JP2011227380A - Objective lens for endoscope, and imaging apparatus - Google Patents

Abstract

An objective lens for an endoscope that can be used satisfactorily in a wide wavelength range.
An endoscope objective lens is configured to satisfy the following conditional expression (1). The endoscope objective lens can be configured to have 5 elements in 4 groups including a cemented lens on the image side from the stop, or 6 elements in 4 groups including cemented lenses on the object side and the image side from the stop.
| Δh−δC | / f <2 (1)
However,
δh: chromatic aberration of magnification of h-line with respect to d-line at maximum half angle of view (unit: μm)
δC: chromatic aberration of magnification of C line with respect to d line at maximum half angle of view (unit: μm)
f: Focal length of the entire system (unit: mm)
[Selection] Figure 1

Description

  The present invention relates to an endoscope objective lens and an imaging device, and more particularly to an endoscope objective lens that can be used in a wide wavelength range, and an imaging device including the endoscope objective lens. .

  Conventionally, a mercury lamp has been used as an illumination light source when using an endoscope objective lens. The mercury lamp has a greenish bluish white color composed of emission lines having a wavelength of 404.7 nm (h line), a wavelength of 435.8 nm (g line), a wavelength of 546.1 nm (e line), a wavelength of 577.0 nm, and a wavelength of 579.1 nm. It emits light, and also emits light in the ultraviolet region with a wavelength of 253.7 nm and a wavelength of 365.0 nm (i-line). However, since most optical materials have low transmittance for i-line, light in the ultraviolet region can be ignored in ordinary optical systems.

  The refractive index of the optical material in the visible range is high on the short wavelength side (blue) and low on the long wavelength side (red). Therefore, in an imaging optical system that does not correct chromatic aberration, the refractive index is higher than that of red light. The focal length for light is shorter. Thus, normally, chromatic aberration is corrected by using various materials having different Abbe numbers for the positive lens and the negative lens. The Abbe number νd for the d-line (wavelength 587.6 nm), which is commonly used, is the refractive index nd for the d-line, the refractive index nF for the F-line (wavelength 486.1 nm), and the C-line (wavelength 656.3 nm). Using the refractive index nC, it is expressed by νd = (nd−1) / (nF−nC). When designing an optical system for the visible range, chromatic aberration is often corrected for the F-line and C-line in general on the d-line basis.

  For example, the inventors of the present application have devised the optical systems described in the following Patent Documents 1 to 3 as an optical system in which correction of chromatic aberration in the visible region is performed well by combining a positive lens and a negative lens having different Abbe numbers. . In particular, Patent Document 2 discloses that it is possible to satisfactorily correct lateral chromatic aberration by satisfying a predetermined conditional expression using the difference between the Abbe numbers of a positive lens and a negative lens.

JP 2008-257108 A JP 2008-152210 A Japanese Patent Application No. 2009-130377

  By the way, in recent years, opportunities to use light having a wavelength of around 400 nm are increasing in diagnosis in the medical field. Therefore, in such a diagnosis, an optical system in which the chromatic aberration is corrected to a shorter wavelength than the conventional optical system in which the chromatic aberration is corrected with respect to the F line and the C line on the d-line basis has been demanded.

  The correction of lateral chromatic aberration will be described in detail with reference to FIGS. Focusing on the three colors of d-line, F-line, and C-line, and looking at the chromatic aberration of magnification accompanying the angle of view of the F-line and C-line on the d-line basis, generally, as shown in FIG. There is a tendency to touch the plus side (over state), and the F line touches the minus side (under state). Note that the vertical axis in FIG. 25 is the angle of view, the horizontal axis is the amount of lateral chromatic aberration with respect to the d-line, the maximum value on the vertical axis corresponds to the maximum half angle of view, and F on the d-line basis. The lateral chromatic aberrations of the line C and the line C are indicated by broken lines with symbols F and C, respectively.

  When correction is performed from the state shown in FIG. 25 and the amount of plus / minus chromatic aberration of magnification with respect to the d-line is reduced, as shown in FIG. 26, the chromatic aberration of magnification of the F-line is in the middle of the maximum half angle of view. In many cases, it is corrected to the plus side until the maximum half angle of view is the same as that of the C line. FIG. 26 is a schematic diagram, and the method of illustration is the same as that of FIG. However, when corrected to the extent shown in FIG. 26, the amount of change in chromatic aberration with respect to the wavelength is severe on the short wavelength side. As indicated by the broken line, the value is significantly positive with respect to the d line. In this case, the size of the image looks different for each color, and the sharpness and resolution of the image are lowered.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an endoscope objective lens that can be used favorably in a wide wavelength range, and an imaging apparatus including the endoscope objective lens. Is.

The endoscope objective lens of the present invention satisfies the following conditional expression (1).
| Δh−δC | / f <2 (1)
However,
δh: chromatic aberration of magnification of h-line with respect to d-line at maximum half angle of view (unit: μm)
δC: chromatic aberration of magnification of C line with respect to d line at maximum half angle of view (unit: μm)
f: Focal length of the entire system (unit: mm)

  The “maximum half angle of view” can be determined by the specifications of the objective lens or the specifications of the system to which the objective lens is applied. For example, when the objective lens is used in combination with an image pickup device arranged on the image plane, the maximum half angle of view may be determined based on the size of the image pickup surface of the image pickup device.

  An endoscope objective lens according to the present invention includes, in order from the object side, a plano-concave lens having a flat surface on the object side or a negative meniscus lens having a convex surface on the object side, and a combination of a positive lens and a negative lens or a negative lens and a positive lens. Positive refractive power as a whole consisting of a first cemented lens composed of a combination of the above, a stop, a positive lens having a convex surface on the image side, and a combination of a positive lens and a negative lens or a combination of a negative lens and a positive lens It is good also as a 4 group 6 piece structure by which the 2nd cemented lens which has is arranged.

ただし、
Ｎθ_１ｈｇ：第１の接合レンズ中の負レンズのｈ線とｇ線間の部分分散比
Ｐθ_１ｈｇ：第１の接合レンズ中の正レンズのｈ線とｇ線間の部分分散比
Ｒ_１Ｃ：第１の接合レンズの接合面の曲率半径
Ｎθ_２ｈｇ：第２の接合レンズ中の負レンズのｈ線とｇ線間の部分分散比
Ｐθ_２ｈｇ：第２の接合レンズ中の正レンズのｈ線とｇ線間の部分分散比
Ｒ_２Ｃ：第２の接合レンズの接合面の曲率半径
Ｂｆ：全系のバックフォーカス
ｄ：最も像側のレンズの中心厚
ｎ：最も像側のレンズのｄ線における屈折率 When the endoscope objective lens of the present invention adopts the above-mentioned four groups and six elements, it is preferable that the following conditional expression (2) is satisfied.

However,
Nθ _1hg : Partial dispersion ratio between the h-line and g-line of the negative lens in the first cemented lens _{Pθ 1hg} : Partial dispersion ratio R _1C between the h-line and g-line of the positive lens in the first cemented lens Radius of curvature Nθ _2hg of the cemented surface of the first cemented lens: partial dispersion ratio _{Pθ 2hg} between the h-line and g-line of the negative lens in the second cemented lens: h-line and g of the positive lens in the second cemented lens Partial dispersion ratio R _2C between the lines: radius of curvature Bf of the cemented surface of the second cemented lens Bf: back focus of the entire system d: center thickness of the lens closest to the image side n: refractive index at the d-line of the lens closest to the image side

  Alternatively, the endoscope objective lens according to the present invention includes, in order from the object side, a negative lens having a concave surface on the image side, a positive lens having a convex surface on the object side, a stop, and a positive lens having a convex surface on the image side. Further, a four-group five-lens configuration in which a cemented lens having a positive refractive power as a whole is formed by bonding a positive lens and a negative lens or by bonding a negative lens and a positive lens.

ただし、
Ｎθ_ｈｇ：接合レンズ中の負レンズのｈ線とｇ線間の部分分散比
Ｐθ_ｈｇ：接合レンズ中の正レンズのｈ線とｇ線間の部分分散比
Ｒ_Ｃ：接合レンズの接合面の曲率半径
Ｂｆ：全系のバックフォーカス
ｄ：最も像側のレンズの中心厚
ｎ：最も像側のレンズのｄ線における屈折率 When the endoscope objective lens of the present invention adopts the above-mentioned four groups and five elements, it is preferable that the following conditional expression (3) is satisfied.

However,
N.theta _hg: a partial dispersion ratio between the h-line and g-line of the negative lens in the cemented lens Pshita _hg: partial dispersion ratio R _C between h-line and g-line of the positive lens in the cemented lens of _curvature of the cemented surface of the cemented lens Radius Bf: Back focus of the entire system d: Center thickness of the lens closest to the image side n: Refractive index at the d-line of the lens closest to the image side

  The partial dispersion ratio between the h-line and the g-line refers to the refractive index at the h-line of the lens as nh, the refractive index at the g-line as ng, the refractive index at the F-line as nF, and the refractive index at the C-line. When nC, it is represented by (nh-ng) / (nF-nC).

  Note that the reference numerals and surface shapes of the lenses described in the four-group five-lens configuration and the four-group six-lens configuration are in the paraxial region when the lens is an aspherical lens. Further, the back focus used in the conditional expressions (2) and (3) is assumed to use an air conversion length.

  An imaging apparatus according to the present invention includes the above-described endoscope objective lens according to the present invention.

In the image pickup apparatus of the present invention, when the image pickup device arranged on the image plane of the endoscope objective lens is further provided, it is preferable that the following conditional expression (4) is satisfied.
| Δh−δC | <3 × P (4)
However,
P: Pitch of pixel array of the image sensor (unit: μm)

In addition, when the pitch of the pixel arrangement of the image sensor is different in the horizontal direction and the vertical direction, for example, P may be determined as follows.
P = (PH × PV) ^1/2
However,
PH: pixel pitch in the horizontal direction of the image sensor PV: pixel pitch in the vertical direction of the image sensor

  Unless otherwise specified, “small chromatic aberration of magnification” and “large chromatic aberration of magnification” in this specification mean that the absolute value of the amount of chromatic aberration of magnification is small and large, respectively.

  According to the endoscope objective lens of the present invention, since it is configured to satisfy the conditional expression (1), it is possible to suppress the lateral chromatic aberration of the h-line having a shorter wavelength than that of the F-line. It is also possible to cope with an increasing light near a wavelength of 400 nm, and it can be used well in a wider wavelength range than a conventional general endoscope objective lens for a visible range.

  According to the imaging apparatus of the present invention, since the endoscope objective lens of the present invention is provided, it is possible to cope with light having a wavelength near 400 nm, which has been increasing in demand in recent years. This makes it possible to use in a wider wavelength range better than when the endoscope objective lens is used.

The figure for demonstrating the magnification chromatic aberration of the objective lens concerning embodiment of this invention Sectional drawing which shows the structure and optical path of the objective lens of Example 1 of this invention Sectional drawing which shows the structure and optical path of the objective lens of Example 2 of this invention Sectional drawing which shows the structure and optical path of the objective lens of Example 3 of this invention Sectional drawing which shows the structure and optical path of the objective lens of Example 4 of this invention Sectional drawing which shows the structure and optical path of the objective lens of Example 5 of this invention FIG. 2 is an aberration diagram of the objective lens according to Example 1 of the present invention, in which (A) is a spherical aberration diagram, (B) is an astigmatism diagram, (C) is a distortion aberration diagram, and (D) is a chromatic aberration diagram of magnification. FIG. 4 is an aberration diagram of the objective lens according to Example 2 of the present invention, in which (A) is a spherical aberration diagram, (B) is an astigmatism diagram, (C) is a distortion aberration diagram, and (D) is a chromatic aberration diagram of magnification. It is each aberration figure of the objective lens of Example 3 of this invention, (A) is a spherical aberration figure, (B) is an astigmatism figure, (C) is a distortion aberration figure, (D) is a magnification chromatic aberration figure. FIG. 6A is an aberration diagram of the objective lens according to Example 4 of the present invention, where FIG. 5A is a spherical aberration diagram, FIG. 5B is an astigmatism diagram, FIG. C is a distortion aberration diagram, and FIG. FIG. 6A is an aberration diagram of the objective lens according to Example 5 of the present invention, where FIG. 6A is a spherical aberration diagram, FIG. 5B is an astigmatism diagram, FIG. C is a distortion aberration diagram, and FIG. Sectional drawing which shows the structure and optical path of the objective lens of the comparative example 1 Sectional drawing which shows the structure and optical path of the objective lens of the comparative example 2 Sectional drawing which shows the structure and optical path of the objective lens of the comparative example 3 Sectional drawing which shows the structure and optical path of the objective lens of the comparative example 4 Sectional drawing which shows the structure and optical path of the objective lens of the comparative example 5 It is each aberration figure of the objective lens of the comparative example 1, (A) is a spherical aberration figure, (B) is an astigmatism figure, (C) is a distortion figure, (D) is a magnification chromatic aberration figure. It is each aberration figure of the objective lens of the comparative example 2, (A) is a spherical aberration figure, (B) is an astigmatism figure, (C) is a distortion aberration figure, (D) is a magnification chromatic aberration figure. It is each aberration figure of the objective lens of the comparative example 3, (A) is a spherical aberration figure, (B) is an astigmatism figure, (C) is a distortion aberration figure, (D) is a magnification chromatic aberration figure. It is each aberration figure of the objective lens of the comparative example 4, (A) is a spherical aberration figure, (B) is an astigmatism figure, (C) is a distortion aberration figure, (D) is a magnification chromatic aberration figure. It is each aberration figure of the objective lens of the comparative example 5, (A) is a spherical aberration figure, (B) is an astigmatism figure, (C) is a distortion aberration figure, (D) is a magnification chromatic aberration figure. The figure which shows the amount of chromatic aberration of magnification of Examples 1-5 of the present invention and Comparative Examples 1-5 The figure which shows schematic structure of the endoscope concerning embodiment of this invention. Cross-sectional view of the main part of the hard end of the endoscope The figure which shows the tendency of general magnification chromatic aberration typically Magnification chromatic aberration diagram for explaining conventional chromatic aberration correction

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a lateral chromatic aberration diagram of an objective lens according to an embodiment of the present invention, where the horizontal axis is the amount of lateral chromatic aberration with reference to the d-line (wavelength 587.6 nm), and the vertical axis is the angle of view. The maximum value on the vertical axis corresponds to the maximum half angle of view. In FIG. 1, the magnification for h-line (wavelength 404.7 nm), g-line (wavelength 435.8 nm), F-line (wavelength 486.1 nm), and C-line (wavelength 656.3 nm) when d-line is the reference. Chromatic aberration is indicated by broken lines with symbols h, g, F, and C, respectively. 1 is an example, and the chromatic aberration of magnification of the objective lens of the present invention is not limited to that shown in FIG.

The objective lens of the present embodiment focuses on the h line and the C line and is configured to satisfy the following conditional expression (1).
| Δh−δC | / f <2 (1)
However,
δh: chromatic aberration of magnification of h-line with respect to d-line at maximum half angle of view (unit: μm)
δC: chromatic aberration of magnification of C line with respect to d line at maximum half angle of view (unit: μm)
f: Focal length of the entire system (unit: mm)

  Conventionally, many visible objective lenses have been corrected for chromatic aberration with respect to the F-line and C-line on the basis of the d-line. In such conventional lenses, as described in the problem section, the g-line based on the d-line is used. , There is a problem that the chromatic aberration of magnification of the h-line becomes large. Considering this point, the objective lens of this embodiment is obtained by performing chromatic aberration correction using h-rays (wavelength 404.7 nm) having a shorter wavelength instead of F-lines (wavelength 486.1 nm).

  By satisfying the conditional expression (1), it is possible to suppress lateral chromatic aberration for the wavelength between the h-line and the C-line. For example, as shown in FIG. 1, when aberration correction is performed so as to satisfy the conditional expression (1) while δh and δC are set to small values on the plus side, the lateral chromatic aberration of the F line with respect to the d line is a small value on the minus side. The lateral chromatic aberration of the g-line with respect to the d-line tends to take a small plus or minus value, and as a result, the lateral chromatic aberration can be reduced in a wide wavelength range.

  According to the objective lens of the present embodiment configured as described above, in addition to the wavelength range that the conventional general visible range objective lens supports, the wavelength range near the wavelength of 400 nm, which has been increasing in demand in recent years, is also available. Good chromatic aberration correction can be ensured in a wide wavelength range including this, and it can be used well in such a wide wavelength range. Light having a wavelength of around 400 nm is useful because it is often used to acquire spectral images that have been applied in the medical field in recent years.

  The objective lens according to the embodiment of the present invention can employ, for example, the lens configuration of the first aspect or the second aspect described below. A configuration example of the first mode is shown in FIGS. 2 to 5, and a configuration example of the second mode is shown in FIG. 6. 2 to 6 correspond to objective lenses of Examples 1 to 5 described later, respectively.

  2 to 6, the left side of the figure is the object side, and the right side is the image side, and the axial light beam 2 and the off-axis light beam 3 corresponding to the maximum half angle of view are also shown. The aperture stop St illustrated in FIGS. 2 to 6 does not represent a shape or size, but a position on the optical axis Z. When the objective lens is applied to the imaging apparatus, it is preferable to provide a cover glass, various filters, a prism, or the like according to the configuration of the imaging apparatus. In FIGS. 2 to 6, these are assumed on the image side of the most image side lens. In this example, the parallel plate-shaped optical member PP is arranged, and the imaging position P is positioned on the image side surface of the optical member PP.

  The first aspect includes, in order from the object side, a plano-concave lens having a flat surface on the object side or a negative meniscus lens having a convex surface on the object side, and a positive lens and a negative lens or a negative lens and a positive lens. A second lens having a positive refractive power as a whole comprising a first cemented lens, a stop, a positive lens having a convex surface on the image side, and a combination of a positive lens and a negative lens or a combination of a negative lens and a positive lens. A four-group six-lens configuration in which cemented lenses are arranged.

  In the configuration example illustrated in FIG. 2, a first lens L21 that is a negative meniscus lens, a second lens L22 that is a plano-convex lens, and a third plano-concave lens that are sequentially from the object side to the image side along the optical axis Z. Bonding of the first cemented lens formed by bonding the lens L23, the aperture stop St, the fourth lens L24 that is a plano-convex lens, the fifth lens L25 that is a biconvex lens, and the sixth lens L26 that is a negative meniscus lens And a second cemented lens having a positive refractive power as a whole.

  In the configuration example shown in FIG. 3, a first lens L21 that is a plano-concave lens, a second lens L22 that is a negative meniscus lens, and a third plano-convex lens in order from the object side to the image side along the optical axis Z. Bonding of the first cemented lens formed by bonding the lens L23, the aperture stop St, the fourth lens L24 that is a plano-convex lens, the fifth lens L25 that is a biconvex lens, and the sixth lens L26 that is a negative meniscus lens And a second cemented lens having a positive refractive power as a whole.

  The configuration examples shown in FIGS. 4 and 5 include a first lens L21 that is a negative meniscus lens, a second lens L22 that is a negative meniscus lens, and a planoconvex lens in order from the object side to the image side along the optical axis Z. A first cemented lens formed by bonding the third lens L23, an aperture stop St, a fourth lens L24 that is a planoconvex lens, a fifth lens L25 that is a biconvex lens, and a sixth lens that is a negative meniscus lens. A second cemented lens having a positive refractive power as a whole, which is formed by bonding L26, is arranged.

  In the first aspect, a cemented lens made up of positive and negative lenses is arranged on both the object side and the image side of the aperture stop St, which is a very advantageous configuration for correcting lateral chromatic aberration. The second cemented lens in the first aspect only needs to be composed of positive and negative lenses, but as in the examples shown in FIGS. 2 to 5, a configuration in which a positive lens and a negative lens are arranged in this order from the object side. Is more effective for good correction of lateral chromatic aberration.

  In addition, it is advantageous to secure a wide angle of view by using a lens that is disposed closest to the object side as a plano-concave lens having a plane on the object side or a negative meniscus lens having a convex surface on the object side. When the objective lens is used without a protective member, the lens arranged closest to the object side may be exposed to the outside. The surface is preferably a flat surface or a convex surface, and is more preferably a flat surface when cost is important.

  By arranging the aperture stop St near the center of the lens system, the lens diameter can be reduced. By adopting the lens configuration of the first aspect, it is possible to realize a lens system in which various aberrations including chromatic aberration of magnification are well corrected while ensuring a wide angle of view and a long back focus with a compact configuration. It becomes easy.

  When the objective lens adopts the four-group six-element configuration of the first aspect, it is preferable that the following conditional expression (2) is further satisfied.

ただし、
Ｎθ_１ｈｇ：第１の接合レンズ中の負レンズのｈ線とｇ線間の部分分散比
Ｐθ_１ｈｇ：第１の接合レンズ中の正レンズのｈ線とｇ線間の部分分散比
Ｒ_１Ｃ：第１の接合レンズの接合面の曲率半径
Ｎθ_２ｈｇ：第２の接合レンズ中の負レンズのｈ線とｇ線間の部分分散比
Ｐθ_２ｈｇ：第２の接合レンズ中の正レンズのｈ線とｇ線間の部分分散比
Ｒ_２Ｃ：第２の接合レンズの接合面の曲率半径
Ｂｆ：全系のバックフォーカス
ｄ：最も像側のレンズの中心厚
ｎ：最も像側のレンズのｄ線における屈折率

However,
Nθ _1hg : Partial dispersion ratio between the h-line and g-line of the negative lens in the first cemented lens _{Pθ 1hg} : Partial dispersion ratio R _1C between the h-line and g-line of the positive lens in the first cemented lens Radius of curvature Nθ _2hg of the cemented surface of the first cemented lens: partial dispersion ratio _{Pθ 2hg} between the h-line and g-line of the negative lens in the second cemented lens: h-line and g of the positive lens in the second cemented lens Partial dispersion ratio R _2C between the lines: radius of curvature Bf of the cemented surface of the second cemented lens Bf: back focus of the entire system d: center thickness of the lens closest to the image side n: refractive index at the d-line of the lens closest to the image side

  Here, the partial dispersion ratio between the h-line and the g-line is the refractive index at the h-line of the lens is nh, the refractive index at the g-line is ng, the refractive index at the F-line is nF, and the refractive index at the C-line. Is represented by (nh-ng) / (nF-nC), where nC is nC. Compared with the above-mentioned definition of Abbe number νd = (nd-1) / (nF-nC), this partial dispersion ratio is characterized in that the partial dispersion, which is the difference in refractive index between h-line and g-line, is used as a numerator. is there.

  The change in refractive index due to wavelength varies depending on the material and appears more prominently on the short wavelength side.If chromatic aberration correction on the shorter wavelength side is required, not only the Abbe number but also the partial dispersion ratio on the short wavelength side It is preferable to consider. From such a viewpoint, the conditional expression (2) defines a suitable degree of correction of chromatic aberration of magnification in the short wavelength region, focusing on the partial dispersion ratio between the h-line and the g-line and the joint surface.

  Here, the first term and the second term of the conditional expression (2) are set to Fθ and Rθ, respectively. Fθ and Rθ can be modified as in the following equations (2A) and (2B).

  The term of Fθ in the conditional expression (2) relates to the first cemented lens on the object side from the aperture stop St. As can be seen from the conditional expression (2A), the positive lens and the negative lens that constitute this cemented lens. This can be divided into a first component composed of a ratio of partial dispersion ratios and a second component obtained by standardizing the absolute value of the radius of curvature of the joint surface with the focal length. These first and second components indicate two conditions that are advantageous for correcting lateral chromatic aberration in the short wavelength region of the cemented lens disposed on the object side from the aperture stop St. In the lens configuration of the first aspect, it is advantageous for correcting the chromatic aberration of magnification that the first component is small and the second component is large.

  The term Rθ in conditional expression (2) relates to the second cemented lens in the rear group convergence system on the image side from the aperture stop St. As can be seen from conditional expression (2B), the negative lens constituting this cemented lens is used. A third component comprising the ratio of the partial dispersion ratio of the lens and the positive lens, a fourth component obtained by normalizing the absolute value of the radius of curvature of the cemented surface with the focal length, the back focus of the entire system, and the lens closest to the image side. It can be considered that it is divided into the fifth component normalized by the focal length, that is, the sum of the air-converted length on the optical axis, that is, the distance from the joint surface to the imaging surface.

  Note that the first cemented lens on the object side and the second cemented lens on the image side from the aperture stop St have opposite effects in terms of correcting the chromatic aberration of magnification, so that the positive and negative lenses of the first component and the third component have positive and negative lenses. The numerator is reversed, and the difference between Fθ and Rθ is obtained on the left side of conditional expression (2).

  These third to fifth components indicate three conditions that are advantageous for correcting lateral chromatic aberration in the short wavelength region of the cemented lens disposed on the image side from the aperture stop St. In the lens configuration of the first aspect, the correction of lateral chromatic aberration is more advantageous as the third component is larger, the fourth component is smaller, and the fifth component is smaller.

  In general, in order to correct lateral chromatic aberration, an optical member responsible for correcting lateral chromatic aberration is disposed at a position distant from the aperture stop St. In particular, on the image side from the aperture stop St, the optical member is disposed at a position close to the imaging surface. The effect is more prominent the more it is. However, in a lens system that requires a long back focus in order to arrange a filter, prism, etc., an optical member for correcting chromatic aberration of magnification cannot be arranged at a position close to the image plane, and correction of chromatic aberration of magnification is not easy. There wasn't. According to the objective lens of the present embodiment, by configuring so as to satisfy the conditional expression (2), it is possible to satisfactorily maintain lateral chromatic aberration in a wide wavelength range while maintaining a back focus with a practically sufficient length. It becomes possible.

  Next, a 2nd aspect is demonstrated. The second aspect includes, in order from the object side, a negative lens having a concave surface on the image side, a positive lens having a convex surface on the object side, a stop, a positive lens having a convex surface on the image side, a positive lens, and a negative lens. This is a four-group five-lens configuration in which a cemented lens having a positive refractive power as a whole is formed by bonding or bonding of a negative lens and a positive lens.

  The configuration example shown in FIG. 5 includes, in order from the object side to the image side along the optical axis Z, a first lens L11 that is a plano-concave lens, a second lens L12 that is a plano-convex lens having a convex surface on the object side, Positive refraction as a whole consisting of an aperture stop St, a third lens L13 that is a positive meniscus lens having a convex surface on the image side, a fourth lens L14 that is a biconvex lens, and a fifth lens L15 that is a negative meniscus lens. A cemented lens having power is arranged.

  The first lens L11 disposed closest to the object side is preferably a negative lens for securing a wide angle and a long back focus, and the object side surface is convex as described in the first aspect. Or since it is preferable to set it as a plane, the image side surface of the first lens L11 is a concave surface. As shown in FIG. 5, when the first lens L11 is a plano-concave lens having a concave surface on the image side, wide angle, long back focus, and low cost can be achieved.

  The objective lens of the second aspect is also advantageous in correcting chromatic aberration by including a cemented lens formed by bonding a positive lens and a negative lens or by bonding a negative lens and a positive lens. Further, by arranging the aperture stop St near the center of the lens system, the lens diameter can be reduced. By adopting the lens configuration of the second aspect, it is possible to realize a lens system in which various aberrations including chromatic aberration of magnification are satisfactorily corrected while ensuring a wide angle of view and a long back focus with a compact configuration. It becomes easy.

ただし、
Ｎθ_ｈｇ：接合レンズ中の負レンズのｈ線とｇ線間の部分分散比
Ｐθ_ｈｇ：接合レンズ中の正レンズのｈ線とｇ線間の部分分散比
Ｒ_Ｃ：接合レンズの接合面の曲率半径
Ｂｆ：全系のバックフォーカス
ｄ：最も像側のレンズの中心厚
ｎ：最も像側のレンズのｄ線における屈折率 When the objective lens adopts the four-group five-element configuration of the second aspect, it is preferable that the following conditional expression (3) is further satisfied.

However,
N.theta _hg: a partial dispersion ratio between the h-line and g-line of the negative lens in the cemented lens Pshita _hg: partial dispersion ratio R _C between h-line and g-line of the positive lens in the cemented lens of _curvature of the cemented surface of the cemented lens Radius Bf: Back focus of the entire system d: Center thickness of the lens closest to the image side n: Refractive index at the d-line of the lens closest to the image side

Conditional expression (3) is similar to the conditional expression (2) described above in terms of the partial dispersion ratio between the h-line and g-line of the positive lens and negative lens constituting the cemented lens on the image side from the aperture stop St and the cemented surface. Paying attention, it prescribes a suitable correction degree of lateral chromatic aberration in the short wavelength region. Conditional expression (3) can be modified as the following expression (3A).

  As can be seen from conditional expression (3A), the left side of conditional expression (3) is the sixth component comprising the ratio of the partial dispersion ratio of the negative lens and positive lens constituting the cemented lens and the absolute radius of curvature of the cemented surface. Focuses on the seventh component whose value is normalized by the focal length and the sum of the back focus of the entire system and the air-converted length on the optical axis of the lens closest to the image, that is, the distance from the junction surface to the imaging surface This can be divided into the eighth component normalized by the distance.

  These sixth to eighth components indicate three conditions that are advantageous for correcting lateral chromatic aberration in the short wavelength region of the cemented lens disposed on the image side from the aperture stop St. In the lens configuration of the second aspect, the correction of lateral chromatic aberration is more advantageous as the sixth component is larger, the seventh component is smaller, and the eighth component is smaller. By configuring so as to satisfy the conditional expression (3), it is possible to satisfactorily maintain lateral chromatic aberration in a wide wavelength range while maintaining a back focus having a practically sufficient length.

  The objective lens of the present embodiment described above can be applied as an objective lens for an endoscope, for example. For an objective lens with a large angle of view, such as an endoscope objective lens, or an objective lens with a large F-number to increase the depth of field, spherical aberration and coma are important factors that determine image quality. However, lateral chromatic aberration may be a major factor in image quality degradation. Since the lateral chromatic aberration appears more prominently toward the image peripheral portion, it is very effective to correct the lateral chromatic aberration well in order to improve the image quality of the image peripheral portion.

When the objective lens of the present embodiment is applied to an imaging device together with an imaging device arranged on the image plane of the objective lens, it is preferable that the following conditional expression (4) is satisfied.
| Δh−δC | <3 × P (4)
However,
δh: chromatic aberration of magnification of h-line with respect to d-line at maximum half angle of view (unit: μm)
δC: chromatic aberration of magnification of C line with respect to d line at maximum half angle of view (unit: μm)
P: Pitch of pixel array of the image sensor (unit: μm)

  Here, when the image sensor is rectangular, the half length of the diagonal line of the rectangle can be set as the maximum image height, and the field angle corresponding to the maximum image height can be determined as the maximum half field angle.

  By constructing the objective lens while paying attention to the pixel pitch of the image sensor so as to satisfy the conditional expression (4), it becomes possible to cope with an image sensor that has been increasing in the number of pixels in recent years, and the sharpness and resolution of the image. Can be improved. In addition, it is possible to reduce the burden of electrical correction that has been conventionally performed to improve image quality.

  When the objective lens is mounted on an imaging device such as an endoscope or in-vehicle camera without a protective member, the lens placed closest to the object should be exposed to body fluids, cleaning fluids, direct sunlight, wind and rain, oils and fats, etc. Become. Therefore, it is preferable to use a material having high water resistance, weather resistance, acid resistance, chemical resistance, etc., for example, weight loss rate rank of powder water resistance, powder acid resistance standard established by the Japan Optical Glass Industry Association. It is preferable to use one having a surface method weather resistance rank of 1.

  Next, numerical examples of the objective lens of the present invention will be described. The lens sectional views of the objective lenses of Examples 1 to 5 are shown in FIGS. Examples 1 to 4 have a configuration of 6 elements in 4 groups, and Example 5 has a configuration of 5 elements in 4 groups.

  Tables 1 to 5 show lens data of the objective lenses of Examples 1 to 5, respectively. In the lens data table of each example, the Si column indicates the i-th (i = 1, 2, 3,...) Surface number that sequentially increases toward the image side with the most object-side component surface as the first surface. The Ri column indicates the radius of curvature of the i-th surface, the Di column indicates the surface spacing on the optical axis Z between the i-th surface and the i + 1-th surface, and the Ndj column is the most object side. The refractive index with respect to the d-line of the j-th (j = 1, 2, 3,...) Optical element that sequentially increases toward the image side with the optical element as the first is shown, and the column of νdj indicates the d of the j-th optical element. The Abbe number with respect to the line is shown, and the columns of θh and gj show the partial dispersion ratio between the h-line and the g-line of the j-th optical element.

  The lens data includes the aperture stop St and the optical member PP, and the term “aperture stop” is also described in the surface number column of the surface corresponding to the aperture stop St. In addition, the focal length and the F value are indicated by f, Fno. It is described as.

  The sign of the radius of curvature is positive when convex on the object side and negative when convex on the image side. “Mm” is used as the unit of the radius of curvature and the surface interval in the lens data, but this is an example, and the optical system can obtain the same optical performance even when proportionally enlarged or reduced. Appropriate units can also be used. In addition, the numerical value of the table | surface described in this specification is rounded by the predetermined digit.

  FIGS. 7A to 7D show aberration diagrams of the spherical aberration, astigmatism, distortion (distortion), and lateral chromatic aberration (chromatic chromatic aberration) of the objective lens of Example 1, respectively. In the respective aberration diagrams of spherical aberration, astigmatism, and distortion, the aberration for the d-line is shown. In the astigmatism diagram, sagittal aberration is indicated by a solid line, and tangential aberration is indicated by a dotted line. In the lateral chromatic aberration diagram, aberrations for the h-line, g-line, F-line, and C-line with respect to the d-line are denoted by broken lines with symbols h, g, F, and C, respectively. Fno. Of spherical aberration diagram. Means F value, and ω in other aberration diagrams means half angle of view.

  Similarly, FIG. 8 (A) to FIG. 8 (D), FIG. 9 (A) to FIG. 9 (D), FIG. 10 (A) to FIG. 10 (D), FIG. 11 (A) to FIG. FIG. 5 shows aberration diagrams of the spherical aberration, astigmatism, distortion (distortion), and lateral chromatic aberration (chromatic chromatic aberration) of the objective lenses of Examples 2 to 5. As can be seen from the respective aberration diagrams, in the first to fifth embodiments, each aberration is corrected well.

  Next, an objective lens of a comparative example will be described. The objective lenses of Comparative Examples 1 to 5 do not satisfy the conditional expression (1) described above. Sectional views of the objective lenses of Comparative Examples 1 to 5 are shown in FIGS. The method shown in FIGS. 12 to 16 is basically the same as that shown in FIGS. 2 to 6, and the axial light beam 2, the off-axis light beam 3 corresponding to the maximum half angle of view, and the optical member PP are also shown. .

  Comparative Examples 1 to 3 have 4 groups and 6 sheets, and Comparative Examples 4 and 5 have 4 groups and 5 sheets. The objective lens of Comparative Example 1 includes, in order from the object side, a first cemented lens formed by bonding a negative meniscus first lens L21 ′, a positive second lens L22 ′, and a negative third lens L23 ′. The aperture stop St, the positive fourth lens L24 ′, and the second cemented lens formed by bonding the positive fifth lens L25 ′ and the negative sixth lens L26 ′ are arranged.

  The objective lens of Comparative Example 2 includes, in order from the object side, a first cemented lens formed by bonding a first lens L21 ′ that is a plano-concave lens, a negative second lens L22 ′, and a positive third lens L23 ′. The aperture stop St, the positive fourth lens L24 ′, and the second cemented lens formed by bonding the positive fifth lens L25 ′ and the negative sixth lens L26 ′ are arranged. The objective lens of Comparative Example 3 includes, in order from the object side, a first cemented lens formed by bonding a first lens L21 ′ that is a plano-concave lens, a negative second lens L22 ′, and a positive third lens L23 ′. The aperture stop St, the positive fourth lens L24 ′, and the second cemented lens formed by bonding the negative fifth lens L25 ′ and the positive sixth lens L26 ′ are arranged.

The objective lens of Comparative Example 4 includes, in order from the object side, a negative meniscus first lens L11 ′, a positive second lens L12 ′, an aperture stop St, a positive third lens L13 ′, and a positive first lens L13 ′. A cemented lens formed by bonding the four lenses L14 ′ and the negative fifth lens L15 ′ is arranged. The objective lens of Comparative Example 5 includes, in order from the object side, a negative meniscus first lens L11 ′, a positive second lens L12 ′, an aperture stop St, a positive third lens L13 ′, and a negative first lens L11 ′. A cemented lens formed by bonding the four lenses L14 ′ and the positive fifth lens L15 ′ is arranged.

  Lens data of the objective lenses of Comparative Examples 1 to 5 are shown in Tables 6 to 10, respectively. The meanings of symbols in Tables 6 to 10 are the same as those in the lens data of Examples 1 to 5 described above.

  17 (A) to 17 (D), 18 (A) to 18 (D), 19 (A) to 19 (D), 20 (A) to 20 (D), and 21. FIGS. 21A to 21D show aberration diagrams of the spherical aberration, astigmatism, distortion (distortion), and lateral chromatic aberration (lateral chromatic aberration) of the objective lenses of Comparative Examples 1 to 5, respectively. The method of displaying the aberration diagrams of Comparative Examples 1 to 5 is the same as that of the aberration diagrams of Examples 1 to 5 described above.

  Table 11 shows the amount of chromatic aberration of magnification (unit: μm) and focal length (unit: mm) for each wavelength on the d-line basis of Examples 1 to 5 and Comparative Examples 1 to 5. Table 12 shows the amount of chromatic aberration of magnification (unit: μm) for each wavelength on the d-line basis of Examples 1 to 5 and Comparative Examples 1 to 5 as the focal length (unit: mm) of each objective lens. The normalized value and the value of (δh−δC) / f are shown. The absolute value of the value of (δh−δC) / f corresponds to the left side of conditional expression (1). In both Table 11 and Table 12, the corresponding bright line spectrum name is described above each wavelength.

  FIG. 22 is a graph showing the numerical values of Examples 1 to 5 and Comparative Examples 1 to 5 in Table 12. The horizontal axis in FIG. 22 is the wavelength, and the vertical axis is a value obtained by normalizing the amount of chromatic aberration of magnification (unit: μm) for each wavelength on the d-line basis with the focal length (unit: mm) of each objective lens. From FIG. 22, it can be seen that, in a wide wavelength range of about 400 nm to 650 nm, Examples 1 to 5 have small lateral chromatic aberration, whereas Comparative Examples 1 to 5 have a large size. Comparative Examples 1 to 3 are overcorrected, Comparative Example 4 is slightly undercorrected, and Comparative Example 5 is undercorrected.

  Tables 13 and 14 show various data of Examples 1 to 5 and Comparative Examples 1 to 5, respectively. The data in Tables 13 and 14 are based on the d-line, all length units are mm, and angle units are degrees.

  The terms described in Table 13 and Table 14 will be described. The “number of components” column shows the lens groups and the number of lenses constituting the entire system. The “front group joint” column shows the power sign and arrangement order of the two lenses constituting the cemented lens on the object side from the aperture stop St, and the “rear group joint” column shows the image side from the aperture stop St. The sign of the power of the two lenses constituting the cemented lens and the order of arrangement are shown. For example, “convex / concave cementing” means a cemented lens in which a positive lens and a negative lens are arrayed in order from the object side. “Joint” means a cemented lens in which a negative lens and a positive lens are arranged in order from the object side.

  In the “back focus” column, the air-converted length on the optical axis from the image side surface of the most image side lens to the image surface is described. The distance in the optical axis direction from the lens surface closest to the object to the object in the “Object distance” column, the radius of curvature of the object surface in the “Object surface curvature radius” column, and the maximum in the “Maximum image height” column In the column “Maximum half angle of view”, the maximum half angle of view is shown, and all values are based on design specifications.

  In the columns of “conditional expression (1) corresponding value”, Fθ, Rθ, “conditional expression (2) corresponding value”, and “conditional expression (3) corresponding value”, the left side of the conditional expression (1), Fθ, Rθ, respectively. , Values corresponding to the left side of conditional expression (2) and the left side of conditional expression (3) are shown.

  The objective lens concerning embodiment of this invention mentioned above can be used conveniently as an objective lens for endoscopes. Next, an endoscope that is an example of an imaging apparatus on which the objective lens according to the embodiment of the present invention is mounted will be described with reference to FIGS. 23 and 24. FIG. 23 is a schematic configuration diagram of the endoscope, and FIG. 24 is a cross-sectional view of a main part of the distal end hard portion of the endoscope.

  The endoscope 100 shown in FIG. 23 mainly includes an operation unit 102, an insertion unit 104, and a connector unit (not shown) for pulling out the universal cord 106. An insertion portion 104 to be inserted into the patient's body is connected to the distal end side of the operation portion 102. From the proximal end side of the operation portion 102, a universal cord for connecting to a connector portion for connecting to a light source device or the like. 106 is pulled out.

  Most of the insertion portion 104 is a flexible portion 107 that bends in an arbitrary direction along the insertion path. The bending portion 108 is provided to direct the distal end hard portion 110 in a desired direction, and the bending operation can be performed by rotating the bending scanning knob 109 provided in the operation portion 102. A distal end hard portion 110 is connected to the distal end of the bending portion 108.

  As shown in FIG. 24, the objective lens 1 according to the present embodiment is disposed inside the distal end hard portion 110. 24 is a cross-sectional view including the optical axis Z of the objective lens 1, and the objective lens 1 shown in FIG. 24 is conceptually illustrated, not a lens shape. . An optical path conversion prism 5 for bending the optical path by 90 degrees is disposed on the image side of the objective lens 1, and an image sensor 10 is bonded to the image side surface of the optical path conversion prism 5. The imaging surface of the imaging device 10 is disposed so as to coincide with the image surface of the objective lens 1, and the objective lens 1 and the imaging device 10 are configured to satisfy the conditional expression (4) described above. In the configuration shown in FIG. 24, the imaging device 10 is arranged by bending the optical path using the optical path conversion prism 5, thereby configuring a direct-view type observation optical system in the lower half of the distal end hard portion 110. A treatment instrument insertion channel 11 is formed in the upper half, and a large number of elements are arranged in a small-diameter insertion portion.

  The present invention has been described with reference to the embodiments and examples. However, the present invention is not limited to the above embodiments and examples, and various modifications can be made. For example, the values of the radius of curvature, the surface spacing, the refractive index, the Abbe number, etc. of each lens component are not limited to the values shown in the above numerical examples, but can take other values.

  For example, in the above conditional expressions (2) and (3), the partial dispersion ratio between the h-line and the g-line is used. You may comprise so that a predetermined | prescribed limit value may be satisfied using the partial dispersion ratio between arbitrary two wavelengths. When correction is required in the short wavelength region, a partial dispersion ratio between any two emission lines in the wavelength region shorter than the d line may be used, and conversely, correction is required in the long wavelength region. In some cases, a partial dispersion ratio between any two emission lines in a wavelength region longer than the d line may be used.

DESCRIPTION OF SYMBOLS 1 Objective lens 2 On-axis light beam 3 Off-axis light beam 5 Optical path conversion prism 10 Image pick-up element 100 Endoscope 102 Operation part 104 Insertion part 106 Universal code 107 Soft part 108 Bending part 109 Curved scanning knob 110 Tip hard part L11, L21 1st Lens L12, L22 Second lens L13, L23 Third lens L14, L24 Fourth lens L15, L25 Fifth lens L26 Sixth lens P Imaging position PP Optical member St Aperture stop Z Optical axis

Claims (7)

An endoscope objective lens characterized by satisfying the following conditional expression (1).
| Δh−δC | / f <2 (1)
However,
δh: chromatic aberration of magnification of h-line with respect to d-line at maximum half angle of view (unit: μm)
δC: chromatic aberration of magnification of C line with respect to d line at maximum half angle of view (unit: μm)
f: Focal length of the entire system (unit: mm)   The endoscope objective lens includes, in order from the object side, a plano-concave lens having a flat surface on the object side or a negative meniscus lens having a convex surface on the object side, and bonding of the positive lens and the negative lens or bonding of the negative lens and the positive lens. The first cemented lens composed of a combination, a stop, a positive lens having a convex surface on the image side, and a positive lens and a negative lens bonded together or a negative lens and a positive lens bonded together have a positive refractive power as a whole. The objective lens for an endoscope according to claim 1, wherein the second objective lens has a four-group, six-lens configuration in which the second cemented lenses are arranged. The endoscope objective lens according to claim 2, wherein the following conditional expression (2) is satisfied.

However,
Nθ _1hg : Partial dispersion ratio between h-line and g-line of the negative lens in the first cemented lens _{Pθ 1hg} : Partial dispersion ratio between h-line and g-line of the positive lens in the first cemented lens R _1C : radius of curvature Nθ _2hg of the cemented surface of the first cemented lens: partial dispersion ratio _{Pθ 2hg} between the h-line and g-line of the negative lens in the second cemented lens: in the second cemented lens Partial dispersion ratio R _2C between the h-line and g-line of the positive lens: curvature radius Bf of the cemented surface of the second cemented lens B: total back focus d: center thickness n of most image side lens: most Refractive index at d-line of image side lens   The endoscope objective lens includes, in order from the object side, a negative lens having a concave surface on the image side, a positive lens having a convex surface on the object side, a stop, a positive lens having a convex surface on the image side, a positive lens, and 2. The internal view according to claim 1, wherein the endoscope has a four-group five-lens configuration in which a cemented lens having a positive refractive power as a whole is formed by bonding a negative lens or a negative lens and a positive lens. Mirror objective lens. The endoscope objective lens according to claim 4, wherein the following conditional expression (3) is satisfied.

However,
N.theta _hg: the partial dispersion ratio between the negative lens of the h-line and g-line in the cemented lens Pshita _hg: partial dispersion ratio R _C between h-line and g-line of the positive lens in the cemented _lens: the cemented lens Radius of curvature Bf of the entire lens system: Back focus of the entire system d: Center thickness of the lens closest to the image side n: Refractive index at the d-line of the lens closest to the image side   An imaging apparatus comprising the endoscope objective lens according to claim 1. An image pickup device disposed on the image plane of the endoscope objective lens;
The imaging apparatus according to claim 6, wherein the following conditional expression (4) is satisfied.
| Δh−δC | <3 × P (4)
However,
P: Pitch of pixel array of the image sensor (unit: μm)

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