JPH1010425A - Endoscope objective lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an endoscope objective lens capable of excellently correcting chromatic aberration of magnification without increasing an entire length. SOLUTION: A first lens group having negative power, a diaphragm, a second lens group having positive power, and a third lens group having positive power at least in the periphery are arrayed in order from an object side, and a surface being the closest to an image side is concave in the second lens group, and the second lens group satisfies the following conditions, that is 0.05<|f/f0|, and f0/0. Provided that (f) defines the focal distance of the entire system, and f0 defines the focal distance of an air lens formed between the surface on a side closest to the image in the second lens group and the surface on a side being the closest to an object in the third lens group.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an endoscope objective lens which is telecentric on the image side.

[0002]

2. Description of the Related Art Generally, an objective lens of an endoscope is arranged such that a principal ray is perpendicular to an end face of a fiber bundle as an image transmission system or a light receiving surface of a CCD sensor as an image pickup device. To ensure a wide angle of view on the side,
A negative lens is arranged in a front group located on the object side of the stop, and a positive lens is arranged in a rear group located on the image side of the stop, and is configured as a wide-angle, telecentric imaging lens system on the image side. In order to satisfy the condition of telecentricity on the image side, the lens closest to the image side needs to be a positive lens having the same diameter as the image plane and relatively large power.

As described above, since the imaging lens system having an asymmetric power distribution with respect to the diaphragm easily causes chromatic aberration of magnification, the positive lens in the rear group is configured as a cemented lens obtained by bonding a positive lens and a negative lens. Conventionally, a method of correcting lateral chromatic aberration by this method has been used.

For example, Japanese Patent Laid-Open No. 4-289811 discloses an endoscope having a five-lens configuration in which a single negative lens is arranged in a front group, a positive lens, a cemented lens, and a positive lens are arranged in a rear group from the stop side. A mirror objective is disclosed. In the endoscope objective lens disclosed in this publication, the occurrence of chromatic aberration of magnification is suppressed by reducing the radius of curvature of the bonding surface of the cemented lens.

[0005]

However, in the conventional endoscope objective lens described above, the radius of curvature of the cemented surface is set small in order to provide the cemented surface with a function of correcting lateral chromatic aberration. In order to secure the thickness (edge thickness) of the peripheral portion of the positive lens constituting the lens, there is a problem that the lens thickness increases and the overall length of the lens increases. Since the endoscope objective lens is provided on the distal end side of the bending portion of the endoscope, it is desirable that the total length of the objective lens be as short as possible in order to reduce the space required for bending.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to provide an endoscope objective lens capable of favorably correcting lateral chromatic aberration without increasing the overall length. I do.

[0007]

In order to achieve the above object, an endoscope objective lens according to the present invention comprises, in order from an object side, a first lens group having a negative power, a diaphragm,
A second lens group having a positive power and a third lens group having a positive power at least at a peripheral portion are arranged and arranged, and the second lens group has a concave surface on the most image side,
In addition, the following condition (1) is satisfied. 0.05 <| f / f0 |, f0 <0 (1) where f is the focal length of the entire system, and f0 is the most image-side surface of the second lens group and the most object-side surface of the third lens group. Is the focal length of the air lens formed between

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an endoscope objective lens according to the present invention will be described below. The endoscope objective lens according to the present invention includes, in order from the object side, a first lens group having a negative power, an aperture, a second lens group having a positive power, and a third lens group having a positive power at least at a peripheral portion. The lens group is arranged. The first lens group includes a plano-concave negative single lens having a flat object side and a concave image side, or a negative meniscus lens having a convex surface facing the object side. Since the first lens group has negative power, a wide angle of view can be obtained.

The second lens group includes a combination of a positive lens and a cemented lens arranged from the stop side, or a combination of two positive lenses and a negative lens. The lens surface closest to the image side of the second lens group is concave. When the second lens group has a cemented lens having a cemented surface having a negative power, the function of correcting lateral chromatic aberration can be assigned to the concave surface closest to the image side of the second lens group and the cemented surface.
The radius of curvature of the cemented surface can be increased, and as a result, the lens thickness of the cemented lens can be reduced, and the overall length of the lens can be reduced.

[0010] In the endoscope objective lens according to the present invention, the air lens formed between the second lens group and the third lens group includes:
It is characterized by satisfying the following condition (1). 0.05 <| f / f0 |, f0 <0 (1) where f is the focal length of the entire system, and f0 is the most image-side surface of the second lens group and the most object-side surface of the third lens group. Is the focal length of the air lens formed between

The power Δt of a thick lens placed in the air
Is given by the following equation (8), where r1 and r2 are the radii of curvature on both surfaces of the lens, d is the lens thickness, and n is the refractive index of the lens. ψt = 1 / f = (n−1) (r1 ⁻¹ −r2 ⁻¹ ) + (n−1) ² d / (nr1r2) (8)

On the other hand, the air lens captures the space between the two lenses as a lens having a refractive index of “1”.
The power ψ0 is such that the radius of curvature of the rear lens surface of the front lens is r1, the radius of curvature of the front lens surface of the rear lens is r2, and the refractive indices of the front and rear lenses are n1 and n, respectively.
2, given by the following equation (9), where d is the lens interval. ψ0 = 1 / f0 = (1-n1) / r1 + (n2-1) / r2- (1-n1) (n2-1) d / (r1r2) (9)

The fact that the air lens formed between the second lens unit and the third lens unit has a negative power (f0 <0) means that the negative lens surface of the second lens unit on the most image side has a negative power. This means that the power is large and the positive power of the lens surface on the object side of the third lens group is small. By giving this air lens negative power, it is possible to suppress the occurrence of chromatic aberration of magnification without increasing the overall length.

When | f / f0 | of the condition (1) is less than 0.05, or when the focal length f0 of the air lens has a positive value, the negative of the most image side surface of the second lens unit is set. Is weakened, the effect of correcting lateral chromatic aberration on this surface is reduced, and the burden on other surfaces is increased.

In order to maintain a telecentric state on the image side, it is desirable to set the above-mentioned air lens so as to satisfy the following condition (2). | F / f0 | <1.00 (2)

If the upper limit of the condition (2) is exceeded, the negative power of the air lens becomes excessive, and the principal ray diverged by the concave surface closest to the image side of the second lens group is changed to the positive power of the third lens group. Can not be refracted in the direction parallel to the optical axis,
Telecentricity cannot be maintained.

The second lens group has a negative lens closest to the image side, and its focal length and Abbe number are respectively fs and vn
It is desirable that the following conditions (3) and (4) are satisfied. 0.05 <| f / fs | <0.8, fs <0 (3) νn <35 (4)

Condition (3) defines the power of the negative lens located closest to the image side of the second lens group. By satisfying the condition (3), chromatic aberration of magnification occurring on the object side surface of the third lens group can be corrected, and occurrence of chromatic aberration of magnification of the entire system can be suppressed. When the value exceeds the upper limit of the condition (3), the negative power of the negative lens disposed closest to the image side of the second lens unit becomes the third power.
The power becomes too large compared to the positive power of the lens group, and the telecentricity cannot be maintained.

Condition (4) defines the dispersion of the negative lens. In an endoscope objective lens having strong positive power as a whole, in order to suppress the amount of chromatic aberration of magnification,
Among the positive and negative lenses constituting the second lens group, it is necessary to reduce the dispersion of the positive lens and increase the dispersion of the negative lens. By satisfying the condition (4), lateral chromatic aberration generated by the positive lens can be favorably corrected. conditions
If the upper limit of (4) is exceeded, the dispersion of the negative lens becomes too small, and the effect of correcting lateral chromatic aberration becomes insufficient.

Further, assuming that the focal lengths of the first lens unit and the second lens unit are f1 and f2, respectively, and the axial focal length of the third lens unit is f3, the following conditions (5), (6) and (7) ) Is desirable. 0.4 <| f / f1 | <1.5, f1 <0 (5) 0.9 <f / f2 <1.8 (6) 0.0 ≦ f / f3 <0.7 (7) )

Condition (5) defines the power of the negative first lens unit. By satisfying this condition, a wide angle of view is ensured, and spherical aberration,
Field curvature can be kept small. When the value goes below the lower limit of the condition (5), the negative power becomes weak, a sufficiently wide angle of view cannot be obtained, and the Petzval sum becomes positive, and the field curvature is insufficiently corrected. When the value exceeds the upper limit of the condition (5), the negative power of the first lens unit becomes excessive, and the spherical aberration and the field curvature become excessively corrected. In addition, since the back focus also increases, the overall length increases, and the entire system cannot be made compact.

Condition (6) defines the power of the positive second lens group. By satisfying this condition, spherical aberration and field curvature can be suppressed to a small value. When the value goes below the lower limit of the condition (6), the positive power of the second lens unit becomes weak, the Petzval sum becomes negative, and the field curvature becomes overcorrected. When the value exceeds the upper limit of the condition (6), the positive power of the second lens unit becomes excessive, and the spherical aberration and the field curvature become insufficiently corrected.

Condition (7) defines the on-axis power of the positive third lens unit. The third lens group is configured to maintain telecentricity by having a positive power at least in the peripheral portion, but has a positive power on the axis to satisfy the condition (7), or has no power. It is desirable. When the value goes below the lower limit of the condition (7), the third lens unit has a negative power, so that telecentricity cannot be maintained. When the value exceeds the upper limit of the condition (7), the positive power of the third lens unit becomes excessive, the spherical aberration and the curvature of field become insufficiently corrected, and the astigmatism becomes large. It cannot be corrected.

[0024]

EXAMPLES Next, eight specific examples satisfying the conditions of the above-described embodiment will be presented.

[0025]

FIG. 1 shows a lens configuration of an endoscope objective lens according to a first embodiment. The specific numerical configuration is shown in Table 1. In the table, FNO. Is the F number, f
Is the focal length of the entire system, m is the magnification, ω is the reference design distance of 10 m
The half angle of view at m, r is the radius of curvature of each lens surface, d is the lens thickness or lens interval, nd is the refractive index of each lens at d-line (588 nm), and νd is the Abbe number of each lens. FIG.
The graph shows chromatic aberration indicated by spherical aberration at d-line, g-line, and C-line, lateral chromatic aberration, astigmatism (S: sagittal, M: meridional), and distortion at g-line and C-line.
The unit of the horizontal axis indicating the amount of distortion is percent (%), and the unit of the horizontal axis indicating other amounts of aberration is mm.

In the first embodiment, the eighth surface, which is the object side surface of the third lens unit, is constituted by a rotationally symmetric aspherical surface. For the aspherical surface, the distance (sag amount) from the tangent plane on the optical axis of the aspherical surface to the coordinate point on the aspherical surface whose height from the optical axis is Y is X, and the curvature of the aspherical surface on the optical axis. Assuming that (1 / r) is C, the conic coefficient is K, the fourth and sixth order aspherical coefficients are A4 and A6, and are represented by the following equation (10). The radius of curvature of the aspheric surface in Table 1 is the radius of curvature on the optical axis, and the conical coefficient and non-arc coefficient of these surfaces are shown in Table 2. ^{X = CY 2 / (1 +} √ (1- (1 + K) C 2 Y 2)) + A4Y 4 + A6Y 6 ... (10)

[0027]

[Table 1]

[0028]

[Table 2]

In the first embodiment, the negative lens represented by the first surface and the second surface is connected to the positive lens represented by the third to seventh surfaces and the cemented lens via the first lens group and the stop S. Is the second
The positive lens represented by the lens group, the eighth and ninth surfaces, is the third lens group. The flat plate between the ninth and tenth surfaces indicates a color correction filter, and the flat plate between the tenth and eleventh surfaces indicates a cover glass of a CCD (not shown) which is an image sensor. The plane coincides with the eleventh plane.

[0030]

Second Embodiment FIG. 3 shows the lens configuration of an endoscope objective lens according to a second embodiment. Table 3 shows a specific numerical configuration. FIG. 4 shows various aberrations due to the above configuration. In the second embodiment, the lens arrangement is the same as in the first embodiment, and the eighth surface is aspheric. However, Example 2
Has an infinite radius of curvature on the axis, that is, a plane, and has a positive power only in the peripheral portion due to the aspheric addition amount shown in Table 4. Even with such a configuration, the off-axis chief ray can be made parallel to the optical axis, and telecentricity can be provided.

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

Third Embodiment FIG. 5 shows a lens configuration of an endoscope objective lens according to a third embodiment. Table 5 shows a specific numerical configuration. FIG. 6 shows various aberrations due to the above configuration.

[0034]

[Table 5]

In the third embodiment, the arrangement of the objective lens itself is the same as in the first and second embodiments, but there is an interval between the filter and the third lens group. The positive lens represented by the eighth and ninth surfaces is the third lens group, the color correction filter is a flat plate between the tenth and eleventh surfaces, and the cover glass is the eleventh lens.
It is represented as a flat plate between the surface and the twelfth surface. In addition,
In this embodiment, no aspheric surface is used.

[0036]

Fourth Embodiment FIG. 7 shows a lens configuration of an endoscope objective lens according to a fourth embodiment. Table 6 shows a specific numerical configuration. FIG. 8 shows various aberrations due to the above configuration. The arrangement of the lenses is the same as in Example 3,
Also in this example, no aspheric surface is used.

[0037]

[Table 6]

[0038]

Fifth Embodiment FIG. 9 shows a lens configuration of an endoscope objective lens according to a fifth embodiment. The specific numerical configuration is shown in Table 7. FIG. 10 shows various aberrations due to the above configuration. The arrangement of the lenses is the same as that of the third embodiment, and no aspheric surface is used in this example.

[0039]

[Table 7]

[0040]

Sixth Embodiment FIG. 11 shows a lens configuration of an endoscope objective lens according to a sixth embodiment. The specific numerical configuration is shown in Table 8. FIG. 12 shows various aberrations due to the above configuration. The arrangement of the lenses is the same as in the third embodiment. In this example, the first surface and the eighth surface are aspherical surfaces, and the aspherical surface coefficients of each surface are shown in Table 9.

[0041]

[Table 8]

[0042]

[Table 9]

[0043]

Seventh Embodiment FIG. 13 shows a lens configuration of an endoscope objective lens according to a seventh embodiment. Table 10 shows a specific numerical configuration. FIG. 14 shows various aberrations due to the above configuration. The arrangement of the lenses is the same as in the third embodiment. In this example, the first surface and the eighth surface are aspherical surfaces,
Table 11 shows the aspheric coefficient of each surface.

[0044]

[Table 10]

[0045]

[Table 11]

[0046]

Eighth Embodiment FIG. 15 shows a lens configuration of an endoscope objective lens according to an eighth embodiment. Table 12 shows a specific numerical configuration. FIG. 16 shows various aberrations due to the above configuration.

[0047]

[Table 12]

In the eighth embodiment, unlike the other embodiments, the second
No cemented lens is provided in the lens group, and all lenses are arranged as single lenses. In this example, the first
A negative lens represented by a surface and a second surface is a first lens group, and two positive lenses and one negative lens represented by a third surface to an eighth surface via a stop S are a second lens group. , The ninth surface and the tenth surface constitute the third lens group. The flat plate between the eleventh and twelfth surfaces is a color correction filter, and the flat plate between the twelfth and thirteenth surfaces is a cover glass.

Table 13 below shows each of Examples 1 to 8 described above.
And the corresponding relationship between the conditional expressions described in the claims.
From this table, it can be understood that all the examples satisfy all the conditional expressions.

[0050]

[Table 13] Example f / f0 f / fs νn f / f1 f / f2 f / f3 1 -0.400 -0.151 21.3 -0.793 1.376 0.200 2 -0.154 -0.106 21.3 -0.748 1.405 0.000 3 -0.556 -0.498 21.3 -1.040 1.260 0.506 4 -0.620 -0.368 21.3 -1.287 1.540 0.329 5 -0.728 -0.330 21.3 -0.942 1.371 0.381 6 -0.475 -0.155 25.4 -0.500 1.107 0.330 7 -0.492 -0.154 25.4 -0.579 1.171 0.319 8 -0.560 -0.710 25.4 -1.124 1.561 0.417

[0051]

As described above, according to the present invention, while maintaining a telecentric state on the image side,
It is possible to provide an endoscope objective lens having a short overall length while favorably correcting lateral chromatic aberration.

[Brief description of the drawings]

FIG. 1 is a lens diagram of an endoscope objective lens according to a first embodiment.

FIG. 2 is a diagram illustrating various aberrations of the endoscope objective lens according to the first example;

FIG. 3 is a lens diagram of an endoscope objective lens according to a second embodiment;

FIG. 4 is a diagram illustrating various aberrations of the endoscope objective lens according to the second example;

FIG. 5 is a lens diagram of an endoscope objective lens according to a third example;

FIG. 6 is a diagram illustrating various aberrations of the endoscope objective lens according to the third example;

FIG. 7 is a lens diagram of an endoscope objective lens according to a fourth example;

FIG. 8 is a diagram illustrating various aberrations of the endoscope objective lens according to the fourth example;

FIG. 9 is a lens diagram of an endoscope objective lens according to a fifth example;

FIG. 10 is a diagram illustrating various aberrations of the endoscope objective lens according to the fifth example;

FIG. 11 is a lens diagram of an endoscope objective lens according to a sixth example;

FIG. 12 is a diagram illustrating various aberrations of the endoscope objective lens according to the sixth example;

FIG. 13 is a lens diagram of an endoscope objective lens according to a seventh example;

FIG. 14 is a diagram illustrating various aberrations of the endoscope objective lens according to the seventh example;

FIG. 15 is a lens diagram of an endoscope objective lens according to Example 8;

FIG. 16 is a diagram illustrating various aberrations of the endoscope objective lens according to the eighth example;

Claims (6)

[Claims] 1. A first lens group having a negative power, a stop, a second lens group having a positive power, and a third lens group having a positive power at least in a peripheral portion are arranged in order from the object side. The endoscope objective lens, wherein the second lens group is arranged, and a surface closest to the image side of the second lens group is concave, and satisfies the following condition (1). 0.05 <| f / f0 |, f0 <0 (1) where f is the focal length of the entire system, and f0 is the most image-side surface of the second lens group and the most object-side surface of the third lens group. Is the focal length of the air lens formed between 2. The endoscope objective lens according to claim 1, wherein the following condition (2) is satisfied. | F / f0 | <1.00 (2) 3. The endoscope objective lens according to claim 1, wherein the second lens group includes a negative lens closest to an image, and satisfies the following conditions (3) and (4). 0.05 <| f / fs | <0.8, fs <0 (3) vn <35 (4) where fs and vn are the focal lengths of the negative lens closest to the image side of the second lens group, respectively. , And Abbe number. 4. The following conditions (5), (6), and (3), wherein the focal lengths of the first and second lens groups are f1 and f2, respectively, and the on-axis focal length of the third lens group is f3. The endoscope objective lens according to claim 3, wherein (7) is satisfied. 0.4 <| f / f1 | <1.5, f1 <0 (5) 0.9 <f / f2 <1.8 (6) 0.0 ≦ f / f3 <0.7 (7) ) 5. The first lens group comprises one negative lens, the second lens group comprises one positive single lens and one cemented lens, and the third lens group 5. The endoscope objective lens according to claim 4, wherein the objective lens comprises one single lens. 6. 6. The first lens group is composed of one negative lens, the second lens group is composed of two positive single lenses and one negative single lens, and The endoscope objective lens according to claim 4, wherein the lens group includes one single lens.