which is not preferable because the resolution of the optical system is lowered. In this respect, the upper limit value of conditional expression (1) is more preferably 1.1 or less.

  On the other hand, when the combined refractive power of the first lens unit L1 and the second lens unit L2 is less than the lower limit value of the conditional expression (1), the refractive power of the third lens unit L3 also decreases. In this case, the occurrence of aberration in the third lens unit L3 is suppressed, but the amount of extension of the first lens unit L1 and the second lens unit L2 during focusing is increased. When the amount of extension of the first lens unit L1 and the second lens unit L2 during focusing is increased, vibration is likely to occur when these lens units L1 and L2 are moved, which may affect the image quality. Absent. Further, when the amount of extension of the first lens unit L1 and the second lens unit L2 is increased, the total length of the optical system in the short distance in-focus state is increased. Therefore, from the viewpoint of realizing a compact optical system, it is required to be not less than the lower limit value of the conditional expression (1). From these viewpoints, the lower limit value of conditional expression (1) is preferably 0.8 or more, and more preferably 0.9 or more.

1-2. Conditional expression (2)
The conditional expression (2) is an expression relating to the ratio of the focal length of the 1a lens group constituting the first lens group L1 and the focal length of the 1b lens group. When this ratio exceeds the upper limit value of conditional expression (2), the refractive power of the 1a lens group becomes weaker than the refractive power of the 1b lens group, so that it is difficult to correct lateral chromatic aberration. Further, the Petzval sum in the entire optical system is increased, and the field curvature is increased, so that the resolution is lowered. From these viewpoints, the upper limit value of conditional expression (2) is more preferably 1.4 or less.

1-3. Conditional expression (3)
The conditional expression (3) is an expression relating to the ratio of the focal length of the second lens unit L2 to the focal length of the second lens unit L2. When the ratio exceeds the upper limit value of conditional expression (3), the ratio of the refractive powers of the 2a lens group and the second lens group L2 becomes large. In this case, the variation in the distance of the spherical aberration is increased, and the resolution at the time of focusing at a short distance is lowered, which is not preferable. From this viewpoint, the upper limit value of conditional expression (3) is preferably 0.65 or less.

  Further, when the ratio is less than the lower limit value of conditional expression (3), the ratio of the refractive powers of the 2a lens group and the second lens group L2 becomes small. In this case, the curvature of field increases and the resolution decreases, which is not preferable. From this viewpoint, the lower limit value of conditional expression (3) is preferably 0.3 or more.

2. Conditional expression (4) and conditional expression (5)
Next, conditional expression (4) and conditional expression (5) will be described. In the optical system according to the present invention, the conditional expressions (1) to (3) are satisfied, and the following conditional expression (4) and conditional expression (5) are satisfied in the second lens unit L2. preferable.

  Conditional expression (4) is an expression relating to the Abbe number of the d-line of the positive lens L21 arranged closest to the object side in the 2a lens group. Conditional expression (5) is an expression relating to the Abbe number of the d-line of the positive lens L22 arranged closest to the image plane in the 2a lens group. By adopting a lens that satisfies these conditional expressions (4) and (5), the balance of chromatic aberration is improved and an image with high resolution can be obtained. On the other hand, when the Abbe number of the d-line of each of the positive lenses L21 and L22 arranged in the 2a lens group is outside the range of the conditional expression (4) or the conditional expression (5), the lateral chromatic aberration and the axial chromatic aberration This is not preferable because the balance is lost and the resolution is lowered. From these viewpoints, the upper limit value of conditional expression (4) is preferably 23 or less, and the lower limit value of conditional expression (5) is preferably 48 or more.

3. Conditional expression (6)
Next, conditional expression (6) will be described. In the optical system according to the present invention, it is more preferable that the following conditional expression (6) is satisfied together with the conditional expressions (1) to (3).

  Conditional expression (6) is a conditional expression relating to the ratio of the optical total length when focusing on the optical system at infinity with respect to the back focus of the optical system. Here, the back focus of the optical system refers to the optical axis between the image plane side surface (surface number 20) of the lens L32 disposed closest to the image plane in the third lens unit L3 and the image plane IMG. Refers to the distance. By satisfying the relationship of the following conditional expression (6), various aberrations including chromatic aberration can be corrected more satisfactorily. In particular, the optical system according to the present invention can be satisfactorily applied to a telephoto optical system having a narrow angle of view (for example, an angle of view of about 12.5 °). By satisfying the conditional expression (6), high magnification The telephoto optical system can obtain a bright and clear image with high resolution. From these viewpoints, the upper limit value of conditional expression (6) is preferably 5.1 or less, and the lower limit value of conditional expression (6) is preferably 3.8 or more.

4). Conditional expression (7)
Finally, conditional expression (7) will be described. In the optical system according to the present invention, it is more preferable that the following conditional expression (7) is satisfied.

  Conditional expression (7) is an expression relating to the ratio of the combined focal length of the first lens unit L1 and the second lens unit L2 to the focal length of the third lens unit L3. That is, the conditional expression (7) is an expression relating to the ratio between the combined refractive power of the first lens unit L1 and the second lens unit L2 and the refractive power of the third lens unit L3. When the upper limit of conditional expression (7) is exceeded, the combined refractive power of the first lens unit L1 and the second lens unit L2 increases, and the amount of extension of the first lens unit L1 and the second lens unit L2 during focusing is as follows. Although the number is reduced, the Petzval sum in the entire optical system is increased and field curvature occurs, resulting in a decrease in resolution.

  On the other hand, when the conditional expression (7) is less than the lower limit value, the field curvature is good. However, the amount of extension of the first lens unit L1 and the second lens unit L2 at the time of focusing increases, and for the same reason as described above, there is a risk of affecting the image quality due to vibration when the lens unit is moved. This is not preferable. Similarly, from the viewpoint of realizing a compact optical system, it is required to be equal to or greater than the lower limit value of the conditional expression (7).

  According to the optical system according to the present invention described above, since the first lens unit L1 and the second lens unit L2 are moved together during focusing, the lens group moving mechanism can have a simple configuration. it can. Further, by satisfying conditional expressions (1) to (3), a bright image with high resolution can be obtained. By satisfying conditional expressions (4) to (6), a bright image with higher resolution can be obtained. For this reason, it can be suitably used as an optical system for a high-pixel image pickup device having a number of pixels exceeding 5 million, and particularly preferably used as a telephoto optical system for a high-pixel image pickup device mounted on a surveillance camera or the like. it can. Further, according to the present invention, the three-group optical system having high optical performance can be configured with a small number (for example, 10) of lenses. Furthermore, various aberrations such as field curvature can be effectively corrected by satisfying the above conditional expression without using an aspheric lens or the like. Therefore, since all the lenses can be spherical lenses, the cost of the optical system can be reduced.

  Next, an Example is shown and this invention is demonstrated concretely. However, the present invention is not limited to the following examples.

  First, the optical system of Example 1 will be described. A specific lens configuration of the optical system of Example 1 is shown in FIG. Since the lens configuration shown in FIG. 1 is as described above, the description thereof is omitted here. In FIG. 1, surface numbers are assigned in order from the object side to the surfaces of the lenses constituting the optical system 100. Further, one surface number (surface number 8) is given to the cemented surfaces of the cemented lenses L14 and L15 constituting the first group b. Table 1 shows specific numerical data of each lens constituting the optical system of Example 1. In Table 1 below, “D16” is a surface (16th surface) on the image surface side of the lens L23 that is disposed closest to the image surface side of the second b lens group in the infinitely focused state and the close-in-focus state. And the distance from the object side surface (the 17th surface) of the lens L31 arranged closest to the object side of the third lens unit L3. The optical system of Example 1 moves the first lens unit L1 and the second lens unit L2 integrally from the image plane side to the object side when focusing on an object at a short distance from infinity. For this reason, it is only the space between the 16th surface and the 17th surface that changes the surface interval in the infinitely focused state and the close-in-focus state. Each numerical data shown in Tables 2 to 4 is the same as that in Table 1.

  2 and 3 show respective aberration diagrams (spherical aberration, astigmatism, distortion aberration, chromatic aberration of magnification) in an infinitely focused state. In each aberration diagram, “F” indicates the aberration at the F line, “C” indicates the aberration at the C line, and “d” indicates the aberration at the d line. In astigmatism, “S” indicates the sagittal direction, and “T” indicates the tangential direction. Further, Table 5 shows numerical values of the conditional expressions (1) to (7). All of the embodiments described below including the first embodiment satisfy the conditional expressions (1) to (7).

  Next, the optical system of Example 2 will be described. FIG. 4 shows a specific lens configuration of the optical system according to the second embodiment. Table 2 shows specific numerical data of each lens constituting the optical system of Example 2. Since the optical system has the same configuration as that of the first embodiment, description thereof is omitted here. FIGS. 5 and 6 show aberration diagrams in the infinitely focused state. Table 5 shows numerical values of the conditional expressions (1) to (7).

  Next, the optical system of Example 3 will be described. FIG. 7 shows a specific lens configuration of the optical system according to the second embodiment. Table 3 shows specific numerical data of each lens constituting the optical system of Example 3. Since the optical system has the same configuration as that of the first embodiment, description thereof is omitted here. FIG. 8 and FIG. 9 show aberration diagrams in the infinitely focused state, respectively. Table 5 shows numerical values of the conditional expressions (1) to (7).

  Finally, the optical system of Example 4 will be described. FIG. 10 shows a specific lens configuration of the optical system according to the second embodiment. Table 4 shows specific numerical data of each lens constituting the optical system of Example 4. Since the optical system has the same configuration as that of the first embodiment, description thereof is omitted here. FIG. 8 and FIG. 9 show aberration diagrams in the infinitely focused state, respectively. Table 5 shows numerical values of the conditional expressions (1) to (7).

  According to the optical system of the present invention, the moving mechanism of the lens group for focusing can have a simple configuration, and a bright image with high resolution can be obtained. For this reason, it can be suitably used as an optical system for a high-pixel image sensor having a number of pixels exceeding 5 million, and in particular, can be suitably used as a telephoto optical system for a high-pixel image sensor.

L1 ... 1st lens group L2 ... 2nd lens group L3 ... 3rd lens group 1a ... 1a lens group 1b ... 1b lens group 2a ... 2a lens group 2b ..2b lens group STOP ... Aperture

Claims (7)

An optical system composed of, in order from the object side, a positive first lens group, a stop, a positive second lens group, and a negative third lens group,
The first lens group includes, in order from the object side, a positive 1a lens group and a negative 1b lens group,
The second lens group includes, in order from the object side, a second a lens group having at least two positive lenses and a second b lens group having at least one negative lens.
An optical system satisfying the following conditional expression:

The optical system according to claim 1, wherein the second lens group satisfies the following conditional expression.

The optical system according to claim 1, wherein the optical system satisfies the following conditional expression.

In the 1b lens group, the image surface side surface of the lens disposed closest to the image surface side is a convex surface,
4. The optical system according to claim 1, wherein in the second-a lens group, an object-side surface of a lens disposed closest to the object side is a convex surface. 5. The said optical system WHEREIN: The optical system as described in any one of Claims 1-4 which satisfies the following conditional expressions.

  The optical system according to any one of claims 1 to 5, wherein when focusing on an object at a short distance from infinity, the first lens group and the second lens group are moved together from the image plane side to the object side.   It comprises the optical system according to any one of claims 1 to 6 and an imaging device, and the optical system forms an image of light incident from the object side on an imaging surface of the imaging device. An imaging device.

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