JP2009300489A - Variable power optical system and imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable power optical system capable of satisfactorily correcting color aberration in a red area, and to provide an imaging apparatus. <P>SOLUTION: The variable power optical system includes, in order from an object side: a first lens group G1 of negative refractive power; a second lens group G2 of positive refractive power as a whole, which moves on an optical axis when varying power or being focused; a third lens group G3 of negative refractive power as a whole, which moves on the optical axis when varying power or being focused and includes at least one negative lens; and a fourth lens group G4 of positive refractive power as a whole, which includes at least one positive lens. The third lens group G3 includes a negative lens that satisfies conditions expressed by formula (1) νd>60 and formula (2) θC, A'<0.001198νd+0.2765. The fourth lens group G4 includes a positive lens that satisfies conditions expressed by formula (1) νd>60 and formula (2) θC, A'<0.001198νd+0.2765. In the formulas, νd represents Abbe's number relative to a line d, and θC, A' represents a partial dispersion ratio where partial dispersion is represented by (nc-nA'), in which nC represents a refractive index relative to a line C and nA' represents a refractive index relative to a line A'. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a variable magnification optical system suitably used as an objective lens for an endoscope, for example, and an imaging apparatus using the same.

  In an optical system using a solid-state imaging device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), the sensitivity of the device is high even in the infrared region. Therefore, the transmittance near the A ′ line (around 750 nm) In many cases, an infrared cut filter of about 50% (half value) is inserted. Nevertheless, since the aberration in the red region can greatly affect the image quality and resolution, it is important to correct the chromatic aberration from the C line (656.3 nm) to the A ′ line (768.2 nm). In an optical system having a moving group, it is desirable that the chromatic aberration is corrected for each group so that the chromatic aberration does not fluctuate during zooming. Conventionally, it has been known that it is effective to use an optical material having anomalous dispersion for correcting chromatic aberration, particularly for correcting a secondary spectrum (see Patent Documents 1 to 4).

JP 2007-163964 A JP 2006-349947 A JP 2006-78964 A JP 2005-345892 A

  However, in each of the above-mentioned patent documents, attention is paid to correction of chromatic aberration in the blue region, and the partial dispersion ratio of the blue region, for example, the partial dispersion ratio θg, F relating to the g-line (435.8 nm) and the F-line (486.1 nm) is set. Only a focused lens system is described, and correction of chromatic aberration in the red region is insufficient. In particular, depending on the material, the tendency of anomalous dispersion may be different between the blue region and the red region, and there is a possibility that sufficient chromatic aberration correction cannot be performed only by focusing on the anomalous dispersion of the blue region.

  The present invention has been made in view of such problems, and an object thereof is to provide a variable magnification optical system and an imaging apparatus capable of performing chromatic aberration correction particularly in a red region better than before.

A variable magnification optical system according to the present invention is a variable magnification optical system including a plurality of lens groups, and includes at least one positive lens, a lens group having positive refractive power, and at least one negative lens. An optical material that satisfies the following conditional expressions (1) and (2) is used for at least one of the positive lens and the negative lens: Is.
νd> 60 (1)
θC, A ′ <0.001198νd + 0.2765 (2)
Where νd is the Abbe number for the d-line, θC, A ′ is the partial dispersion ratio when the partial dispersion is (nC−nA ′) (nC is the refractive index for the C-line, and nA ′ is the refractive index for the A′-line). And

In the variable power optical system according to the present invention, the lens configuration is optimized by using an appropriate optical material while paying attention to the partial dispersion ratio of the red region, so that the chromatic aberration particularly in the red region is better than before. It is corrected. In the variable magnification optical system according to the present invention, in particular, a lens (a positive lens in a lens group having a positive refractive power or a lens having a negative refractive power) that has a relatively strong power in the lens group and is likely to cause chromatic aberration. By using an optical material having anomalous dispersion advantageous for correcting chromatic aberration satisfying the above conditions for the negative lens in the group, effective chromatic aberration correction can be performed.
Further, by properly adopting and satisfying the following preferable configuration, the correction of chromatic aberration is made more effective.

In the zoom optical system according to the present invention, it is preferable that the following conditional expressions are further satisfied.
0.330 <θC, A ′ <0.360 (3)

  In the variable power optical system according to the present invention, an infrared cut filter having a transmittance half value of 750 nm ± 20 nm may be provided between the lens system and the imaging surface.

  Further, the variable magnification optical system according to the present invention includes, in order from the object side, a first lens unit that is fixed at the time of zooming and focusing and has negative refractive power, and an optical axis at the time of zooming and focusing. A second lens group having a positive refractive power, and a third lens group having a negative refractive power, which moves on the optical axis during zooming and focusing and includes at least one negative lens. And a fourth lens group that is fixed at the time of zooming and focusing, includes at least one positive lens, and has positive refractive power. The variable magnification optical system according to the present invention may be used as an objective lens for an endoscope.

  In the configuration in which the first lens group to the fourth lens group are disposed, the third lens group includes a negative lens using an optical material that satisfies the conditions of the conditional expressions (1) and (2). Preferably it is.

  It is preferable that the fourth lens group includes a positive lens using an optical material that satisfies the conditions of the conditional expressions (1) and (2).

An image pickup apparatus according to the present invention includes a variable magnification optical system according to the present invention and an image pickup element that outputs an image pickup signal corresponding to an optical image formed by the variable magnification optical system.
In the imaging apparatus according to the present invention, a high-quality imaging signal is obtained based on a high-quality optical image obtained by the variable magnification optical system of the present invention, and a high-quality captured image is obtained based on the imaging signal.

  According to the variable magnification optical system of the present invention, the lens configuration is optimized by using an appropriate optical material by paying attention to the partial dispersion ratio of the red region, and thus the chromatic aberration correction particularly in the red region is conventionally performed. Can be done better.

  According to the imaging apparatus of the present invention, since the high-performance variable magnification optical system of the present invention is used as an imaging lens, a high-quality captured image can be obtained.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
1A and 1B show a first configuration example of a variable magnification optical system according to an embodiment of the present invention. This configuration example corresponds to the lens configuration of a first numerical example (FIGS. 7A and 7B) described later. 1A corresponds to the arrangement of the optical system when the object distance is on the far side (standard state), and FIG. 1B corresponds to the arrangement of the optical system when the object distance is on the near side (close state). Similarly, sectional configurations of second to third configuration examples corresponding to lens configurations of second to third numerical examples described later are shown in FIGS. 2 (A), 2 (B) to 3 (A), Shown in (B). In FIGS. 1 (A), (B) to FIGS. 3 (A), (B), the symbol Ri increases sequentially toward the image side (imaging side), with the most object side component surface being the first. In this way, the radius of curvature of the i-th surface that is labeled is shown. The symbol Di indicates the surface interval on the optical axis Z1 between the i-th surface and the i + 1-th surface. In addition, about code | symbol Di, it attaches | subjects code | symbol only to the surface interval of the part which changes with zooming and focusing. Since the basic configuration is the same for each configuration example, the following description is based on the first configuration example shown in FIGS. 1 (A) and 1 (B).

  This variable power optical system is used, for example, as an objective lens provided at the distal end portion of an endoscope. For example, it can be suitably used as an objective lens of an endoscope in which a solid-state imaging device such as a CCD is arranged in parallel to the axial direction (longitudinal direction). FIG. 25 shows a usage example of such an objective lens 100 for an endoscope. As shown in FIG. 25, the solid-state imaging device 200 is arranged in parallel to the axial direction (longitudinal direction) Z at the distal end portion of the endoscope. An optical path conversion prism 102 is inserted between the objective lens 100 and the solid-state imaging device 200. The optical axis Z1 of the objective lens 100 is bent by approximately 90 degrees in the direction of the solid-state imaging device 200 by the optical path conversion prism 102. The solid-state imaging device 200 is arranged so that its imaging surface coincides with the imaging surface after the optical path of the objective lens 100 is bent. Other optical members 101 such as a cover glass and an infrared cut filter may be disposed between the optical path conversion prism 102 and the solid-state imaging device 200. When an infrared cut filter is provided, it is preferable to use, for example, a filter having a half-value transmittance of 750 nm ± 20 nm.

  As shown in FIGS. 1A and 1B, the variable magnification optical system includes a first lens group G1, a second lens group G2, and a third lens in order from the object side along the optical axis Z1. A group G3 and a fourth lens group G4 are provided. The optical aperture stop St is disposed, for example, between the second lens group G2 and the third lens group G3. A prism GP corresponding to the above-described optical path conversion prism 102 (FIG. 25) is disposed between the fourth lens group G4 and the imaging plane.

  This variable magnification optical system is configured to perform variable magnification (change in focal length) and focus by moving the second lens group G2 and the third lens group G3 on the optical axis. The first lens group G1 and the fourth lens group G4 are always fixed during zooming and focusing. As shown in FIG. 1B, the variable magnification optical system has the second lens group G2 on the object side and the third lens group with respect to the long distance side (standard state) shown in FIG. By moving G3 to the image side, it is possible to focus on the closer side (close state) and to obtain a larger imaging magnification. When used as the objective lens 100 for an endoscope, the object distance on the far side is about 15 mm, for example, and the object distance on the near side is about 2 mm, for example.

  In this variable magnification optical system, the first lens group G1 has a negative refractive power as a whole. For example, as shown in FIGS. 1A and 1B, the first lens group G1 may be composed of a negative lens L11 and a cemented lens made up of a negative lens L12 and a positive lens L13 in this order from the object side. it can. In addition, as shown in the second configuration example of FIGS. 2A and 2B and the third configuration example of FIGS. 3A and 3B, the gap between the negative lens L11 and the cemented lens is shown. A plane lens L14 may be disposed.

  The second lens group G2 has a positive refractive power as a whole. For example, as shown in FIGS. 1A and 1B, the second lens group G2 can be composed of a single positive lens L21.

  The third lens group G3 includes at least one negative lens and has a negative refractive power as a whole. For example, as shown in FIGS. 1A and 1B, the third lens group G3 can be composed of a single negative lens L31.

  The fourth lens group G4 includes at least one positive lens and has a positive refractive power as a whole. For example, as shown in FIGS. 1A and 1B, the fourth lens group G4 may be composed of a positive lens L41 and a cemented lens made up of a positive lens L42 and a negative lens L43 in order from the object side. it can.

In this variable magnification optical system, the following conditional expressions (1) and (2) are applied to at least one of a positive lens in a lens group having a positive refractive power and a negative lens in a lens group having a negative refractive power. The lens includes a satisfactory optical material having a predetermined anomalous dispersion. The significance of satisfying these conditions will be described later.
νd> 60 (1)
θC, A ′ <0.001198νd + 0.2765 (2)
Where νd is the Abbe number with respect to the d-line. θC, A ′ is a partial dispersion ratio (nC is the refractive index for the C line, and nA ′ is the refractive index for the A ′ line) when the partial dispersion is (nC−nA ′).

  In the lens configuration examples in FIGS. 1A and 1B, for example, an optical material that satisfies the conditional expressions (1) and (2) is preferably used for the negative lens L31 in the third lens group G3. . In addition, it is preferable that an optical material satisfying the conditional expressions (1) and (2) is used for the positive lens L42 in the fourth lens group G4.

Here, when the conditional expressions (1) and (2) are satisfied, it is preferable that the following conditional expressions are further satisfied.
0.330 <θC, A ′ <0.360 (3)

Next, operations and effects of the variable magnification optical system configured as described above will be described.
In this variable magnification optical system, focusing on the partial dispersion ratio in the red region and optimizing the lens configuration using an appropriate optical material, the chromatic aberration in the red region in particular is corrected better than before. The In this variable power optical system, a lens that is relatively strong in the lens group and is likely to cause chromatic aberration (a positive lens in a lens group having a positive refractive power as a whole, or a negative refractive power as a whole). By using an optical material having anomalous dispersion advantageous for correction of chromatic aberration satisfying the above conditions for the negative lens in the lens group having the lens group, effective chromatic aberration correction is performed.

  The above conditional expression relates to anomalous dispersion. Hereinafter, the main dispersion (nF−) of the difference between the refractive indices of the F-line and C-line of the partial dispersion (nx−ny), which is the difference between the refractive indexes of two wavelengths, will be explained. The ratio (nx−ny) / (nF−nC) to nC) is referred to as the partial dispersion ratio (θx, y). On the graph with the partial dispersion ratio (θx, y) on the vertical axis and the Abbe number νd on the horizontal axis, when a straight line connecting two standard glass types is taken as a reference line, it is generally at a position away from this reference line. The distributed optical material is referred to as “anomalous dispersion” optical material.

For example, when a straight line connecting two glass types manufactured by OHARA INC., NSL7 (nd = 1.51112, νd = 60.5) and PBM2 (nd = 1.62004, νd = 36.3) is used as a reference line, For the partial dispersion ratio θC, A 'in the long wavelength (red) region,
From NSL7 = 0.3492 and PBM2 = 0.3198, the equation of the reference line is
θC, A′≈0.001215νd + 0.2757 (A)
It becomes. Note that νd = (nd−1) / (nF−nC) (nd is the refractive index with respect to the d-line).
On the other hand, regarding the partial dispersion ratio θg, F in the short wavelength (blue) region,
From NSL7 = 0.5436 and PBM2 = 0.5828, the reference line equation is
θg, F≈−0.001620νd + 0.6416 (B)
It becomes. Many glass types are distributed along this reference line, but glass materials with anomalous dispersion are located farther from this straight line and are more effective in correcting chromatic aberration and reducing the secondary spectrum.

  FIG. 4 plots the relationship between the partial dispersion ratio θg, F in the blue region and the Abbe number νd for some representative optical materials manufactured by OHARA INC. And Sumita Optical Glass Co., Ltd. . Further, the reference line according to the above formula (B) is shown by a solid line. FIG. 5 plots the relationship between the partial dispersion ratios C and A ′ in the red region and the Abbe number νd for several similar representative optical materials. Further, the reference line according to the above formula (A) is shown by a solid line.

  Here, taking FSL5 (nd = 1.48749, νd = 70.2), which is a low dispersion glass material and also has anomalous dispersion in the blue region, as an example, the partial dispersion calculated by equation (A) The ratio θC, A ′ is 0.361, and the partial dispersion ratio θg, F calculated from the equation (B) is 0.528. On the other hand, the partial dispersion ratio θC, A ′ of the actual FSL5 is 0.3633 and 0.5300. Therefore, the amount of the short wavelength anomalous dispersion is +0.002, which is located on the + side (left side of the reference line in the graph of FIG. 4) as in the case of other glass materials having anomalous dispersion. In the region of (2), it is distributed at a position of +0.002 in the opposite direction (to the right side of the reference line in the graph of FIG. 5) from other anomalous dispersion materials. That is, the FSL5 is on the same side as the FPL53, FPL51, PHM53, GFK70, etc. with respect to the reference line on the graph of θg, F−νd in FIG. 4, whereas θC, A′−νd in FIG. On the graph, it exists on the side opposite to the FPL 53 or the like with respect to the reference line.

  Some of the optical materials, such as FSL5, have different anomalous dispersibility tendencies in the blue region and the red region. Therefore, if an attempt is made to remove chromatic aberration by selecting a glass material by paying attention only to the anomalous dispersion in the blue region, there may be a case where sufficient chromatic aberration correction cannot be performed in the red region. Therefore, in the present embodiment, attention is paid to the partial dispersion ratio θC, A ′ in the red region from the C line to the A ′ line, satisfying the conditional expressions (1), (2) and (3). By selecting the lens material, the chromatic aberration in the red region can be corrected well. In FIG. 5, the region highlighted with diagonal lines is a region that satisfies the conditional expressions (1) and (2) and the conditional expression (3).

It should be noted that the material may be selected based on the partial dispersion ratio θC, t focusing on the longer wavelength t-line (1014 nm). That is, a material that satisfies the following conditional expression may be selected. θC, t represents a partial dispersion ratio (nC is a refractive index for the C line, and nt is a refractive index for the t line) when the partial dispersion is (nC−nt).
νd> 60 (1)
θC, t <0.004700νd + 0.5460 (4)
0.760 <θC, t <0.855 (5)

  In the same manner as in FIGS. 4 and 5, FIG. 6 shows partial dispersion ratios θC, t in a longer wavelength region for some representative optical materials manufactured by OHARA INC. And Sumita Optical Glass Co., Ltd. A plot of the relationship with the Abbe number νd is shown. A reference line determined by NSL7 and PBM2 is shown by a solid line. In FIG. 6, the region highlighted with diagonal lines is a region that satisfies the conditional expressions (1) and (4) and the conditional expression (5).

  As described above, according to the variable magnification optical system according to the present embodiment, the lens configuration is optimized using an appropriate optical material by paying attention to the partial dispersion ratio of the red region. In particular, chromatic aberration correction in the red region can be performed better than before. Further, according to the imaging apparatus equipped with the variable magnification optical system according to the present embodiment, since the variable magnification optical system is used as an imaging lens, a high-quality captured image can be obtained.

  Next, specific numerical examples of the variable magnification optical system according to the present embodiment will be described. Hereinafter, a plurality of numerical examples will be described together.

  7A and 7B show specific lens data corresponding to the configuration of the variable magnification optical system shown in FIGS. 1A and 1B. In particular, FIG. 7A shows the basic lens data, and FIG. 7B shows other data relating to zooming and focusing. In the surface number Si column in the lens data shown in FIG. 7A, the surface of the component closest to the object side is the first in the variable-power optical system according to Example 1, and increases sequentially toward the image side. The numbers of the i-th (i = 1, 2, 3,. In the column of the radius of curvature Ri, the value (mm) of the radius of curvature of the i-th surface from the object side is shown in correspondence with the reference symbol Ri in FIG. Similarly, the column of the surface interval Di indicates the interval (mm) on the optical axis between the i-th surface Si and the i + 1-th surface Si + 1 from the object side. The ndj column indicates the refractive index value for the d-line (587.6 nm) of the j-th (j = 1, 2, 3...) optical element from the object side. The column of νdj shows the Abbe number value for the d-line of the j-th optical element from the object side.

  FIG. 7A also shows values of the partial dispersion ratio θC, A ′ and the partial dispersion ratio θC, t. In the variable magnification optical system according to Example 1, the negative lens L31 in the third lens group G3 and the positive lens L42 in the fourth lens group G4 have the conditional expressions (1), (2) and ( It is an optical material with anomalous dispersion that satisfies 3). These lenses also satisfy the conditional expressions (1) and (4) and the conditional expression (5). The part corresponding to these optical materials having anomalous dispersion is marked with “*”.

  In the variable magnification optical system according to Example 1, the second lens group G2 and the third lens group G3 move on the optical axis as the magnification and focus are changed. , D7, D8, and D10 are variable. In FIG. 7B, as data at the time of zooming of these surface distances D5, D7, D8, and D10, data on the long distance side (standard state) corresponding to the lens arrangement of FIG. Data on the short distance side (proximity state) corresponding to the lens arrangement of FIG.

  Similar to the variable magnification optical system according to Example 1 above, specific lens data corresponding to the configuration of the variable magnification optical system shown in FIGS. Shown in (A) and (B). Similarly, specific lens data corresponding to the configuration of the variable magnification optical system shown in FIGS. 3A and 3B is shown as Example 3 in FIGS. 9A and 9B. Also in Example 2 and Example 3, since the second lens group G2 and the third lens group G3 move on the optical axis with zooming and focusing, the front and back surface distances D7, The values of D9, D10, and D12 are variable.

  In Example 2 and Example 3, as in Example 1, the negative lens L31 in the third lens group G3 and the positive lens L42 in the fourth lens group G4 are the conditional expressions (1), (2) and The optical material has anomalous dispersion that satisfies the conditional expression (3). The lens also satisfies the conditional expressions (1) and (4) and the conditional expression (5).

  Lens data of the variable magnification optical system according to Example 4 and Example 5 are shown in FIGS. 10 (A) and 10 (B) and FIGS. 11 (A) and 11 (B). In Examples 4 and 5, the basic rough lens configuration is the same as that of Example 1, and the cross-section of the lens is substantially the same as that shown in FIGS. However, the portion corresponding to the optical material having anomalous dispersion is different. In Example 4, only the negative lens L31 in the third lens group G3 satisfies the predetermined conditions (conditional expressions (1) to (5)) regarding the anomalous dispersion in the present embodiment. In Example 5, only the positive lens L42 in the fourth lens group G4 satisfies a predetermined condition regarding anomalous dispersion.

  Further, lens data of the variable magnification optical system according to the comparative example are shown in FIGS. In this comparative example, the basic rough lens configuration is the same as that of the first embodiment, and the cross section of the lens is substantially the same as that shown in FIGS. However, in this comparative example, an optical material that satisfies a predetermined condition regarding anomalous dispersion in the present embodiment is not used.

  FIGS. 13A to 13D respectively show spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration on the long distance side in the variable magnification optical system according to Example 1. FIG. 14A to 14D show similar aberrations on the short distance side. Each aberration diagram shows an aberration with the d-line as a reference wavelength. The spherical aberration diagram and the lateral chromatic aberration diagram also show aberrations for the F-line, C-line, g-line, and A′-line. In the astigmatism diagram, the solid line indicates the sagittal direction and the broken line indicates the tangential direction. FNO. Indicates an F value, and ω indicates a half angle of view.

  Similarly, various aberrations of the variable magnification optical system according to Example 2 are shown in FIGS. 15A to 15D (far distance side) and FIGS. 16A to 16D (short distance side). Similarly, various aberrations regarding the variable magnification optical systems according to Examples 3 to 5 are shown in FIGS. 17 to 22 (A) to (D). Further, various aberrations of the variable magnification optical system according to the comparative example are shown in FIGS. 23A to 23D (far distance side) and FIGS. 24A to 24D (short distance side).

  The present embodiment is for achieving good correction of chromatic aberration, and is particularly effective for correcting lateral chromatic aberration and axial chromatic aberration in the standard state (long distance side). In the case of axial chromatic aberration, the imaging magnification is large in the close state (short distance side). Therefore, in the case of the same aperture diameter, the effective F value is darker than the standard state and the F value is brighter in the standard state. It is necessary to correct better than the proximity state. In the comparative example, since an optical material having a predetermined anomalous dispersion is not used, correction of lateral chromatic aberration in a standard state is insufficient. In the fifth embodiment, an optical material having a predetermined anomalous dispersion is used for the fourth lens group G4, and the correction of the chromatic aberration of magnification is good, but the axial chromatic aberration is increased in the standard state. On the other hand, in Example 4 in which an optical material having a predetermined anomalous dispersion is used for the third lens group G3, the axial chromatic aberration is improved in the standard state, but the lateral chromatic aberration is insufficiently corrected. In Examples 1 to 3, optical materials having predetermined anomalous dispersion are used for both the third lens group G3 and the fourth lens group G4, and lateral chromatic aberration and axial chromatic aberration are corrected well.

  In addition, this invention is not limited to the said embodiment and each Example, A various deformation | transformation implementation is possible. For example, the radius of curvature, the surface interval, and the refractive index of each lens component are not limited to the values shown in the above numerical examples, and may take other values.

  Further, the present invention is not limited to the four-group variable magnification optical system as shown in the above embodiment, but can be applied to other variable magnification optical systems. In particular, the present invention includes a lens group including at least one positive lens and having a positive refractive power as a whole, and a lens group including at least one negative lens and having a negative refractive power as a whole. In a variable magnification optical system, an optical element having anomalous dispersion satisfying the above-described predetermined condition for at least one of a positive lens in a lens group having a positive refractive power and a negative lens in a lens group having a negative refractive power It only needs to include a lens in which the material is used.

1 is a lens cross-sectional view corresponding to Example 1, illustrating a first configuration example of a variable magnification optical system according to an embodiment of the present invention. FIG. 2 is a lens cross-sectional view corresponding to Example 2 and illustrating a second configuration example of a variable magnification optical system according to an embodiment of the present invention. FIG. FIG. 6 is a lens cross-sectional view illustrating a third configuration example of a variable magnification optical system according to an embodiment of the present invention and corresponding to Example 3; It is explanatory drawing which shows the relationship between partial dispersion ratio (theta) g, F of the blue area | region from g line to F line, and Abbe number (nu) d. It is explanatory drawing which shows the relationship between partial dispersion ratio (theta) C, A 'of the red area | region from C line to A' line, and Abbe number (nu) d. It is explanatory drawing which shows the relationship between partial dispersion ratio (theta) C, t of the red area | region from C line to t line, and Abbe number (nu) d. 2A and 2B are diagrams illustrating lens data of a variable magnification optical system according to Example 1. FIG. 3A illustrates basic lens data, and FIG. FIG. 4 is a diagram illustrating lens data of a zoom optical system according to Example 2, where (A) shows basic lens data and (B) shows data related to zooming. FIG. 10 is a diagram illustrating lens data of a variable magnification optical system according to Example 3, wherein (A) illustrates basic lens data, and (B) illustrates data relating to variable magnification. FIG. 10 is a diagram illustrating lens data of a variable magnification optical system according to Example 4, wherein (A) illustrates basic lens data, and (B) illustrates data relating to variable magnification. FIG. 10 is a diagram illustrating lens data of a zoom optical system according to Example 5, where (A) shows basic lens data and (B) shows data related to zoom. It is a figure which shows the lens data of the zooming optical system which concerns on a comparative example, (A) shows basic lens data, (B) shows the data regarding zooming. FIG. 4 is an aberration diagram showing various aberrations of the variable magnification optical system according to Example 1 in a long distance state, where (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. FIG. 4 is an aberration diagram showing various aberrations in the short distance state of the variable magnification optical system according to Example 1, where (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. FIG. 6 is an aberration diagram showing various aberrations in the long distance state of the variable magnification optical system according to Example 2, wherein (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. FIG. 6 is an aberration diagram showing various aberrations in the short distance state of the variable magnification optical system according to Example 2, wherein (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. FIG. 6 is an aberration diagram showing various aberrations in the long distance state of the variable magnification optical system according to Example 3, wherein (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. FIG. 6 is an aberration diagram showing various aberrations in the short distance state of the variable magnification optical system according to Example 3, wherein (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. FIG. 10 is an aberration diagram showing various aberrations in the long distance state of the variable magnification optical system according to Example 4, wherein (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. FIG. 10 is an aberration diagram showing various aberrations in the short distance state of the variable magnification optical system according to Example 4, wherein (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. FIG. 10 is an aberration diagram showing various aberrations in the long distance state of the variable magnification optical system according to Example 5, where (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. FIG. 10 is an aberration diagram showing various aberrations in the short distance state of the variable magnification optical system according to Example 5, wherein (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Indicates. It is an aberration diagram which shows the various aberrations in the long distance state of the variable magnification optical system which concerns on a comparative example, (A) is spherical aberration, (B) is astigmatism, (C) is distortion, (D) is lateral chromatic aberration. Show. It is an aberration diagram which shows the various aberrations in the short distance state of the variable magnification optical system which concerns on a comparative example, (A) is spherical aberration, (B) is astigmatism, (C) is distortion, (D) is magnification chromatic aberration. Show. It is a block diagram which shows an example of the objective optical system in an endoscope.

Explanation of symbols

  GC ... optical member, G1 ... first lens group, G2 ... second lens group, G3 ... third lens group, G4 ... fourth lens group, GP ... prism, St ... aperture stop, Ri ... ith from the object side The radius of curvature of the lens surface, Di: the surface distance between the i-th and i + 1-th lens surfaces from the object side, Z1,.

Claims (8)

A variable magnification optical system having a plurality of lens groups,
A lens group including at least one positive lens and having a positive refractive power;
A lens group including at least one negative lens and having a negative refractive power,
An optical material that satisfies the following conditional expressions (1) and (2) is used for at least one of the positive lens and the negative lens.
νd> 60 (1)
θC, A ′ <0.001198νd + 0.2765 (2)
However,
νd: Abbe number with respect to d-line θC, A ′: Partial dispersion ratio when partial dispersion is (nC−nA ′). nC is the refractive index for the C line, and nA ′ is the refractive index for the A ′ line. The variable magnification optical system according to claim 1, wherein the lens satisfying the conditional expressions (1) and (2) further satisfies the following conditional expression.
0.330 <θC, A ′ <0.360 (3) 3. The variable magnification optical system according to claim 1, wherein an infrared cut filter having a transmittance half value of 750 nm ± 20 nm is provided between the lens system and the imaging plane. From the object side,
A first lens group having a negative refractive power that is fixed during zooming and focusing;
A second lens group that moves on the optical axis during zooming and focusing and has a positive refractive power;
A third lens group that moves on the optical axis during zooming and focusing, includes at least one negative lens, and has negative refractive power;
4. A fourth lens group fixed at the time of zooming and focusing and including at least one positive lens and having a positive refractive power is provided. 2. A variable magnification optical system according to item 1. 5. The variable magnification optical system according to claim 4, wherein the third lens group includes a negative lens using an optical material that satisfies the conditions of the conditional expressions (1) and (2). . The zoom lens according to claim 4 or 5, wherein the fourth lens group includes a positive lens using an optical material that satisfies the conditions of the conditional expressions (1) and (2). Optical system. The variable power optical system according to any one of claims 1 to 6, wherein the zoom lens system is used as an objective lens for an endoscope. A variable magnification optical system according to any one of claims 1 to 7,
An imaging device comprising: an imaging element that outputs an imaging signal corresponding to an optical image formed by the zoom optical system.

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