JP2017116763A - Optical and imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical system achieving a wider angle and a larger diameter and capable of achieving high optical performance, and an imaging apparatus including the optical system.SOLUTION: An optical system comprises, in order from an object side, a first lens group G1 having positive refractive power and a second lens group G2 having positive refractive power, fixes the first lens group G1 in an optical axis direction, and moves the second lens group G2 to perform focusing from an infinity object to a finite distance object. The first lens group G1 includes at least three negative lenses, a positive lens LA, and a positive lens LB. A predetermined condition is satisfied.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical system and an imaging apparatus, and more particularly, to an optical system having a large aperture suitable for wide-angle imaging and an imaging apparatus including the optical system.

  Conventionally, imaging devices using solid-state imaging devices such as digital still cameras and digital video cameras have become widespread. Along with this, the performance of optical systems used in the imaging apparatus has been rapidly improved. In recent years, in addition to the improvement in performance of optical systems, there has been a great demand for an increase in the diameter and widening of optical systems. In particular, there is a great demand for a large aperture and wide angle for a single focus optical system. For example, in a single focus optical system, a bright optical system with Fno smaller than 2.8 is required. Similarly, in a single focus optical system, an optical system capable of wide-angle imaging having an angle of view (2ω) exceeding 60 degrees is required.

  As such an optical system, for example, a wide-angle lens described in Patent Document 1 is known. The wide-angle lens described in Patent Document 1 includes a first lens group having a positive refractive power and a second lens group having a positive refractive power, and the second lens is in a state where the position of the first lens group is fixed. The group is moved to the rear object side to focus on a finite distance object. The wide-angle lens has an angle of view of 83 ° to 84 ° and an Fno of about 1.4, and realizes a wide angle and a large aperture.

JP 2011-59290 A

However, the wide-angle lens described in Patent Document 1 has insufficient correction of axial chromatic aberration and lateral chromatic aberration, and cannot be said to have sufficiently improved performance.
An object of the present invention is to provide an optical system capable of realizing a wide angle and a large aperture and realizing high optical performance, and an image pickup apparatus including the optical system.

  In order to solve the above problems, an optical system of the present invention includes, in order from the object side, a first lens group having a positive refractive power and a second lens group having a positive refractive power, and the first lens. The group is fixed in the optical axis direction, and the second lens group is moved to focus from an object at infinity to a finite distance object. The first lens group includes at least three negative lenses and a positive lens. Including LA and positive lens LB, the following conditions are satisfied.

νd1b1 <35.0 (1)
νd1b2> 65.0 (2)

However,
νd1b1 is an Abbe number with respect to the d-line of the positive lens LA included in the first lens group,
νd1b2 is an Abbe number with respect to the d-line of the positive lens LB included in the first lens group.

  The imaging apparatus of the present invention includes the optical system according to the present invention, and an image sensor that converts an optical image formed by the optical system into an electrical signal on the image side of the optical system. Features.

  According to the present invention, it is possible to provide an optical system capable of realizing a wide angle and a large aperture and realizing high optical performance, and an imaging apparatus including the optical system.

It is sectional drawing which shows the lens structural example at the time of infinity focusing of the optical system of Example 1 of this invention. FIG. 6 is a longitudinal aberration diagram of the optical system of Example 1 when focusing on infinity. However, spherical aberration, astigmatism and distortion are shown in order from the left toward the drawing. The same applies to the longitudinal aberration diagrams. 3 is a lateral aberration diagram illustrating coma aberration when the optical system of Example 1 is focused at infinity. FIG. FIG. 4 is a longitudinal aberration diagram of the optical system of Example 1 when focusing on 0.5 m. 6 is a lateral aberration diagram illustrating coma aberration when the optical system of Example 1 is focused on 0.5 m. FIG. It is sectional drawing which shows the lens structural example at the time of infinity focusing of the optical system of Example 2 of this invention. 6 is a longitudinal aberration diagram of the optical system of Example 2 when focusing on infinity. FIG. 6 is a lateral aberration diagram illustrating coma aberration when the optical system of Example 2 is focused at infinity. FIG. 6 is a longitudinal aberration diagram of the optical system of Example 2 when focusing on 0.5 m. FIG. 6 is a lateral aberration diagram illustrating coma aberration when the optical system of Example 2 is focused on 0.5 m. FIG. It is sectional drawing which shows the lens structural example at the time of infinity focusing of the optical system of Example 3 of this invention. 6 is a longitudinal aberration diagram of the optical system of Example 3 when focusing on infinity. FIG. FIG. 12 is a lateral aberration diagram illustrating coma aberration of the optical system according to practical example 3 when focusing on infinity. 6 is a longitudinal aberration diagram of the optical system of Example 3 when focusing on 0.5 m. FIG. 6 is a lateral aberration diagram illustrating coma aberration when the optical system of Example 3 is focused on 0.5 m. FIG.

  Hereinafter, embodiments of an optical system and an imaging apparatus according to the present invention will be described.

1. Optical system 1-1. Configuration of Optical System An optical system according to an embodiment of the present invention includes, in order from the object side, a first lens group having a positive refractive power and a second lens group having a positive refractive power, and the first lens group. Is fixed in the optical axis direction, and the second lens group is moved to focus from an object at infinity to a finite distance object. The first lens group includes at least three negative lenses, a positive lens LA, and a positive lens. Including the lens LB, and satisfies the conditions described later. First, after describing the configuration of the optical system according to the present embodiment, items related to conditional expressions will be described.

  The optical system includes a first lens group having a positive refractive power and a second lens group having a positive refractive power in order from the object side. By giving a positive refractive power to the second lens group disposed on the image side, the light beam can be converged on the image side group. Therefore, it is possible to increase the diameter of the optical system.

  The optical system can be used as a wide-angle lens capable of wide-angle imaging. In general, in a wide-angle lens, an off-axis light beam incident at a large incident angle is converged by a first lens group. For this reason, the incident angle of the off-axis light beam with respect to the lens surface of the lens disposed closest to the object side in the first lens group is large, and aberration is likely to occur, and the amount of aberration is likely to increase. Therefore, in the optical system, the first lens group includes at least three negative lenses, the positive lens LA, and the positive lens LB, thereby suppressing the occurrence of aberration, widening the angle, and high optical performance. Realization was possible. Further, since the off-axis light beam can be corrected well, high optical performance can be realized in the entire screen.

  Further, in the optical system, the first lens group is fixed in the optical axis direction, and only the second lens group is moved, thereby focusing from an infinite object to a finite distance object. Since the first lens group has a positive refractive power, the light beam converged in the first lens group can be incident on the second lens group. Therefore, it is easy to reduce the diameter of the second lens group. Further, by using only the second lens group as a moving group at the time of focusing, it is possible to reduce aberration fluctuations due to focusing from infinity to a short distance. Therefore, good optical performance can be realized in the entire focus area regardless of the object distance. In addition, by reducing the diameter of the second lens group, it is possible to reduce the weight of the focus group and to perform a quick focusing operation. At the same time, the focus drive mechanism can be reduced in size and weight. Hereinafter, the configuration of each lens group will be described in more detail.

(1) First lens group The first lens group has a positive refractive power, and its specific configuration is particularly limited as long as it includes at least three negative lenses, a positive lens LA, and a positive lens LB. is not. However, it is preferable that the first lens group has at least one object-side concave surface in which the object-side surface is concave on the object side. On the object-side concave surface arranged in the first lens group, the degree of influence of off-axis aberration on on-axis aberration is large. In other words, the amount of off-axis aberration, particularly the amount of coma, can be reduced without affecting the on-axis aberration. Therefore, it becomes easy to obtain an optical system having better optical performance.

  At this time, it is preferable that the lens arranged closest to the object side among the at least three negative lenses included in the first lens group is a meniscus lens having a convex shape on the object side. By disposing such a meniscus lens on the most object side of the first A lens group, even when off-axis rays are incident on the meniscus lens at a large incident angle, the occurrence of various aberrations can be suppressed. It becomes easy to realize high optical performance. At this time, it is more preferable that the meniscus lens has a negative refractive power.

  The first lens group is preferably composed of a first A lens group having negative refractive power and a first B lens group having positive refractive power in order from the object side. At this time, it is preferable that the surface disposed closest to the object side in the first B lens group is the object-side concave surface having the largest curvature among the object-side concave surfaces included in the first lens group.

  By configuring the first lens group as described above, a negative refractive power can be disposed on the object side of the optical system, and a positive refractive power can be disposed on the image side. That is, by adopting a so-called retrofocus type refractive power arrangement, it becomes easy to widen the angle of the optical system. Further, by adopting the refractive power arrangement, the front lens diameter can be reduced. In addition, by adopting a retrofocus type refractive power arrangement, it is possible to ensure a predetermined back focus required for the optical system such as an interchangeable lens of a single-lens reflex camera even when the optical system is widened. Becomes easier.

  At this time, it is preferable that the first A lens group includes at least one positive lens and the first B lens group includes at least one negative lens. By adopting this configuration, it is possible to satisfactorily correct axial chromatic aberration and lateral chromatic aberration, and to realize high performance of the optical system.

i) First A Lens Group Here, it is preferable that at least one positive lens included in the first A lens group is disposed on the image side with respect to the widest air gap in the first A lens group. By arranging the positive lens in the first A lens group in this way, negative refractive power can be arranged on the object side of the first A lens group and positive refractive power can be arranged on the image side. That is, even within the first A lens group, a retrofocus type refractive power arrangement can be further adopted, and it becomes easy to widen the angle of the optical system. In addition, by adopting the refractive power arrangement, it is effective in further reducing the front lens diameter.

  In the first A lens group, it is preferable that the surface disposed closest to the object side is an object-side convex surface that is convex on the object side. In this case, it is effective for correcting distortion and image surface aberration.

ii) 1B lens group In the 1B lens group, the surface arranged closest to the object side is an object side concave surface that is concave on the object side, and has the largest curvature among the object side concave surfaces included in the first lens group. A surface is preferred. By doing so, off-axis aberrations, in particular, sagittal coma aberration peculiar to a large aperture wide-angle lens can be corrected well.

  In addition, it is preferable that the lens component having the object-side concave surface disposed closest to the object side of the first B lens group is a negative lens component having negative refractive power. Here, the lens component refers to a single lens, a compound lens obtained by molding aspherical resin on one or both surfaces of the lens, and the like. The lens component may be a cemented lens in which two or more lenses are cemented. However, in the case of a cemented lens, a lens disposed closest to the object side of the cemented lens is referred to as a lens component.

  In addition, it is preferable that the first lens group includes the positive lens LA and the positive lens LB. When the first B lens group includes these two positive lenses, sagittal coma flare can be corrected more satisfactorily.

(2) Second Lens Group The specific lens configuration of the second lens group is not particularly limited as long as it has a positive refractive power when viewed as a whole. However, in order to solve the problems according to the present invention, the following configuration is preferable.

  In the second lens group, it is preferable that the surface arranged closest to the object side is an object-side convex surface convex toward the object side. In the optical system according to the present invention, the first lens group has a positive refractive power. Therefore, the light beam converged by the first lens group is incident on the second lens group. At this time, the surface of the second lens group that is disposed closest to the object side is the object-side convex surface, which makes it easy to correct spherical aberration. Therefore, better optical performance can be realized. At the same time, the number of lenses constituting the second lens group can be reduced, and it becomes easy to reduce the size of the optical system. Further, by configuring the second lens group with a smaller number of lenses, the focus group can be reduced in size and weight, and a quick focusing operation can be performed.

  Furthermore, the lens component including the object-side convex surface is preferably a positive lens component having a positive refractive power. In this case, since the height of the light beam incident on the second lens group can be reduced, the spherical aberration can be corrected more effectively.

  The second lens group may be divided into two or more partial lens groups, and each partial lens group may be moved during focusing. At this time, the subject of the present invention can be achieved even if the movement amounts of the respective partial lens groups are different. Note that if each partial lens group is moved with a different amount of movement during focusing, it becomes easier to perform better aberration correction over the entire focus area regardless of the object distance.

(3) Aperture stop In the optical system according to the present invention, it is preferable to have an aperture stop in the second lens group. Since the diameter of the axial light beam incident on the second lens group is converged by the first lens group as described above, the aperture stop can be downsized. In addition, the driving mechanism for driving the aperture stop can be reduced in size and weight. Therefore, it becomes easy to reduce the size and weight of the optical system. Note that the second lens group may be divided into partial lens groups as described above before and after the aperture stop.

1-2. Lens Group Configuration The optical system only needs to include the first lens group and the second lens group described above, and may include a subsequent lens group on the image side of the second lens group. For example, a third lens group having positive or negative refractive power can be disposed on the image side of the second lens group. At this time, the specific lens configuration of the subsequent lens group is not particularly limited.

  Further, the number of lens groups constituting the optical system is not particularly limited, but from the viewpoint of further miniaturization of the optical system, the optical system is configured by the first lens group and the second lens group. More preferably.

1-3. Operation at the time of focusing In the optical system according to the present invention, the first lens group is fixed in the optical axis direction, and the second lens group is moved along the optical axis, so that the object from the infinity object to the finite distance object is aligned. As long as focusing is performed, the fixing / moving of the subsequent lens group following the second lens group is not particularly limited. However, when the lens group is provided on the image side of the second lens group from the viewpoint of reducing the weight of the focus group, the subsequent lens group is preferably fixed in the optical axis direction during focusing.

1-4. Conditional Expressions Next, conditions that should be satisfied by the optical system or conditions that are preferably satisfied will be described. The optical system shall satisfy the following conditions.

Conditional expression (1): νd1b1 <35.0
Conditional expression (2): νd1b2> 65.0

However,
νd1b1 is an Abbe number with respect to the d line of the positive lens LA included in the first lens group,
νd1b2 is an Abbe number with respect to the d-line of the positive lens LB included in the first lens group.

1-4-1. Conditional expression (1)

  Conditional expression (1) defines the Abbe number with respect to the d-line of the positive lens LA included in the first lens group. The positive lens LA may be any positive lens among the positive lenses included in the first lens group. By satisfying conditional expression (1), it becomes easy to correct lateral chromatic aberration, and an optical system having good optical performance can be obtained.

  In obtaining the above effect, the upper limit of conditional expression (1) is preferably 33.0, more preferably 32.0, and even more preferably 28.0.

1-4-2. Conditional expression (2)
Conditional expression (2) defines the Abbe number with respect to the d-line of the positive lens LB included in the first lens group. The positive lens LB is any positive lens included in the first lens group, and is a lens different from the positive lens LA. When the conditional expression (2) is satisfied, the axial chromatic aberration can be easily corrected, and an optical system having good optical performance can be obtained.

  In obtaining the above effect, the upper limit of conditional expression (2) is preferably 68.0.

  In the optical system, the first lens group includes both the positive lens LA that satisfies the conditional expression (1) and the positive lens LB that satisfies the conditional expression (2), thereby correcting axial chromatic aberration and lateral chromatic aberration. Correction can be performed at the same time. Note that the arrangement order of the positive lens LA and the positive lens LB in the first lens group is not particularly limited. That is, the positive lens LA may be disposed on the object side of the positive lens LB, or may be disposed on the image side. In any case, the above-described effect is achieved. However, when the first lens group includes a first A lens group having negative refractive power in order from the object side and a first B lens group having positive refractive power, the positive lens LA and the positive lens LB are It is preferably included in the 1B lens group.

1-4-3. Conditional expression (3)
In the optical system, it is preferable that the following condition is satisfied when the first lens group includes the first A lens group and the first B lens group.

  Conditional expression (3): −0.95 <f1a / f1b <−0.40

However,
f1a is the focal length of the 1A lens group,
f1b is the focal length of the first B lens group.

  Conditional expression (3) defines the ratio of the focal length of the first A lens group to the focal length of the first B lens group. When the conditional expression (3) is satisfied, the field curvature and the coma can be corrected well. Also, sagittal coma flare can be corrected well. For these reasons, it becomes easy to obtain an optical system having higher optical performance.

  On the other hand, if the numerical value of the conditional expression (3) is greater than or equal to the upper limit value, that is, if the refractive power of the first A lens group is increased beyond an appropriate range, it is difficult to correct field curvature and coma aberration, It is not preferable. On the other hand, if the numerical value of the conditional expression (3) is less than or equal to the lower limit value, that is, if the refractive power of the first B lens group becomes stronger than an appropriate range, it is difficult to correct sagittal coma flare.

  In obtaining the above effect, the lower limit value of conditional expression (3) is more preferably −0.90, still more preferably −0.85, even more preferably −0.80, − Most preferably, it is 0.75. The upper limit value of conditional expression (3) is more preferably −0.45, and further preferably −0.50.

1-4-4. Conditional expression (4)
When the first lens group is composed of the first A lens group and the first B lens group, the object side concave surface having the largest curvature among the object side concave surfaces included in the first lens group satisfies the following conditions: Is preferred.

  Conditional expression (4): 0.48 <Cr1bf / f1a <1.70

However,
Cr1bf is the radius of curvature of the object-side concave surface having the largest curvature among the object-side concave surfaces included in the first lens group, and f1a is the same as described above.

  Conditional expression (4) is an expression related to the object-side concave surface having the largest curvature among the object-side concave surfaces included in the first lens group, that is, the object-side concave surface arranged closest to the object side in the first B lens group. Specifically, the ratio of the radius of curvature of the object-side concave surface to the focal length of the first A lens group is defined. When the conditional expression (4) is satisfied, the light beam incident on the object-side concave surface having the largest curvature among the object-side concave surfaces included in the first lens group does not generate large refraction. That is, it is possible to suppress the occurrence of spherical aberration on the object-side concave surface and to increase the influence of off-axis aberrations on on-axis aberrations. Therefore, it becomes easier to correct off-axis aberrations, particularly coma.

  In obtaining the above effect, the lower limit of conditional expression (4) is more preferably 0.50. The upper limit value of conditional expression (4) is more preferably 1.50, further preferably 1.30, and still more preferably 1.20.

1-4-5. Conditional expression (5)
In the optical system, when the first lens group is composed of the first A lens group and the first B lens group, an air lens composed of an air interval between the first A lens group and the first B lens group is described below. It is preferable to satisfy the following conditions.

Conditional expression (5):
−0.5 <(Cr1ar + Cr1bf) / (Cr1ar−Cr1bf) <1.0

However,
Cr1ar is the radius of curvature of the surface arranged closest to the image side in the 1A lens group,
Cr1bf is the radius of curvature of the object-side concave surface arranged closest to the object side in the first B lens group.

  Conditional expression (5) is an expression relating to the radius of curvature of the object side surface of the air lens and the radius of curvature of the image side surface, which is the air gap between the first A lens group and the first B lens group in the optical system. However, the image side surface of the air lens is an object-side concave surface arranged closest to the object side in the first B lens group, and the curvature of the object-side concave surface is the most of the object-side concave surfaces included in the first lens group. large.

  When the conditional expression (5) is satisfied, the air lens formed by the air gap between the first A lens group and the first B lens group has a biconvex shape, and correction of coma aberration and sagittal coma flare is facilitated and better. It becomes easy to obtain an optical system having excellent optical performance.

  In obtaining the above effect, the lower limit of conditional expression (5) is more preferably −0.3, and further preferably −0.2. The upper limit value of conditional expression (5) is more preferably 0.9.

1-4-6. Conditional expression (6)
In the optical system, when the first lens group includes the first A lens group and the first B lens group, it is preferable that the first B lens group and the first lens group satisfy the following conditions.

  Conditional expression (6): 0.10 <f1b / f1 <0.47

  However, f1b and f1 are the same as the above.

  Conditional expression (6) defines the ratio of the focal length of the first lens group to the focal length of the first lens group. When the conditional expression (6) is satisfied, the refractive power of the first B lens group is within an appropriate range. For this reason, even when the angle of the optical system is increased, the occurrence of spherical aberration and sagittal coma flare can be suppressed, and it becomes easy to obtain an optical system having better optical performance. Further, since the optical system can be configured with a small number of lenses, the optical system can be configured compactly.

  On the other hand, when the numerical value of conditional expression (6) is less than the lower limit, that is, when the refractive power of the 1B lens group becomes stronger than the appropriate range, the generation amount of spherical aberration and sagittal coma flare increases. This is not preferable. Further, when the numerical value of conditional expression (6) is equal to or higher than the upper limit, that is, when the refractive power of the first B lens group is weakened beyond an appropriate range, the retrofocus type lens configuration is weakened, and the wide angle and radial direction are reduced. It becomes difficult to achieve both downsizing. In addition, it is difficult to ensure a predetermined back focus required for a single-lens reflex camera or the like.

  In obtaining the above effect, the lower limit of conditional expression (6) is more preferably 0.12, and further preferably 0.14. The upper limit value of conditional expression (6) is more preferably 0.46.

1-4-7. Conditional expression (7)
In the optical system, when the first lens group includes the first A lens group and the first B lens group, the first A lens group preferably satisfies the following conditions.

Conditional expression (7): −0.40 <f1a / f1 <−0.05
However, f1a and f1 are the same as the above.

  Conditional expression (7) is an expression that defines the focal length of the first A lens group with respect to the focal length of the entire optical system. When the conditional expression (7) is satisfied, it is possible to satisfactorily correct field curvature and coma. Also, sagittal coma flare can be corrected well. For these reasons, it becomes easy to obtain an optical system having higher optical performance. In this case, since the lens structure is a retrofocus type, it is easy to achieve both wide angle and radial size reduction, and it is easy to secure a predetermined back focus required for a single-lens reflex camera or the like. Become.

  On the other hand, when the numerical value of the conditional expression (7) is greater than or equal to the upper limit value, that is, when the refractive power of the first A lens group is increased beyond an appropriate range, correction of field curvature and coma becomes difficult, It is not preferable. On the other hand, when the numerical value of the conditional expression (7) is equal to or lower than the lower limit value, that is, when the refractive power of the first B lens group is weakened beyond an appropriate range, the retrofocus type lens configuration is weakened, and the wide angle and radial direction are reduced. It becomes difficult to achieve both downsizing. In addition, it is difficult to ensure a predetermined back focus required for a single-lens reflex camera or the like.

  In obtaining the above effect, the lower limit value of conditional expression (7) is more preferably −0.38, and further preferably −0.34. Further, the upper limit of conditional expression (7) is more preferably −0.06, and further preferably −0.07.

1-4-8. Conditional expression (8)
In the optical system, it is preferable that the first lens group and the second lens group satisfy the following conditions.

  Conditional expression (8): 3.2 <f1 / f2 <9.0

However,
f1 is the same as described above, and f2 is the focal length of the second lens group.

  Conditional expression (8) defines the ratio of the focal length of the first lens group to the focal length of the second lens group. When the conditional expression (8) is satisfied, the refractive power of the second lens group is within an appropriate range, so that aberration fluctuation when the second lens group is moved during focusing can be suppressed. In addition, the amount of movement of the second lens group during focusing can be within an appropriate range. Therefore, good imaging performance can be realized over the entire focus area, and the entire optical system can be configured compactly.

  On the other hand, when the numerical value of the conditional expression (8) exceeds the upper limit value, that is, when the refractive power of the second lens group becomes stronger than an appropriate range, the aberration associated with the movement of the second lens group during focusing. The fluctuation becomes large, and it becomes difficult to obtain good imaging performance depending on the object distance. Further, in order to obtain good imaging performance, the number of lenses required for aberration correction increases, so that it is difficult to make the optical system compact. On the other hand, when the numerical value of the conditional expression (8) is equal to or lower than the lower limit value, that is, when the refractive power of the second lens group is weakened beyond an appropriate range, the amount of movement of the second lens group during focusing increases. The total length of the optical system becomes long, which is not preferable.

  In obtaining the above effect, the lower limit of conditional expression (8) is more preferably 3.4, and even more preferably 3.5. Further, the upper limit value of conditional expression (8) is more preferably 8.0, still more preferably 7.5, and even more preferably 7.0.

1-4-9. Conditional expression (9)
In the optical system according to the present invention, it is preferable that the surface arranged closest to the object side in the second lens group satisfies the following conditions.

  Conditional expression (9): 0 <Cr2f / f1

However,
Cr2f is the radius of curvature of the surface disposed closest to the object side in the second lens group, and f1 is the same as described above.

  Conditional expression (9) defines the ratio of the radius of curvature of the surface disposed closest to the object side in the second lens group to the focal length of the first lens group. When the conditional expression (9) is satisfied, the surface disposed closest to the object side in the second lens group is an object-side convex surface convex toward the object side. Therefore, as described above, correction of spherical aberration is facilitated, and better optical performance can be realized. At the same time, the number of lenses constituting the second lens group can be reduced, and it becomes easy to reduce the size of the optical system.

  On the other hand, when the numerical value of the conditional expression (9) is less than or equal to the lower limit value, that is, when the surface arranged closest to the object side in the second lens group is a flat surface or concave on the object side, the spherical aberration is corrected. It becomes difficult and not preferable. Also, in this case, since the second lens group is a focus group, aberration fluctuation during focusing becomes large, which is not preferable.

  In obtaining the above effect, it is more preferable that the optical system satisfies the following conditional expression (9) ′.

  Conditional expression (9) ': 0.02 <Cr2f / f1 <20.0

  In order to obtain the above effect, the lower limit value of conditional expression (9) ′ is more preferably 0.1, and still more preferably 0.3. The upper limit value of conditional expression (9) ′ is more preferably 5.0, still more preferably 4.0, even more preferably 3.0, and most preferably 2.7. preferable.

  At this time, as described above, it is preferable that the lens component including the object-side convex surface is a positive lens component having a positive refractive power from the viewpoint that the spherical aberration can be corrected more favorably.

1-4-10. Conditional expression (10)
In the optical system, it is preferable that the second lens group satisfies the following conditions.

  Conditional expression (10): 1.75 <f2 / f <3.00

However,
f2 is the focal length of the second lens group, and f is the same as above.

  Conditional expression (10) defines the ratio of the focal length of the second lens group to the focal length of the entire optical system. When the conditional expression (10) is satisfied, the refractive power of the second lens group is within an appropriate range, and aberration fluctuation when the second lens group is moved during focusing can be suppressed. In addition, the amount of movement of the second lens group during focusing can be within an appropriate range. Therefore, good imaging performance can be realized over the entire focus area, and the entire optical system can be configured compactly.

  On the other hand, when the numerical value of the conditional expression (10) exceeds the upper limit value, that is, when the refractive power of the second lens group becomes stronger than an appropriate range, the aberration associated with the movement of the second lens group during focusing. Fluctuations increase, making it difficult to correct spherical aberration and chromatic aberration, and it becomes difficult to obtain good imaging performance depending on the object distance. In addition, in order to obtain good imaging performance, the number of lenses required to correct these aberrations increases, making it difficult to make the optical system compact. On the other hand, when the numerical value of the conditional expression (10) is equal to or lower than the lower limit value, that is, when the refractive power of the second lens group is weakened beyond an appropriate range, the amount of movement of the second lens group during focusing increases. The total length of the optical system becomes long, which is not preferable.

  In obtaining the above effect, the lower limit of conditional expression (10) is more preferably 1.78. Further, the upper limit value of conditional expression (10) is more preferably 2.70, still more preferably 2.50, and even more preferably 2.30.

1-4-11. Conditional expression (11)
In the optical system, it is preferable that the second lens group includes at least one air lens having negative refractive power, and the air lens having negative refractive power satisfies the following conditions.

  Conditional expression (11): -0.5 <(R1 + R2) / (R1-R2) <0.5

However,
R1 is the radius of curvature of the object side surface of the air lens having negative refractive power included in the second lens group;
R2 is the radius of curvature of the image side surface of the air lens having negative refractive power included in the second lens group.

  Conditional expression (11) is an expression relating to the curvature radius of the object side surface and the curvature radius of the image side surface of the air lens having negative refractive power included in the second lens group. When the conditional expression (11) is satisfied, the second lens group includes a biconvex air lens having approximate curvature radii on both sides, and the second lens group has a double Gauss type lens configuration. . Therefore, it is easy to correct spherical aberration, coma aberration, and sagittal coma flare with a small number of lenses, and it becomes easy to obtain an optical system having better optical performance. Further, by adopting this configuration, it is possible to reduce the amount of aberration generated in the second lens group, so that it is possible to reduce aberration fluctuations accompanying the movement of the second lens group during focusing. The biconvex air lens has negative refractive power.

  In obtaining the above effect, the lower limit of conditional expression (11) is more preferably −0.3, and even more preferably −0.2. The upper limit value of conditional expression (11) is more preferably 0.3, and further preferably 0.2.

1-4-12. Conditional expression (12)
In the optical system, it is preferable that the surface arranged closest to the object side in the first lens group satisfies the following conditions.

  Conditional expression (12): 0 <Cr1f / f

However,
Cr1f is the radius of curvature of the surface arranged closest to the object side in the first lens group,
f is the focal length of the entire optical system.

  Conditional expression (12) defines the ratio of the radius of curvature of the surface disposed closest to the object side in the first lens group to the focal length of the optical system. When the conditional expression (12) is satisfied, the surface disposed closest to the object side in the first lens group is the object-side convex surface that is convex on the object side, and as described above, correction of distortion and curvature of field is performed. It is effective and better optical performance can be realized. At the same time, the number of lenses constituting the first lens group can be reduced, and it becomes easy to reduce the size of the optical system.

  On the other hand, when the numerical value of the conditional expression (12) is less than or equal to the lower limit value, that is, when the surface arranged closest to the object side in the first lens group is flat or concave on the object side, distortion aberration and image surface It is difficult to correct the curvature, which is not preferable.

  In obtaining the above effect, it is more preferable that the optical system satisfies the following conditional expression (12) ′.

  Conditional expression (12) ': 0.2 <Cr1f / f <200.0

  In order to obtain the above effect, the lower limit value of conditional expression (12) ′ is more preferably 0.5, and even more preferably 1.0. The upper limit value of conditional expression (12) ′ is more preferably 100.0, further preferably 30.0, even more preferably 8.0, and most preferably 4.0. preferable.

1-4-13. Conditional expression (13)
In the optical system, among the at least three negative lenses included in the first lens group, the negative lens L1n satisfies the following condition when the negative lens disposed closest to the object side is the negative lens L1n. Is preferred.

Conditional expression (13): νd1n <40
However,
νd1n represents the Abbe number of the negative lens L1n with respect to the d-line.

  Conditional expression (13) defines the Abbe number with respect to the d-line of the negative lens L1n. By satisfying the conditional expression (13), for example, it is possible to satisfactorily correct lateral chromatic aberration even when attempting to increase the aperture while maintaining an angle of view of about 80 °, and a compact optical device with high optical performance. The wide-angle lens can be realized.

  In obtaining the above effect, the upper limit value of conditional expression (13) is more preferably 35.3, further preferably 34.0, further preferably 32.0, and 30.0. Most preferably it is.

1-4-14. Conditional expression (14)
In the optical system, of the at least three negative lenses included in the first lens group, when the negative lens disposed on the image side of the negative lens L1n is a negative lens L2n, the negative lens L2n is as follows. It is preferable to satisfy the conditions.

  Conditional expression (14): νd2n> 54.0

However,
νd2n represents the Abbe number of the negative lens L2n with respect to the d-line.

  Conditional expression (14) defines the Abbe number with respect to the d-line of the negative lens L2n. By configuring the first lens group using the negative lens L2n that satisfies the conditional expression (14), it becomes easy to reduce the lateral chromatic aberration, particularly the widths of the F-line and the C-line, and higher optical performance can be obtained. It becomes easy to realize. On the other hand, if the numerical value of the conditional expression (14) is less than or equal to the lower limit value, it is difficult to reduce the lateral chromatic aberration, particularly the width of the F-line and C-line, which is not preferable. However, the negative lens L2n may be any negative lens other than the negative lens L1n arranged closest to the object among the negative lenses included in the first lens group.

  In obtaining the above effect, the lower limit value of conditional expression (14) is more preferably 58.0, still more preferably 63.0, still more preferably 66.0, and 70.0. Most preferably it is.

2. Next, an imaging apparatus according to the present invention will be described. An imaging apparatus according to the present invention includes the optical system according to the present invention, and an imaging element that is provided on an image side of the optical system and converts an optical image formed by the optical system into an electrical signal. It is characterized by that. Here, there is no limitation in particular in an image pick-up element etc., Solid-state image pick-up elements, such as a CCD sensor and a CMOS sensor, etc. can be used. The imaging device according to the present invention is suitable for an imaging device using these solid-state imaging devices such as a digital camera and a video camera. The imaging device may be a lens-fixed imaging device in which a lens is fixed to a housing. The optical system according to the present invention can ensure the back focus required for interchangeable lens imaging devices such as single-lens reflex cameras and mirrorless single-lens cameras, so that the interchangeable lenses of these interchangeable lens imaging devices can be secured. Is preferred.

  Next, an Example is shown and this invention is demonstrated concretely. However, the present invention is not limited to the following examples. The optical system of each example given below is an imaging optical system used for an imaging device (optical device) such as a digital camera, a video camera, or a silver salt film camera. In each lens cross-sectional view, the left side is the object side and the right side is the image side in the drawing.

(1) Configuration of Optical System FIG. 1 is a lens cross-sectional view showing the lens configuration of the optical system of Example 1 according to the present invention when focusing on infinity. The optical system includes, in order from the object side, a first lens group G1 having a positive refractive power and a second lens group G2 having a positive refractive power. When focusing from an object at infinity to an object at a short distance, the second lens group G2 moves to the object side along the optical axis while the first lens group G1 is fixed in the optical axis direction. The aperture stop S is disposed inside the second lens group G2.

  The first lens group G1 includes, in order from the object side, a first A lens group G1A having a negative refractive power and a first B lens group G1B having a positive refractive power.

  The first A lens group G1A includes, in order from the object side, a negative meniscus lens L1 having a convex surface facing the object side, a positive bilens lens L2, a negative meniscus lens L3 having a convex surface facing the object side, and an object side. And a negative meniscus lens L4 having a convex surface. The negative meniscus lens L4 is a glass mold type aspherical lens whose both surfaces are aspherical, or an aspherical lens by grinding.

  In order from the object side, the first B lens group G1B includes a negative meniscus lens L5 having a convex surface directed toward the image side, a biconvex positive lens L6, a biconvex positive lens L7, and a positive surface having a convex surface directed toward the image side. It is composed of a cemented lens having a positive refractive power formed by cementing a meniscus lens L8. The most object-side lens surface of the first B lens group G1B (the object side surface of the negative meniscus lens L5) is the object-side concave surface having the largest curvature among the object-side concave surfaces included in the first lens group G1.

  The second lens group G2 includes a biconvex positive lens L9, a cemented lens having a negative refractive power obtained by cementing a biconvex positive lens L10 and a biconcave negative lens L11, an aperture stop S, and an image side. A positive meniscus lens L12 having a convex surface facing the lens and a negative meniscus lens L13 having a convex surface facing the image side, a birefringent positive lens L14, and a positive lens having a convex surface facing the image side. And a meniscus lens L15. The positive meniscus lens L15 is a glass mold type aspheric lens having an aspheric surface on the object side or an aspheric lens by grinding.

  In FIG. 1, “IP” is an image plane, and specifically indicates an imaging plane of a solid-state imaging device such as a CCD sensor or a CMOS sensor, or a film plane of a silver salt film. Further, a cover glass or the like (reference numeral is omitted) is provided on the object side of the IP. This also applies to the lens cross-sectional views shown in the second and third embodiments.

(2) Numerical Examples Next, numerical examples to which specific numerical values of the optical system are applied will be described. Table 1 shows lens data of the optical system. In Table 1, “surface number” is the order of the lens surfaces counted from the object side, “r” is the radius of curvature of the lens surfaces, “d” is the distance on the optical axis of the lens surfaces, and “nd” is the d-line (wavelength “νd” indicates the Abbe number with respect to the d-line. “ASP” displayed in the column next to the surface number indicates that the lens surface is an aspheric surface, and “S” indicates an aperture stop.

  Table 2 shows the specifications of the optical system, aspherical data, variable distance on the optical axis, and focal length of each lens group. The original table shows the focal length “f”, F number “Fno.”, Half angle of view “ω”, and image height “Ym” of the entire optical system when an object at infinity is in focus.

The aspheric surface data indicates an aspheric coefficient when the aspheric shape is defined by the following equation. However, in the table, “E−a” indicates “× 10 ^−a ”. In the following equation, “x” is the amount of displacement from the reference plane in the optical axis direction, “r” is the paraxial radius of curvature, “H” is the height from the optical axis in the direction perpendicular to the optical axis, “k” "Is a conic coefficient, and" An "is an n-order aspheric coefficient.

  The variable interval indicates the interval between the lens surfaces when focusing on infinity and focusing on 0.5 m. Table 7 shows numerical values of the conditional expressions (1) to (14). The unit of length in each table is “mm”, and the unit of angle of view is “°”. Since the items related to these tables are the same in the tables shown in the second and third embodiments, the description thereof will be omitted below.

  2 and 3 show longitudinal aberration diagrams and lateral aberration diagrams when the optical system is focused at infinity. 4 and 5 show longitudinal aberration diagrams and lateral aberration diagrams when the optical system is focused on 0.5 m.

  In each longitudinal aberration diagram, spherical aberration, astigmatism, and distortion aberration are shown in order from the left toward the drawing. In the graph showing spherical aberration, the vertical axis is the ratio to the open F value, the horizontal axis is defocused, the solid line is the d line (wavelength λ = 587.6 nm), and the dotted line is the g line (wavelength λ = 435.8 nm). The spherical aberration at is shown. In the diagram showing astigmatism, the vertical axis represents the image height, the horizontal axis defocused, the solid line represents the sagittal image plane (ds) for the d line, and the dotted line represents the meridional image plane (dm) for the d line. In the diagram showing distortion aberration, the vertical axis represents image height and the horizontal axis represents%, and represents distortion aberration.

  In each lateral aberration diagram, 100% image point (1.0Ω), 90% image point (0.9Ω), 70% image point (0.7Ω), and 50% image of the maximum image height in order from the top. Lateral aberrations are shown at point (0.5Ω) and on-axis point (0.0Ω). In each lateral aberration diagram, the horizontal axis represents the distance from the principal ray on the pupil plane, the solid line indicates the d-line and the dotted line indicates the coma aberration at the g-line.

  Since the matters relating to these drawings are the same in the longitudinal aberration diagram and the lateral aberration diagram shown in Example 2 and Example 3, the description thereof will be omitted below.

The back focus “BF” when the optical system is focused at infinity is as follows. However, the following values are values including a cover glass having a thickness of 2 mm, and the same applies to back focus shown in other examples.
BF = 39.0331

(1) Configuration of Optical System FIG. 6 is a lens cross-sectional view showing the lens configuration of the optical system of Example 2 according to the present invention when focusing on infinity. The optical system includes, in order from the object side, a first lens group G1 having a positive refractive power and a second lens group G2 having a positive refractive power. When focusing from an object at infinity to an object at a short distance, the second lens group G2 moves to the object side along the optical axis while the first lens group G1 is fixed in the optical axis direction. The aperture stop S is disposed inside the second lens group G2.

  The first lens group G1 includes, in order from the object side, a first A lens group G1A having a negative refractive power and a first B lens group G1B having a positive refractive power.

  The first A lens group G1A includes, in order from the object side, a negative meniscus lens L1 having a convex surface facing the object side, a negative meniscus lens L2 having a convex surface facing the object side, a biconvex positive lens L3, and a biconcave shape. It consists of a cemented lens having a negative refractive power formed by cementing a negative lens L4. The negative meniscus lens L2 is a glass mold type aspherical lens whose both surfaces are aspherical, or an aspherical lens by grinding.

  The first B lens group G1B includes, in order from the object side, a negative lens cemented lens in which a biconcave negative lens L5 and a biconvex positive lens L6 are cemented, a biconvex positive lens L7, and a biconvex lens. It consists of a cemented lens having a negative refractive power formed by cementing a positive lens L8 having a shape and a negative lens L9 having a biconcave shape. The most object side lens surface of the first B lens group G1B (the object side surface of the negative lens L5) is the object side concave surface having the largest curvature among the object side concave surfaces included in the first lens group G1.

  The second lens group G2 includes a biconvex positive lens L10, a negative meniscus lens L11 having a convex surface facing the object side, an aperture stop S, a biconcave negative lens L12, and a biconvex positive lens L13. And a cemented lens having a positive refractive power obtained by cementing a positive meniscus lens L14 having a convex surface facing the object side, and a biconvex positive lens L15. The positive meniscus lens L14 is a glass mold type aspheric lens having an aspherical surface on the image side, or an aspheric lens by grinding.

(2) Numerical Examples Next, numerical examples to which specific numerical values of the optical system are applied will be described. Table 3 shows lens data of the optical system, and Table 4 shows a specification table of the optical system, aspherical data, a variable interval on the optical axis, and a focal length of each lens group. Table 7 shows numerical values of the conditional expressions (1) to (14). 7 to 10 are longitudinal aberration diagrams and lateral aberration diagrams when the optical system is focused at infinity and when focused to 0.5 m.

Further, the back focus when the optical system is focused at infinity is as follows.
BF = 39.0353

(1) Configuration of Optical System FIG. 11 is a lens cross-sectional view showing the lens configuration of the optical system of Example 3 according to the present invention when focusing on infinity. The optical system includes, in order from the object side, a first lens group G1 having a positive refractive power and a second lens group G2 having a positive refractive power. When focusing from an object at infinity to an object at a short distance, the second lens group G2 moves to the object side along the optical axis while the first lens group G1 is fixed in the optical axis direction. The aperture stop S is disposed inside the second lens group G2.

  The first lens group G1 includes, in order from the object side, a first A lens group G1A having a negative refractive power and a first B lens group G1B having a positive refractive power.

  The first A lens group G1A includes, in order from the object side, a positive meniscus lens L1 having a convex surface facing the object side, a negative meniscus lens L2 having a convex surface facing the object side, and a negative meniscus lens L3 having a convex surface facing the object side. And a cemented lens having a negative refractive power formed by cementing a positive meniscus lens L4 having a convex surface toward the image side and a biconcave negative lens L5. The negative meniscus lens L3 is a glass mold type aspherical lens whose both surfaces are aspherical, or an aspherical lens by grinding.

  The first B lens group G1B includes, in order from the object side, a cemented lens having negative refractive power obtained by cementing a biconcave negative lens L6 and a biconvex positive lens L7, and a biconvex positive lens L8. Has been. The most object-side lens surface of the first B lens group G1B (the object side surface of the negative lens L6) is the object-side concave surface having the largest curvature among the object-side concave surfaces included in the first lens group G1.

  The second lens group G2 includes a biconvex positive lens L9, a negative meniscus lens L10 having a convex surface facing the object side, an aperture stop S, a biconcave negative lens L11, and a biconvex positive lens L12. And a cemented lens having a positive refractive power obtained by cementing a negative meniscus lens L13 having a convex surface facing the image side, and a biconvex positive lens L14. The negative meniscus lens L13 constituting the cemented lens is a glass mold type aspheric lens having an aspheric surface on the image side, or an aspheric lens by grinding.

(2) Numerical Examples Next, numerical examples to which specific numerical values of the optical system are applied will be described. Table 5 shows lens data of the optical system, and Table 6 shows a specification table of the optical system, aspherical data, a variable interval on the optical axis, and a focal length of each lens group. Table 7 shows numerical values of the conditional expressions (1) to (14). Further, FIGS. 12 to 15 are longitudinal aberration diagrams and lateral aberration diagrams when the optical system is focused at infinity and when focused to 0.5 m.

Further, the back focus when the optical system is focused at infinity is as follows.
BF = 39.0376

  In the above-described embodiments, the case where the optical system has a two-group configuration has been described as an example. However, the present invention is not limited to this. For example, the same effect can be obtained even if the optical system includes a third lens group that follows the second lens group. Even if the optical system is composed of the first lens group and the second lens group, an optical element such as a cover glass or a filter may be included between the second lens group and the image plane. Needless to say, an optical element that does not substantially affect the optical system, such as a single lens having almost no refractive power, may be included.

  According to the present invention, it is possible to provide an optical system capable of realizing a wide angle and a large aperture and realizing high optical performance, and an imaging apparatus including the optical system.

G1 ... 1st lens group G1A ... 1A lens group G1B ... 1B lens group G2 ... 2nd lens group S ... Aperture stop IP ... Image plane

Claims (18)

In order from the object side, a first lens group having a positive refractive power and a second lens group having a positive refractive power are provided.
The first lens group is fixed in the optical axis direction, and the second lens group is moved to focus from an infinite object to a finite distance object,
The first lens group includes at least three negative lenses, a positive lens LA, and a positive lens LB.
An optical system characterized by satisfying the following conditions.
νd1b1 <35.0 (1)
νd1b2> 65.0 (2)
However,
νd1b1 is an Abbe number with respect to the d-line of the positive lens LA included in the first lens group,
νd1b2 is an Abbe number with respect to the d-line of the positive lens LB included in the first lens group.   The first lens group has at least one object-side concave surface in which the object-side surface is concave on the object side, and has, in order from the object side, a first A lens group having negative refractive power and a positive refractive power. 2. The first B lens group, and the surface disposed closest to the object side in the first B lens group is the object side concave surface having the largest curvature among the object side concave surfaces included in the first lens group. The optical system described in 1.   The optical system according to claim 2, wherein the first lens group includes the positive lens LA and the positive lens LB. The optical system according to claim 2 or 3, wherein the first A lens group and the first B lens group satisfy the following conditions.
−0.95 <f1a / f1b <−0.40 (3)
However,
f1a is the focal length of the first A lens group;
f1b is a focal length of the first B lens group. The optical system according to any one of claims 2 to 4, wherein an object-side concave surface having the largest curvature among the object-side concave surfaces included in the first lens group satisfies the following condition.
0.48 <Cr1bf / f1a <1.70 (4)
However,
Cr1bf is a radius of curvature of the object-side concave surface having the largest curvature among the object-side concave surfaces included in the first lens group. The optical system according to any one of claims 2 to 5, wherein an air lens including an air interval between the first A lens group and the first B lens group satisfies the following condition.
−0.5 <(Cr1ar + Cr1bf) / (Cr1ar−Cr1bf) <1.0 (5)
However,
Cr1ar is a radius of curvature of the surface arranged closest to the image side in the first A lens group,
Cr1bf is a radius of curvature of the object-side concave surface disposed closest to the object side in the first B lens group. The optical system according to any one of claims 2 to 6, wherein the first B lens group and the first lens group satisfy the following conditions.
0.10 <f1b / f1 <0.47 (6)
However,
f1b is a focal length of the first B lens group,
f1 is a focal length of the first lens group. The optical system according to any one of claims 2 to 7, wherein the first A lens group satisfies the following condition.
−0.40 <f1a / f1 <−0.05 (7)
However,
f1a is the focal length of the first A lens group;
f1 is a focal length of the first lens group. The optical system according to any one of claims 1 to 8, wherein the first lens group and the second lens group satisfy the following condition.
3.2 <f1 / f2 <9.0 (8)
However,
f1 is a focal length of the first lens group;
f2 is a focal length of the second lens group. 10. The optical system according to claim 1, wherein a surface disposed closest to the object side in the second lens group satisfies the following condition.
0 <Cr2f / f1 (9)
However,
Cr2f is the radius of curvature of the surface arranged closest to the object side in the second lens group. The optical system according to any one of claims 1 to 10, wherein the second lens group satisfies the following condition.
1.75 <f2 / f <3.00 (10)
However,
f2 is a focal length of the second lens group;
f is the focal length of the entire optical system. The second lens group includes at least one air lens having negative refractive power, and the air lens having negative refractive power satisfies the following condition. The optical system described.
-0.5 <(R1 + R2) / (R1-R2) <0.5 (11)
However,
R1 is a radius of curvature of the object side surface of the air lens having negative refractive power included in the second lens group;
R2 is the radius of curvature of the image side surface of the air lens having negative refractive power included in the second lens group. The optical system according to any one of claims 1 to 12, wherein a surface disposed closest to the object side in the first lens group satisfies the following condition.
0 <Cr1f / f (12)
However,
Cr1f is the radius of curvature of the surface arranged closest to the object side in the first lens group,
f is the focal length of the entire optical system.   The negative lens arranged closest to the object side among at least three negative lenses included in the first lens group is a meniscus lens having a convex shape on the object side. The optical system according to item. The negative lens L1n satisfies the following conditions when the negative lens arranged closest to the object among the at least three negative lenses included in the first lens group is a negative lens L1n. Item 15. The optical system according to any one of Items 14.
νd1n <40 (13)
However,
νd1n is an Abbe number with respect to the d-line of the negative lens L1n. Of the at least three negative lenses included in the first lens group, when the negative lens disposed on the image side relative to the negative lens disposed closest to the object side is a negative lens L2n, the negative lens L2n is: The optical system according to any one of claims 1 to 14, which satisfies the following condition.
νd2n> 54.0 (14)
However,
νd2n is an Abbe number with respect to the d-line of the negative lens L2n.   The optical system according to any one of claims 1 to 16, wherein the optical system includes the first lens group and the second lens group.   An optical system according to any one of claims 1 to 17, and an image sensor for converting an optical image formed by the optical system into an electrical signal on the image side of the academic system. An imaging apparatus characterized by the above.

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