JPH0792385A - Objective lens for endoscope - Google Patents

Abstract

PURPOSE:To provide a lens having excellent performance and whose magnification and color aberration are compensated without increasing the number of constitutional lenses. CONSTITUTION:The objective lens is constituted of three lens groups; a 1st lens group constituted of one negative lens, a 2nd lens group constituted of one positive lens and a 3rd lens group constituted of one positive lens and one negative lens, and at least one of the positive lens as the 2nd lens and the positive lens in the 3rd group is provided with Abbe number >=57.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an objective lens for medical and industrial endoscopes.

[0002]

2. Description of the Related Art As an endoscope objective lens having a three-group structure of a negative lens, a positive lens, and a positive / negative cemented lens, Japanese Patent Application Laid-Open No. 63-281112 filed by the present applicant.
There is an issue. Although this lens has a three-group configuration, it has large chromatic aberration of magnification. For each of the embodiments shown in the JP-63-281112, when calculating the ratio of the maximum amount of lateral chromatic aberration (T _M) and focal length _{(f), 0.008 <T M} / f
It becomes <0.017. For example, assuming an objective lens having a focal length of 1 mm, the amount of lateral chromatic aberration T _M is 8 to 17 μm, which is very large, which is about the same as or larger than the individual fiber diameter of the medical fiberscope. This chromatic aberration of magnification is one of the major factors that cause a reduction in image contrast.

In order to improve this point, for example, JP-A-63-2811, as disclosed in JP-A-63-293525.
An attempt has also been made to divide the positive second lens in No. 12 into two pieces and correct lateral chromatic aberration at the boundary surface thereof. According to each of the conventional examples, 0.004 <T _M
/F<0.009, the chromatic aberration of magnification is about half that of JP-A-63-281112, and a clear improvement effect can be seen, but the cost is increased because the number of lens components is increased by one. It has the drawback of becoming expensive.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an endoscope objective lens with excellent performance in which chromatic aberration of magnification is corrected without increasing the number of lens components, that is, without incurring the drawback of high cost. .

[0005]

SUMMARY OF THE INVENTION An endoscope objective lens according to the present invention comprises a first lens group composed of one negative lens, a second lens group composed of one positive lens, and a lens composed of one positive lens and one negative lens. Three
It is composed of three lens groups of the lens group, and at least one of the positive lens of the second lens and the positive lens in the third group has an Abbe number of 57 or more.

The present invention preferably further satisfies the following conditional expression. _{(1) -1.5 <f 1 /f<-0.8} (2) -0.8 <R 4 /f<-0.4 (3) 0.05 <d 2 /f<0.3 However F _{1 is} the focal length of the first lens, f is the focal length of the entire system, R _{4 is} the radius of curvature of the image-side surface of the second lens, d ₂ is the distance between the first lens and the second lens.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is based on the following idea. In the conventional example, a glass material having a high refractive index is used for the positive lens in order to reduce the Petzval sum and correct the field curvature. However, a glass material with a high refractive index
Inevitably, the Abbe number is relatively small, that is, the dispersion is large, so that the chromatic aberration of magnification remains large. The present invention improves on this point and is characterized in that a glass material having an Abbe number of 57 or more and a small dispersion is used for at least one of the two positive lenses in the lens structure. As a result, the amount of residual chromatic aberration of magnification is first suppressed. In order to correct lateral chromatic aberration to a higher degree, it is more preferable to use a glass material having an Abbe number of 62 or more.

On the other hand, a glass material having a large Abbe number generally has a low refractive index, as is clear from a glass catalog supplied by a glass material manufacturer. Therefore, when such a glass material is used as a positive lens, the Petzval sum increases. To do.

Conditional expression (1) (-1.5 <f ₁ / f <-
0.8, f ₁ : focal length of the first lens, f: focal length of the entire system) are conditions for suppressing the increase of Petzval sum and keeping it small to correct the field curvature. By setting the negative power of the first lens to be relatively strong, a large amount of negative Petzval is generated, and it is possible to prevent the Petzval sum of the entire system from increasing positively. If the lower limit of conditional expression (1) is exceeded, the negative power of the first lens becomes too weak, and the above effect cannot be obtained. When the value exceeds the upper limit, the negative power becomes excessively strong, which causes an increase in spherical aberration and chromatic aberration.

Conditional expression (2) (-0.8 <R ₄ / f <-
0.4, R _4: the curvature of the image side surface of the second lens radius),
This is a necessary condition for the radius of curvature of the image-side surface of the second lens in order to keep the spherical aberration in the under state. In the lens of the present invention, the field curvature does not become completely zero even if the conditional expression (1) is satisfied. The on-axis and off-axis balance, i.e., the uniformity of image quality throughout the field of view, is determined by the amount of field curvature and spherical aberration. No matter how close the spherical aberration is to zero in the state where the field curvature remains to improve the axial performance, the balance of the entire visual field is lacking. In the present invention, since the positive lens is made of a glass material having a relatively low refractive index, the Petzval sum is large and the field curvature is likely to be large. Therefore, in order to balance the entire visual field, it is necessary to generate spherical aberration to some extent and appropriately maintain the under state. Conditional expression (2) is a condition for keeping the spherical aberration in the under state. If the lower limit of this conditional expression (2) is exceeded, the radius of curvature becomes too loose, and the effect of making the spherical aberration under is lost, and the spherical aberration becomes excessive in the entire lens, and the off-axis performance cannot be balanced. When the value exceeds the upper limit, the radius of curvature becomes conversely large and the spherical aberration becomes excessively under.

Conditional expression (3) (0.05 <d ₂ / f <0.
3, d ₂ : the distance between the first lens and the second lens) is required in addition to the conditional expression (2) for the distance between the first lens and the second lens in order to appropriately maintain the spherical aberration amount. . In the present invention, as described above, the negative power of the first lens is set to be strong in order to reduce the Petzval sum. Therefore,
In the first lens, various aberrations, especially spherical aberration and chromatic aberration, become excessive. These over-aberrations are canceled by the under-aberrations produced by the second lens having a positive power, but conditional expression (3) is required for the distance between these two lenses in order to cause appropriate cancellation. To be done. If the range of the conditional expression (3) is not satisfied, the appropriate aberrations cannot be canceled out as described above, spherical aberration, chromatic aberration, etc. remain, and the image quality is degraded. Further, the lower limit of the conditional expression (3) is necessary to secure the physical distance between the first lens and the second lens.

[Embodiment 1] FIG. 1 is a lens configuration diagram of a first embodiment of the present invention. The rightmost plane-parallel plate (surface Nos. 8 and 9) in this lens configuration diagram represents the cover glass and filters attached to the CCD camera used in the electronic endoscope. It should be noted that the presence or absence of this plane-parallel plate does not change the gist of the present invention. Table 1 shows specific numerical data of this lens system, and FIG. 2 shows various aberrations. In the various aberration diagrams, SA is spherical aberration, SC is sine condition, d line, g line,
The c-line indicates chromatic aberration and chromatic aberration of magnification indicated by spherical aberration, S indicates sagittal, and M indicates meridional at each wavelength.

In the table and drawings, F _NO is the F number, f is the focal length, ω is the half angle of view, f _B is the back focus, M is the lateral magnification, r _i is the radius of curvature of each lens surface, and d _i is the lens. The thickness or lens spacing, N is the refractive index, and ν is the Abbe number.

[0014]

[Table 1] F _NO = 1: 5.6 f = 1.44 ω = 56 ° f _B = 0.15 M = -0.096 surface NO rd N ν 1 ∞ 0.57 1.51633 64.1 2 0.855 0.13--3 -5.076 0.99 1.49700 81.6 4 -0.728 0.23 --5 2.601 0.30 1.84666 23.8 6 1.190 1.17 1.72916 54.7 7 ∞ 0.38--8 ∞ 1.50 1.51633 64.1 9 ∞---

[Embodiment 2] FIG. 3 is a lens configuration diagram of Embodiment 2 of the endoscope objective lens of the present invention. Table 2 shows specific numerical data of this lens system, and FIG. 4 shows various aberrations thereof.

[0016]

[Table 2] F _NO = 1: 5.6 f = 1.41 ω = 56 ° f _B = 0.12 M = -0.094 NO rd N ν 1 ∞ 0.57 1.51633 64.1 2 0.881 0.16--3 -5.670 1.06 1.56907 71.3 4 -0.824 0.19- -5 2.815 0.30 1.84666 23.8 6 1.250 1.06 1.72916 54.7 7 ∞ 0.46--8 ∞ 1.50 1.51633 64.1 9 ∞---

[Embodiment 3] FIG. 5 is a lens configuration diagram of Embodiment 3 of the endoscope objective lens of the present invention. Table 3 shows specific numerical data of this lens system, and FIG. 6 shows various aberrations thereof.

[0018]

[Table 3] F _NO = 1: 5.6 f = 1.41 ω = 56 ° f _B = 0.12 M = -0.094 surface NO rd N ν 1 ∞ 0.57 1.51633 64.1 2 0.858 0.16--3 -5.816 1.06 1.59240 68.3 4 -0.846 0.16 --5 2.851 0.30 1.84666 23.8 6 1.246 1.06 1.72916 54.7 7 ∞ 0.48--8 ∞ 1.50 1.51633 64.1 9 ∞---

[Fourth Embodiment] FIG. 7 is a lens configuration diagram of a fourth embodiment of the endoscope objective lens of the present invention. Table 4 shows specific numerical data of this lens system, and FIG. 8 shows various aberrations thereof.

[0020]

[Table 4] F _NO = 1: 5.6 f = 1.41 ω = 56 ° f _B = 0.12 M = -0.094 surface NO rd N ν 1 ∞ 0.57 1.51633 64.1 2 0.854 0.17--3 -6.289 1.06 1.61800 63.4 4 -0.878 0.16 --5 2.865 0.30 1.84666 23.8 6 1.243 1.06 1.72916 54.7 7 ∞ 0.47--8 ∞ 1.50 1.51633 64.1 9 ∞---

[Fifth Embodiment] FIG. 9 is a lens configuration diagram of a fifth embodiment of the endoscope objective lens of the present invention. Table 5 shows specific numerical data of this lens system, and various aberrations thereof are shown in FIG.
Shown in.

[0022]

[Table 5] F _NO = 1: 5.6 f = 1.42 ω = 56 ° f _B = 0.12 M = -0.094 plane NO rd N ν 1 ∞ 0.57 1.51633 64.1 2 0.873 0.18--3 -2.592 0.80 1.43875 95.0 4 -0.682 0.05 --5 2.430 0.30 1.84666 23.8 6 1.395 1.28 1.56907 71.3 7 -2.465 0.62--8 ∞ 1.50 1.51633 64.1 9 ∞---

[Sixth Embodiment] FIG. 11 is a lens configuration diagram of a sixth embodiment of the endoscope objective lens of the present invention. Table 6 shows specific numerical data of this lens system, and its various aberrations are shown in FIG.
2 shows.

[0024]

[Table 6] F _NO = 1: 5.6 f = 1.39 ω = 56 ° f _B = 0.01 M = -0.092 surface NO rd N ν 1 ∞ 0.57 1.51633 64.1 2 0.874 0.32--3 -3.860 0.83 1.56907 71.3 4 -0.797 0.17 --5 2.860 0.93 1.56907 71.3 6 -1.272 0.24 1.84666 23.8 7 -3.099 0.75--8 ∞ 1.50 1.51633 64.1 9 ∞---

[Seventh Embodiment] FIG. 13 is a lens configuration diagram of a seventh embodiment of the endoscope objective lens of the present invention. Specific numerical data of this lens system is shown in Table 7, and its various aberrations are shown in FIG.
4 shows.

[0026]

[Table 7] F _NO = 1: 5.6 f = 1.40 ω = 56 ° f _B = -0.01 M = -0.093 surface NO rd N ν 1 20.000 0.69 1.51633 64.1 2 0.811 0.16--3 -3.113 0.96 1.56907 71.3 4 -0.740 0.27--5 2.585 0.30 1.84666 23.8 6 1.202 1.07 1.72916 54.7 7 13.459 0.34--8 ∞ 1.50 1.51633 64.1 9 ∞---

Table 8 shows the values corresponding to the conditional expressions of Examples 1 to 7.

Table 8 Conditional Expression (1) Conditional Expression (2) Conditional Expression (3) Example 1 -1.171 -0.515 0.114 Example 2 -1.208 -0.582 0.113 Example 3 -1.176 -0.599 0.115 Example 4 -1.171 -0.621 0.118 Example 5 -1.188 -0.479 0.127 Example 6 -1.188 -0.560 0.223 Example 7 -1.183 -0.528 0.114

As is clear from Table 8, the numerical values of Examples 1 to 7 are all conditional expressions (1) to (3).
Are satisfied. Further, the endoscope objective lens of the present invention,
The chromatic aberration of magnification is small, and other aberrations are relatively well corrected.

[0029]

According to the present invention, it is possible to obtain an endoscope objective lens having a simple structure composed of three lens groups and having excellent chromatic aberration of magnification and excellent performance.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram showing a first embodiment of an endoscope objective lens according to the present invention.

FIG. 2 is a diagram of various types of aberration of the lens system in FIG.

FIG. 3 is a lens configuration diagram showing a second embodiment of the endoscope objective lens according to the present invention.

FIG. 4 is a diagram of various types of aberration of the lens system in FIG.

FIG. 5 is a lens configuration diagram showing a third embodiment of the endoscope objective lens according to the present invention.

FIG. 6 is a diagram of various types of aberration of the lens system in FIG.

FIG. 7 is a lens configuration diagram showing a fourth example of the endoscope objective lens according to the present invention.

FIG. 8 is a diagram of various types of aberration of the lens system in FIG.

FIG. 9 is a lens configuration diagram showing a fifth embodiment of the endoscope objective lens according to the present invention.

FIG. 10 is a diagram of various types of aberration of the lens system in FIG.

FIG. 11 is a lens configuration diagram showing a sixth embodiment of the endoscope objective lens according to the present invention.

12 is a diagram of various types of aberration in the lens system of FIG.

FIG. 13 is a lens configuration diagram showing a seventh embodiment of the endoscope objective lens according to the present invention.

14 is various aberrations of the lens system of FIG.

Claims (2)

[Claims] 1. A three-lens group consisting of a first lens group consisting of one negative lens, a second lens group consisting of one positive lens, and a third lens group consisting of one positive and one negative lens respectively. An objective lens for an endoscope, wherein at least one of the positive lens of the second lens and the positive lens in the third lens group has an Abbe number of 57 or more. 2. The endoscope objective lens according to claim 1, further satisfying the following conditional expressions (1) to (3). _{(1) -1.5 <f 1 /f<-0.8} (2) -0.8 <R 4 /f<-0.4 (3) 0.05 <d 2 /f<0.3 However F _{1 is} the focal length of the first lens, f is the focal length of the entire system, R _{4 is} the radius of curvature of the image-side surface of the second lens, d _{2 is} the distance between the first lens and the second lens.