JP3441567B2 - Endoscope objective lens - Google Patents

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 A small-diameter endoscope that is inserted into a narrow body cavity such as the bronchus, biliary tract, or pancreatic duct for observation and treatment has been put into practical use, but it is necessary to observe a thinner lumen. An endoscope objective lens having a smaller outer diameter is required for the purpose of reducing the pain of the patient.

As a small-diameter endoscope objective lens, for example, a first lens having a negative power in order from the object side, as in Japanese Patent Application Laid-Open No. 7-84179 filed by the present applicant,
It is known that the aperture stop, a second lens having a positive power, and a third lens having a positive power have a single-lens structure of three lenses. This type has the advantage that the F-number is small and the depth of focus is deep, but on the other hand, field curvature, lateral chromatic aberration, etc. are relatively large, and resolution was not sufficient.

As one having the same three-sheet structure, Japanese Unexamined Patent Publication No.
No. 107470, a single lens composed of a first lens having negative power in order from the object side, a second lens having positive power, an aperture stop, and a third lens having positive power. There are three. However, although this type can correct lateral chromatic aberration, it has large field curvature and astigmatism.

On the other hand, according to the type using a cemented lens for the positive lens of the rear group as in Japanese Patent Publication No. 60-4963, various aberrations such as field curvature and lateral chromatic aberration are well corrected. However, in the cemented lens, the thickness of the peripheral portion of the positive lens (edge thickness) and the center thickness of the negative lens must be kept to be a predetermined thickness, so that the problem that the total length becomes long is avoided. Absent. If the total length of the objective lens becomes long, the distal end portion of the endoscope also becomes long, which deteriorates the bending operability and is not preferable. Also, the cemented lens is
In addition to the problem that the number of processes such as centering and bonding of the lens newly increases during processing and manufacturing, there is also the problem that it is difficult to determine the lens orientation because the curvature of both sides of the biconvex lens that composes the cemented lens is almost the same. Can happen.

Japanese Unexamined Patent Application Publication No. HEI 2-
Like No. 176611, the first lens having negative power in order from the object side, the second lens having positive power, the aperture stop, the third lens having positive power, and the positive power. A lens including a fourth lens is also known. However, in Japanese Patent Laid-Open No. 2-176611, LD
/ F = about 5 to 6 (LD; total lens length, f: focal length), and there is a problem that the total lens length is long. Further, since the object-side surface of the third lens is substantially flat or convex, coma and astigmatism are largely generated. In addition, since the positive power of the third lens is strong, large spherical aberration occurs on the second surface thereof. In other words, the fourth lens works only for making a telecentric optical system, and is not very useful for aberration correction.

[0007]

SUMMARY OF THE INVENTION The present invention is based on the above-mentioned awareness, and in an endoscope objective lens having a four-element structure, various aberrations, particularly chromatic aberration of magnification, are well corrected without using a cemented lens, and the total lens length is improved. An object is to provide an endoscope objective lens that is short and has high optical performance.

[0008]

SUMMARY OF THE INVENTION The present invention is a negative, positive, positive, positive single lens 4
In an endoscope objective lens consisting of a single piece, if a convex meniscus lens is arranged as the third lens on the image side and a convex meniscus lens is arranged on the object side as the fourth lens, the total lens length is shortened, and field curvature and The present invention has been completed by finding that it is possible to obtain a lens system in which lateral chromatic aberration is corrected well. That is, the present invention is, in order from the object side, a first lens having a negative refractive power, a second lens having a positive refractive power, an aperture stop, a third lens having a positive refractive power, and a positive lens. In an objective lens system of an endoscope composed of four single lenses of a fourth lens having a refractive power of, the third lens is composed of a meniscus lens convex to the image side, and the fourth lens is convex to the object side. It consists of a meniscus lens ,
Furthermore, it is necessary to satisfy the following conditional expressions (1) and (2).
It has a feature. (1) 0 <f / r ₈ <0.5 (2) -1.2 <f / r ₅ <-0.1 where f is the focal length of the entire objective lens system, and r _{8 is} the fourth lens of the fourth lens. 2 is the radius of curvature of the surface, r _{5 is the} radius of curvature of the first surface of the third lens .

In the endoscope objective lens of the present invention, if at least one of the four lenses forming the objective lens is made of an aspherical surface, good aberration correction can be performed.

[0010]

In order to obtain better performance, it is desirable to satisfy the following conditional expressions (3) to (6). (3) 0.1 <| f / f ₁ | <1.0 (4) ν ₂ <45 (5) n ₃ > 1.70 (6) n ₄ > 1.65 where f ₁ ; first lens , Ν ₂ ; Abbe number of the second lens, n ₃ ; refractive index of the third lens, n ₄ ; refractive index of the fourth lens.

Further, in order to further reduce the chromatic aberration of magnification,
It is desirable to satisfy the following conditional expressions (7) and (8). (7) ν ₃ > 40 (8) ν ₄ > 40 where ν _{3 is} the Abbe number of the third lens, and ν ₄ is the Abbe number of the fourth lens.

[0013]

DETAILED DESCRIPTION OF THE INVENTION Negative, positive, diaphragm, positive, positive single lens 4
When a positive meniscus lens having a convex surface on the object side is used as the fourth lens in the endoscope objective lens composed of a single sheet, the negative power of the first lens becomes small due to the power distribution, and in addition, the final surface Is a concave surface, the total lens length (distance from the first surface of the first lens to the image surface) can be shortened. Further, since the final surface is concave, various aberrations can be reduced.

Further, the second lens disposed between the first lens and the aperture stop is a high-dispersion positive lens on the object side in order to cancel the chromatic aberration of magnification generated by the first, third and fourth lenses. It is preferable to have a strong convex surface. Further, since the positive lenses, the second lens and the third lens, are arranged in a substantially symmetrical system with the aperture stop interposed therebetween, the occurrence of coma and distortion is reduced.

Conditional expression (1) is a condition relating to the curvature of the second surface (final surface) of the fourth lens. If the curvature of the final surface is increased beyond the upper limit of this conditional expression, the angle of the principal ray of the off-axis light flux emitted from the final lens becomes large, and in the case of a fiberscope, light amount loss is caused, which is not preferable. If the curvature becomes weaker than the lower limit, coma, astigmatism, etc. will increase.

Conditional expression (2) is a condition relating to the curvature of the first surface of the third lens. When the curvature becomes strong beyond the lower limit of the conditional expression (2), spherical aberration and field curvature largely become under. If the spherical aberration becomes large, the resolution will be deteriorated, which is not preferable. If the curvature exceeds the upper limit and the curvature becomes weak, coma, astigmatism, and the like largely occur, and correction cannot be performed. Further, the first surface of the third lens has a negative value, that is, a concave surface, as shown in this conditional expression. When the first surface of the third lens is concave, coma and astigmatism can be corrected well.

Conditional expression (3) represents the range of the power of the first lens. If the power is reduced below the lower limit, the required angle of view cannot be obtained. In addition, the field curvature and the axial chromatic aberration are undercorrected. When the power exceeds the upper limit and becomes large, spherical aberration and field curvature are overcorrected. In addition, the back focus becomes long, which is not preferable.

Conditional expression (4) is a condition regarding the Abbe number of the second lens. When a glass material having a large Abbe number is used for the second lens having a positive refractive power, axial chromatic aberration increases but lateral chromatic aberration decreases. Therefore, by appropriately selecting the glass material of the second lens, it is possible to achieve an appropriate balance between the axial chromatic aberration and the lateral chromatic aberration. Conditional expression (5)
If the Abbe's number becomes larger than the upper limit of, the correction of lateral chromatic aberration, which is one of the effects of the endoscope objective lens of the present invention, becomes insufficient.

Conditional expressions (5) and (6) are conditions relating to the refractive indices of the third lens and the fourth lens. When the lower limit of conditional expression (5) is exceeded and the refractive index of the third lens having a positive refractive power becomes small, field curvature becomes large. If the lower limit of conditional expression (6) is exceeded and the refractive index of the positive fourth lens decreases, then the field curvature similarly increases.

Conditional expressions (7) and (8) are conditions concerning the Abbe numbers of the third lens and the fourth lens. If the lower limits of these conditional expressions are exceeded, lateral chromatic aberration cannot be corrected well.

The present invention will be described with reference to specific examples. Examples 1 to 6 are examples in which four constituent lenses do not have an aspherical surface, and Examples 7 and 8 are examples in which an aspherical surface is included. [Embodiment 1] FIGS. 1 and 2 show a first embodiment of the endoscope objective lens of the present invention. FIG. 1 is a lens configuration diagram of this embodiment. In order from the object side, a negative first lens 11, a positive second lens 12, a diaphragm S, a positive third lens 13, and a positive fourth lens 14 are arranged in this order. The third lens 13 is a meniscus lens having a convex surface on the image side, and the fourth lens 14 is a meniscus lens having a convex surface on the object side. FIG. 2 is a diagram showing various aberrations of this lens system.

Table 1 shows specific numerical data of this lens system. In the following tables and drawings, FE is the effective F value at the reference object distance (distance from the object to the first surface of the first lens) 5 mm, F is the focal length of the entire objective lens system, and M is the paraxial lateral magnification. , W is the half angle of view at the reference object distance, FB is the back focus, r is the radius of curvature of each lens surface, d is the lens thickness or lens spacing, N is the refractive index for the d line, and ν is the Abbe number for the d line. Show. SA is spherical aberration, SC is sine condition, d-line,
The g-line and the C-line represent chromatic aberration indicated by spherical aberration, S represents sagittal, and M represents meridional at each wavelength.

[0023]

[Table 1] FE = 1: 2.3 f = 0.50 M = -0.096 W = 59.7 ° FB = 0.36 No. rd N ν 1 ∞ 0.30 1.51633 64.1 2 0.441 0.08-- 3 0.666 0.30 1.92286 21.3 4 ∞ 0.03-- Aperture ∞ 0.03-- 5 -0.691 0.40 1.88300 40.8 6 -0.537 0.05-- 7 0.870 0.46 1.88 300 40.8 8 2.276---

[Second Embodiment] FIGS. 3 and 4 show a second embodiment of the endoscope objective lens according to the present invention. Table 2 shows lens data, and FIG. 3 is a lens configuration diagram. The basic lens configuration is the same as that of the first embodiment. FIG. 4 is a diagram showing various aberrations.

[Table 2] FE = 1: 2.3 f = 0.56 M = -0.110 W = 49.8 FB = 0.45 No. rd N ν 1 ∞ 0.30 1.88300 40.8 2 0.518 0.08-- 3 0.489 0.30 1.92286 21.3 4 ∞ 0.03-- Aperture ∞ 0.03-- 5 -0.634 0.40 1.88300 40.8 6 -0.564 0.05-- 7 0.893 0.46 1.72916 54.7 8 3.017---

[Embodiment 3] FIGS. 5 and 6 show a third embodiment of the endoscope objective lens according to the present invention. Table 3 shows lens data, and FIG. 5 shows a lens configuration diagram. The basic lens configuration is the same as that of the first embodiment. FIG. 6 is a diagram showing various aberrations.

[Table 3] F _E = 1: 2.2 f = 0.48 M = -0.093 W = 60.6 ° FB = 0.33 No. rd N ν 1 ∞ 0.30 1.51633 64.1 2 0.514 0.10--3 0.694 0.30 1.92286 21.3 4 ∞ 0.03--Aperture ∞ 0.03--5 -0.541 0.35 1.69680 55.5 6 -0.404 0.05--7 0.806 0.41 1.88300 40.8 8 1.502-- -

[Embodiment 4] FIGS. 7 and 8 show a fourth embodiment of the endoscope objective lens according to the present invention. Table 4 shows lens data, and FIG. 7 shows a lens configuration diagram, and the basic lens configuration is the same as that of the first embodiment. FIG. 8 is a diagram showing various aberrations.

[Table 4] FE = 1: 2.2 f = 0.49 M = -0.093 W = 60.2 ° FB = 0.35 No. rd N ν 1 ∞ 0.30 1.51633 64.1 2 1.500 0.17-- 3 1.159 0.30 1.92286 21.3 4 ∞ 0.03-- Aperture ∞ 0.03-- 5 -0.608 0.35 1.88300 40.8 6 -0.456 0.05-- 7 0.766 0.32 1.88 300 40.8 8 1.266---

[Fifth Embodiment] FIGS. 9 and 10 show a fifth embodiment of the endoscope objective lens according to the present invention. Table 5 shows the lens data, and FIG. 9 shows the lens configuration diagram.
This is the same as in the first embodiment. FIG. 10 is a diagram showing various aberrations.

[Table 5] FE = 1: 2.2 f = 0.49 M = -0.093 W = 60.2 ° FB = 0.34 No. rd N ν 1 ∞ 0.30 1.51633 64.1 2 1.830 0.19-- 3 1.245 0.30 1.92286 21.3 4 ∞ 0.03-- Aperture ∞ 0.03-- 5 -0.610 0.35 1.88300 40.8 6 -0.459 0.05-- 7 0.769 0.34 1.88 300 40.8 8 1.266---

[Sixth Embodiment] FIGS. 11 and 12 show a sixth embodiment of the endoscope objective lens according to the present invention. Table 6 shows lens data, and FIG. 11 is a lens configuration diagram, and the basic lens configuration is the same as that of the first embodiment. FIG. 12 is a diagram showing various aberrations.

[Table 6] FE = 1: 2.2 f = 0.48 M = -0.091 W = 60.2 ° FB = 0.28 No. rd N ν 1 ∞ 0.30 1.51633 64.1 2 0.441 0.13-- 3 0.533 0.30 1.88 300 40.8 4 ∞ 0.03-- Aperture ∞ 0.03-- 5 -0.624 0.40 1.88300 40.8 6 -0.489 0.05-- 7 0.722 0.40 1.69680 55.5 8 1.120---

[Embodiment 7] FIGS. 13 and 14 show a seventh embodiment of the endoscope objective lens according to the present invention. Table 7 shows lens data, and FIG. 13 is a lens configuration diagram. The basic lens configuration is the same as in Example 1, but the second surface (final surface) of the fourth lens is a rotationally symmetric aspherical surface. FIG. 14 is a diagram showing various aberrations.

[Table 7] FE = 1: 2.2 f = 0.44 M = -0.081 W = 60.0 ° FB = 0.32 No. rd N ν 1 ∞ 0.45 1.51633 64.1 2 0.602 0.22--3 0.715 0.30 1.92286 21.3 4 ∞ 0.03--Aperture ∞ 0.03--5 -0.642 0.36 1.88300 40.8 6 -0.452 0.05--7 0.961 0.34 1.88300 40.8 8 * 1.943- --* Is a rotationally symmetric aspherical surface. However, the rotationally symmetric aspherical surface is defined by the following equation. x = Ch ² / {1+ [1- (1 + K) C ² h ² ] ^1/2 } + A4h ⁴ + A6h ⁶ + A8h ⁸ + ... (C is curvature (1 / r), h is (Height from the optical axis, K is the conic coefficient) Aspherical data; No.8: K = -0.84661 × 10, A4 = 0.40136, A6 = -0.32335 × 10, A8 = 0.39710 × 10 ² , A10 = -0.11827 × 10 ³

[Embodiment 8] FIGS. 15 and 16 show the eighth embodiment of the endoscope objective lens according to the present invention. Table 8 is the lens data, and FIG. 15 is a lens configuration diagram. The basic lens configuration is the same as that of the example 1, but the first surface of the first lens is a rotationally symmetric aspherical surface. FIG. 16 is a diagram showing various aberrations.

[Table 8] FE = 1: 2.2 f = 0.42 M = -0.078 W = 59.9 ° FB = 0.30 No. rd N ν 1 * ∞ 0.40 1.51633 64.1 2 0.291 0.08--3 0.567 0.30 1.92286 21.3 4 ∞ 0.03--Aperture ∞ 0.03--5 -0.921 0.35 1.88300 40.8 6 -0.452 0.05--7 1.126 0.44 1.88300 40.8 8 4.545- --* Is a rotationally symmetric aspherical surface. Aspherical data; No.1: K = 0, A4 = 0.26540, A6 = -0.23453, A8 = -0.32681, A10
= 0.73374

Table 9 shows the values corresponding to the conditional expressions of Examples 1 to 8.

[Table 9]             Conditional expression (1) Conditional expression (2) Conditional expression (3)   Example 1 0.220 -0.724 0.586         2 0.186 -0.883 0.959         3 0.320 -0.887 0.486         4 0.387 -0.806 0.168         5 0.387 -0.803 0.138         6 0.429 -0.769 0.559         7 0.227 -0.685 0.375         8 0.092 -0.456 0.738             Conditional expression (4) Conditional expression (5) Conditional expression (6)   Example 1 21.3 1.883 1.883         2 21.3 1.883 1.729         3 21.3 1.729 1.883         4 21.3 1.883 1.883         5 21.3 1.883 1.883         6 40.8 1.883 1.729         7 21.3 1.883 1.883         8 21.3 1.883 1.883

As is clear from Table 9, the numerical values of Examples 1 to 8 all satisfy the conditional expressions (1) to (6). Further, in Examples 7 and 8 in which the aspherical surface is used, the distortion aberration is corrected particularly well. Further, the total lens length LD of each example was LD / f = 3 to 4, which was shortened to about 60% as compared with JP-A-2-176611.

[0033]

As described above, according to the endoscope objective lens of the present invention, various aberrations are satisfactorily corrected and the total lens length is shortened by the construction of four single lenses without using a cemented lens. , A lens with high optical performance can be obtained.

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

FIG. 15 is a lens configuration diagram showing an eighth embodiment of the endoscope objective lens according to the present invention.

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

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Claims (3)

(57) [Claims] 1. A first lens having a negative refractive power, a second lens having a positive refractive power, an aperture stop, a third lens having a positive refractive power, and a positive lens in order from the object side. In an endoscope objective lens composed of four single lenses of a fourth lens having a refractive power, the third lens is a meniscus lens having a convex surface on the image side, and the fourth lens is a meniscus lens having a convex surface on the object side. And satisfying the following conditional expressions (1) and (2):
Endoscope objective lens. (1) 0 <f / r ₈ <0.5 (2) -1.2 <f / r ₅ <-0.1 where r _{8 is the} radius of curvature of the second surface of the fourth lens, and f is the objective lens. The focal length of the entire system, r ₅ : the radius of curvature of the first surface of the third lens. 2. The endoscope objective lens according to claim 1.
And an objective lens for an endoscope that further satisfies the following conditional expressions (3) to (6). (3) 0.1 <| f / f ₁ | <1.0 (4) ν ₂ <45 (5) n ₃ > 1.70 (6) n ₄ > 1.65 where f ₁ ; first lens , Ν ₂ ; Abbe number of second lens, n ₃ ; refractive index of third lens, n ₄ ; refractive index of fourth lens. 3. The endoscope objective lens according to claim 2.
And an objective lens for an endoscope that further satisfies the following conditional expressions (7) and (8). (7) ν ₃ > 40 (8) ν ₄ > 40 However, ν ₃ ; Abbe number of the third lens, ν ₄ ; Abbe number of the fourth lens.