JP2009251520A - Camera head optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a camera head optical system used for photographing by being attached to the eyepiece part of an endoscope, which is optimized to a compact imaging element having a negative oblique incident characteristic and a short exit pupil distance without using a relay lens. <P>SOLUTION: In the optical system 30 for the camera head, which has an imaging lens 32 used for photographing by being attached to the eyepiece part 20 of an endoscope, the small imaging element 33 with the short exit pupil distance is optimized using a field lens 34 of negative power. As a range in which high effectiveness is achieved when a field lens 34 of a negative power is used for oblique incidence, the imaging lens 32 satisfies expression (1) given below: f×tan(-tw)/h>1.0 ... (1), wherein f is a focal distance of an imaging lens, h is the maximum image height on an imaging face, and tw is an angle at which the light ray of the center of an oblique luminous flux enters an imaging face at the maximum image height. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a camera head optical system, and more particularly to an imaging optical system that performs imaging by connecting to an eyepiece of an endoscope.

  Diagnosis is performed by connecting a detachable camera head to the eyepiece of the fiberscope and projecting an image in the body cavity by the objective lens of the scope on the monitor. Such a camera head includes a solid-state imaging device such as a CCD and an imaging lens for guiding an image from the eyepiece to the solid-state imaging device.

  For the purpose of reducing the size and weight of the camera head, such solid-state imaging devices tend to be downsized, and for the purpose of improving sensitivity, microlenses corresponding to the respective pixels are arranged on the surface of the solid-state imaging device. Since the microlens has a thickness, if a light beam having an angle optimized for the microlens is not incident, the light beam incident on the imaging element is blocked. In particular, when the off-axis light beam is blocked and the amount of light is insufficient, a phenomenon called shading occurs, and the peripheral image becomes darker or the periphery appears colored with respect to the center. This is more conspicuous as the image sensor is miniaturized in that the thickness of the microlens is greatly effective.

  Conventionally, for example, as shown in Patent Documents 1 and 2, it is optimized for an optical system in which an off-axis light beam is perpendicularly incident on an image sensor and an exit pupil is at infinity, so-called image-side telecentric optical system. There were many things. However, since the accuracy of the microlens has improved, an imaging device that has been optimized for an optical system whose exit pupil is close to the object side in recent years (hereinafter, this is referred to as having a negative oblique incidence characteristic) is also used. This is often used as the application of the leading edge imaging optical system of a video scope because it has appeared and can be easily downsized. Examples of such an optical system include Patent Documents 3 and 4 and many others.

  If an image sensor having such negative oblique incidence characteristics is used for the image sensor used in the camera head, the image sensor used in the video scope and the camera head can be shared, and image processing in the processor Advantages such as a common method for the video scope and the camera head are born. Therefore, it is desirable that the image sensor used for the camera head also has negative oblique incidence characteristics.

  However, an image-side telecentric optical system as shown in Patent Document 1 cannot be used for an imaging element having negative oblique incidence characteristics because of the occurrence of shading as described above. Although it is possible to solve this problem by transmitting an image using a relay lens and having a negative oblique incidence characteristic, there is a problem that the total length of the lens system becomes long and the camera head becomes large. .

Moreover, if an optical system as shown in Patent Documents 5 and 6 is used, negative oblique incidence characteristics can be realized. However, if a lens system is configured in accordance with the size of a small image sensor, The optical system must be very small, which is problematic in terms of workability. In addition, since it is necessary to reduce the aperture, it cannot be called an optimized optical system in that a bright image cannot be obtained. In view of the fact that all lenses use aspherical lenses, it is not possible to reduce the cost.
JP 2006-53218 A Japanese Patent Laying-Open No. 2005-99080 JP 2001-91832 A JP 2002-28126 A JP 2005-557518 A JP 2007-3768 A

  The present invention has been made in view of such problems of the prior art, and an object of the present invention is to provide a small imaging device having negative oblique incidence characteristics and a short exit pupil distance without using a relay lens. It is to constitute a camera head optical system that performs photography by attaching to an eyepiece portion of an optimized endoscope.

  In order to achieve the above, the camera head optical system of the present invention uses a negative power field lens in an optical system of a camera head provided with an imaging lens that is attached to an eyepiece of an endoscope and performs photographing. It is characterized by this.

  Hereinafter, the reason and action of the above configuration in the camera head optical system of the present invention will be described.

  According to the configuration described above, the light beam that converges immediately before the image sensor can be changed to divergent light, and the light beam that matches the image sensor with negative oblique incident angle characteristics can be incident on the image sensor. As a result, the shading problem can be solved without using a relay lens, and an increase in the size of the camera head can be avoided.

In order to obtain an optical system having an exit pupil at a position close to the image plane, the following condition (1):
f · tan (−tw) / h> 1.0 (1)
It is desirable that is satisfied. Here, f is the focal length of the imaging lens, h is the maximum image height on the imaging surface, and tw is the angle at which the central ray of the oblique ray bundle enters the imaging surface at the maximum image height.

  This is a value obtained as a range having a large effect when a negative power field lens is used for realizing oblique incidence, and this will be described below.

  First, since the camera head forms an image of light emitted from the eyepiece lens on the solid-state imaging device, the light incident on the camera head is −1 diopter (imaged at a position of 1000 mm from the exit end) or −2 diopter. Etc. (image formation at a position of 500 mm from the exit end). On the other hand, since the focal length of the camera head is at most about 10 mm, it can be approximated to approximately infinity. Therefore, it can be considered that the object-side telecentric light is incident on the camera head.

  The light beam incident on the camera head is regulated in brightness by a diaphragm located between the eyepiece lens of the scope and the imaging lens of the camera head, and enters the solid-state image sensor. For the sake of simplicity, the eyepiece lens and the imaging lens of the camera head are thin lenses, and this is shown in FIG.

  Let the maximum image height of the solid-state imaging device 33 be h, and pay attention to the light beam 35 at the center of the light beam among the light beams incident on this height h. Since the light beam 35 is refracted by the imaging lens 32 of the camera head 30, the distance from the solid-state imaging device 33 to the exit pupil position is clearly larger than the focal length f of the imaging lens 32. If the incident angle of the light beam 35 is tw, the distance from the solid-state imaging device surface to the exit pupil position is given by h / tan (−tw), and can be expressed as f <h / tan (−tw). (In the case of FIG. 1, the signs are defined as h> 0, f> 0, tw <0). F = h / tan (−tw) only when the position of the diaphragm 31 between the eyepiece 20 of the fiber scope 10 and the imaging lens 32 is close to the position of the imaging lens 32. This is shown in FIG.

  Therefore, even if a solid-state imaging device optimized for oblique incidence is used, if it is desired to satisfy f <h / tan (−tw), in principle, neither a relay lens nor a field lens is required, and a negative power field is required. The effect of disposing the lens is limited to facilitating correction of aberrations associated with oblique incidence.

  However, if the diaphragm 31 is disposed between the eyepiece 20 and the imaging lens 32, it is impossible for the optical system as shown in FIG. 2 to satisfy f ≧ h / tan (−tw). In order to realize oblique incidence, a new optical system must be added. If a relay lens is arranged here, this can be realized, but the total length of the optical system becomes long, and the camera head 30 becomes large. Further, the relay of the image with the positive lens generates a field curvature, which is disadvantageous in that it is difficult to flatten the image.

  However, the arrangement of the field lens 34 having a negative power contributes to downsizing because oblique incidence can be realized without causing such a problem as shown in FIG. That is, the incident angle tw of the central light beam 35 with respect to the solid-state imaging device 33 is increased in absolute value by the negative power field lens 34, and h / tan (−tw) becomes smaller, so that f ≧ h / tan ( -Tw) can be satisfied.

further,
f · tan (−tw) / h> 1.1 (1-1)
It is desirable that the relationship is satisfied. This is because, in most cases, the angle characteristics of the solid-state imaging device have a margin of incident angle in a certain range other than the optimized incident angle. The range varies depending on the product, but is usually about 3 °. For example, the optical system shown in Patent Document 3 performs zooming by moving a lens with respect to one image sensor, and the incident angle at the maximum height increases from −7.8 ° at the wide end and the tele end. Although it changes by about 2.8 ° from 10.6 °, the solid-state image sensor used at this time should not generate shading at any incident angle, and has characteristics only at a certain angle. If it is, it becomes an obstacle at the time of use.

  Therefore, if the oblique incident angle of the solid-state imaging device satisfies f <h / tan (−tw) in this range, it is not necessary to use the negative power field lens. The above equation (1-1) gives a condition that it is effective to use a field lens having a negative power even in consideration of such a margin. This will be described below.

Since oblique incidence is considered now, it is assumed that tw is a finite angle and the incident angle is changed by a minute angle δ. Then, the exit pupil position P from the solid-state imaging device 33 is h / tan (−tw + δ), so the tangent addition theorem:
tan (a + b) = (tana + tanb) / (1-tana tanb)
Using,
P = h {1-tan (−tw) tan δ} / {tan (−tw) + tan δ}
It becomes. Expand this as δ is small,
P≈ {h / tan (−tw)} × [1-tan δ {1 / tan (−tw) + tan (−tw)}]
Is obtained.

Now, since this value becomes the largest when δ is negative, tan (−3 °) = − 0.052 when substituting −3 °, which is a margin, here. Substituting this, the formula of arithmetic mean and geometric mean to make the above formula independent of tw:
1/2 × (a + b) ≧ √ (ab)
The above formula is rewritten,
P≈ {h / tan (−tw)} × [1 + 0.052 {1 / tan (−tw) + tan (−tw)}]
≧ {h / tan (−tw)} × 1.104
It becomes. When the focal length f is larger than this value, even if there is an allowance of 3 °, the advantage of not using a relay lens by arranging a negative field lens for oblique incidence is great. Using such P and rewriting f ≧ P, the condition for giving the lowest limit is
f · tan (−tw) / h> 1.1 (1-1)
Is obtained.

The purpose of placing the negative power field lens 34 is to realize oblique incidence characteristics, but there is a so-called field flattener in which a concave lens is arranged in the vicinity of the image sensor. This also corrects the curvature of field with a negative lens in the vicinity of an image sensor having a relatively low image height. It is convenient for the purpose of aberration correction to give such an effect to the above field lens, and Petzval conditions:
_k
Ptz = Σ 1 / f _i n _i = 0
^{i = 1}
However, it is desirable to be satisfied to some extent. For that,
_k
| Σ1 / n _i f _i | ≦ (2F · p) / (F ² p ² + h ² ) (2)
^{i = 1}
Satisfaction is one measure. Where n _i is the refractive index of the i-th lens element, f _i is the focal length of the i-th lens element, F is the F value of the imaging lens, h is the maximum image height on the imaging surface, and p is the imaging surface The pitch of the image sensor at. This will be described below.

Since the Petzval sum Ptz gives the curvature of field curvature, the paraxial field curvature at the maximum image height h is
1 / Ptz-√ {(1 / Ptz) ² −h ² }
It becomes. On the other hand, when the F value is F, the amount of movement of the imaging surface that is considered to form an image without blurring the light incident on the imaging element with the pixel pitch p is one side Fp. Therefore, when the image plane is in the best focus state, a condition that the field curvature at the maximum image height is smaller than Fp can be adopted as a condition that the image is not blurred even at the maximum image height. That is,
Fp ≧ 1 / Ptz−√ {(1 / Ptz) ² −h ² }
By transforming this, the following equation is obtained.

_{k
Ptz = | Σ 1 / n i} f i | ≦ (2F · p) / (F 2 p 2 + h 2) ·· (2)
^{i = 1}
Further, in this configuration, it is desirable to adopt a configuration of an aperture-negative lens group-positive lens group-negative power field lens from the object side to the image side. Since the diaphragm is arranged at the head on the object side, in order to correct the spherical surface and coma aberration generated by the field flattener, it is necessary to greatly apply the correction by the positive lens. Therefore, it is desirable for the positive lens to have a high ray height.

  In a camera head optical system used by connecting to a fiberscope, a filter is inserted for reasons such as moire removal. Using the most object side as a negative lens and conforming to the retrofocus type is convenient in terms of large back focus and easy filter insertion.

When taking the configuration of the stop-negative lens group-positive lens group-field lens, in addition to this,
_{| Φ n | / φ p <} φ p / | φ f |, φ p / | φ f | <1 ··· (3)
It is preferable to satisfy the following condition. However, the respective powers of the negative lens group, the positive lens group, and the negative power field lens are φ _n , φ _p , and φ _f .

  The size of the refraction angle by the lens is determined by the power of the lens and the height of the light beam. From the above configuration, the ray height is higher in the positive lens group than in the negative lens group, and lower in the field lens than in the positive lens group. In order to correct the incident angle, the field lens needs to have a high power. For this reason, the ray height tends to be small. Therefore, since the ray height in the positive lens group is relatively higher than the ray height in the field lens, the aberration generated in the field lens can be corrected with a weaker power than that in the field lens. In this way, since the aberration correction is omitted between the field lens and the positive lens group, the negative lens group should be arranged for the purpose of raising the height of the light beam in the positive lens group without generating aberration. . In general, in the camera head optical system, a light beam having a relatively high light beam height is incident, so that the light beam heights of the negative lens unit and the positive lens unit are not greatly different, and the negative lens unit is large. There is no need to have power. Therefore, the ratio between the power of the positive lens group and the power of the negative lens group may be smaller than the ratio between the power of the positive lens group and the power of the field lens.

  For the reasons described here, it is desirable that the lens element closest to the object side in the negative lens group has a meniscus shape with the concave surface facing the object side. This is because it becomes close to a concentric configuration, and the occurrence of aberrations such as astigmatism can be suppressed.

  Furthermore, disposing a negative lens having a large power in front of the image sensor causes a large chromatic aberration. From the viewpoint of correcting this, it is desirable that the positive lens group includes at least one cemented lens of a convex lens and a concave lens. Since paraxial chromatic aberration is proportional to the square of the ray height, it is effective to correct it at a location where the ray height is high. However, the ray height is highest in the positive lens group as described above. .

  Further, if the above configuration is adopted, it is preferable that the lens element closest to the image side in the positive lens group has a meniscus shape with the convex surface facing the object side. In the positive lens group, a high light ray height needs to be lowered in the field lens. A meniscus lens with a convex surface facing the object side located in the position from the positive lens group to the field lens works as a surface close to an aplanatic surface, so it suppresses the generation of spherical and coma aberrations and reduces the beam height. enable.

  In the camera head optical system, a filter is inserted to remove moire. It is desirable to place a filter for removing moire in front of the negative field lens. This is because if a flat filter is inserted at the entrance pupil position, the reflected light beam is likely to become a ghost, so it is better to place the filter at a position away from the entrance pupil. Further, since the height of the light beam is low, the effective diameter can be reduced, the appearance standard of the crystal filter unit is relaxed, and the processability is improved.

  In addition, it is desirable that the moire removal filter is a joint of three quartz plates. The fiberscope connected to the camera head has three directions for generating moire because the image guide fiber is in a hexagonal close-packed form. Therefore, in principle, it is most efficient to remove moire in each direction using three filters. When the camera head is downsized, the filter tends to be thin, and a sufficient edge thickness may not be ensured. Therefore, it is easy to handle burrs for assembly one by one, but assembly can be easily performed by joining them together and handling them as one component.

Alternatively, the filter for removing moire may be composed of at least two quartz plates, and a member having the effect of a λ / 4 plate may be sandwiched between the quartz plates. The negative field lens disposed on the image plane side of the filter enlarges the amount of separation by the crystal filter and enters the imaging element, so that the thickness of the crystal filter is reduced. As a result, the wavelength dependence of the filter appears, so that the separation by the crystal is not uniform. this,
(N _e −n _o ) × d = (2N + 1) × λ / 4
It is possible to reduce the non-uniformity of separation by using a member that satisfies the above. In n _o and n _e is the refractive index relates to ordinary light and extraordinary light, respectively glass material, d is the optical axis direction of the thickness of the member, N represents 0 or any integer, lambda is the wavelength of light incident on the imaging device is there. Furthermore, even when these are joined, the advantage that the above-mentioned handling becomes easy does not change.

Further, the member having the effect of the λ / 4 plate is preferably the same crystal plate as the filter for removing moire. Crystal is a glass material having a lower refractive index among the crystalline materials can be employed as a member having the aforementioned advantage of such lambda / 4 plate, the difference in refractive index between ordinary light and extraordinary light (n _e -n _o) also 0.009 And smaller than other birefringent materials. Therefore, even if the effect as the same λ / 4 plate is aimed, the thickness d can be increased, and at the same time, the tolerance of the thickness of the member can be relaxed. Therefore, the standard can be relaxed, which contributes to low cost.

  Alternatively, the member having the effect of the λ / 4 plate may be a birefringent polymer. Compared to other birefringent materials, it is inexpensive and also has good affinity with adhesives.

  According to the present invention, it is possible to provide an optical system of a camera head that is optimized for an image pickup device that is small in size and has negative oblique incidence characteristics without causing a problem of shading.

  Examples 1 to 9 of the camera head optical system according to the present invention will be described below with reference to FIGS. In these drawings, the surface numbers and the surface intervals of the optical surfaces are omitted for the sake of simplicity.

  In any of the embodiments shown in FIGS. 4 to 12, the luminous flux whose brightness is defined by the aperture stop 100 that defines the luminous flux diameter is a negative lens group 110, a positive lens group 120, and a negative power field lens. An image is formed on an imaging surface 140 of a solid-state imaging device (for example, CCD; Charge Coupled Device or CMOS; Complementary Metal Oxide Semiconductor) by an imaging lens composed of 130. Regarding the filters, in addition to a cover glass 210 for improving water tightness, a filter group 220 to be inserted for the purpose of removing moire and a cover glass 230 for a solid-state image sensor are arranged. The cover glass 210 may be disposed inclined with respect to the optical axis for the purpose of removing flare. Moreover, the filter group 220 and the cover glass 230 of a solid-state image sensor may contain multiple sheets. Reference numerals 310a and 310b are flare stops for removing flare.

  Although the numerical data of these examples will be described later, these numerical data are obtained when the incident light beam is -2 diopters and the focal length is normalized to 7 mm. The surface number is the surface number of the optical surface counted from the front surface of the cover glass 210 by “No”, the radius of curvature is “r”, the surface interval or air interval is “d”, and the d-line The refractive index is indicated by “nd” and the Abbe number is indicated by “vd”. The radius of curvature and the surface spacing are in mm.

<Example 1>
FIG. 4 shows a lens cross-sectional view of Example 1, and FIG. 13 shows various aberration diagrams thereof. In the various aberration diagrams of FIG. 13, (a) is spherical aberration, (b) is coma aberration at an image height ratio of 0.75, (c) is coma aberration at an image height ratio of 1, (d) is distortion aberration, (e ) Indicates astigmatism, and (f) indicates magnification aberration. same as below.

  In the numerical data of Example 1 to be described later, the fourth surface to the sixth surface are a negative lens group 110 including a cemented lens of a biconcave negative lens and a biconvex positive lens, and the seventh surface to the eleventh surface are convex plano positive lenses. A positive lens group 120 including a biconvex positive lens and a cemented lens of a negative meniscus lens having a concave surface facing the object side, and the fourteenth to fifteenth surfaces disposed in front of the cover glass 230 of the solid-state imaging device. Is a negative power field lens 130 made up of a biconcave negative lens, which realizes a negative oblique incidence characteristic. A diaphragm 100 that determines the diameter of the light beam is disposed on the third surface, and flare diaphragms 310a and 310b are disposed on the thirteenth and sixteenth surfaces. The first surface to the second surface are the cover glass 210, and the seventeenth surface to the eighteenth surface are the cover glass 230 of the solid-state imaging device. The twelfth to thirteenth surfaces, which are parallel flat plates having a large thickness, are a filter group 220, which includes a quartz filter for removing moire, an infrared cut filter for color correction, A filter or the like having a coating for cutting laser light of a specific wavelength such as LD cut or YAG cut may be inserted. This also applies to the following examples, and is not particularly specified.

<Example 2>
FIG. 5 shows a lens cross-sectional view of Example 2, and FIG. 14 shows aberration diagrams thereof. In the numerical data of Example 2 to be described later, the fourth to fifth surfaces have a negative lens group 110 including negative meniscus lenses having a concave surface facing the object side, and the sixth to twelfth surfaces have a concave surface facing the object side. A positive lens group 120 comprising a positive meniscus lens, a cemented lens of a biconvex positive lens, a negative meniscus lens having a concave surface facing the object side, and a positive meniscus lens having a convex surface facing the object side, and a solid-state imaging device The fifteenth to sixteenth surfaces arranged in front of the cover glass 230 are negative power field lenses 130 made of plano-concave negative lenses, thereby realizing negative oblique incidence characteristics. A diaphragm 100 that determines the diameter of the light beam is disposed on the third surface, and flare diaphragms 310a and 310b are disposed on the fourteenth and seventeenth surfaces. The first surface to the second surface are the cover glass 210, and the eighteenth surface to the nineteenth surface are the cover glass 230 of the solid-state imaging device. The thirteenth to fourteenth surfaces, which are parallel flat plates having a large thickness, are the filter group 220.

<Example 3>
FIG. 6 shows a lens cross-sectional view of Example 3, and FIG. 15 shows various aberration diagrams thereof. In numerical data of Example 3 to be described later, the fourth to sixth surfaces are a negative lens group 110, a negative lens group 110, a negative meniscus lens having a concave surface facing the object side, and a positive meniscus lens having a concave surface facing the object side. The seventh surface to the tenth surface are a positive lens group 120 including a biconvex positive lens and a positive meniscus lens having a convex surface facing the object side. The thirteenth surface disposed in front of the cover glass 230 of the solid-state imaging device. ˜14th surface is a negative power field lens 130 made of a concave flat negative lens, thereby realizing a negative oblique incidence characteristic. A diaphragm 100 for determining the diameter of the light beam is disposed on the third surface, and flare diaphragms 310a and 310b are disposed on the twelfth surface and the fifteenth surface. The first surface to the second surface are the cover glass 210, and the sixteenth surface to the seventeenth surface are the cover glass 230 of the solid-state imaging device. The eleventh to twelfth surfaces, which are parallel flat plates having a large thickness, are the filter group 220.

<Example 4>
FIG. 7 shows a lens cross-sectional view of Example 4, and FIG. 16 shows various aberration diagrams thereof. In numerical data of Example 4 to be described later, the fourth to fifth surfaces are negative meniscus lenses having a concave surface facing the object side to the negative lens group 110, the sixth to twelfth surfaces are biconvex positive lenses, and the object This is a positive lens group 120 composed of a positive meniscus lens having a convex surface facing the side, and a cemented lens of a biconvex positive lens and a negative meniscus lens having a concave surface facing the object side, in front of the cover glass 230 of the solid-state imaging device. The arranged fifteenth to sixteenth surfaces are a negative power field lens 130 made of a biconcave negative lens, thereby realizing a negative oblique incidence characteristic. A diaphragm 100 that determines the diameter of the light beam is disposed on the third surface, and flare diaphragms 310a and 310b are disposed on the fourteenth and seventeenth surfaces. The first surface to the second surface are the cover glass 210, and the eighteenth surface to the nineteenth surface are the cover glass 230 of the solid-state imaging device. The thirteenth to fourteenth surfaces, which are parallel flat plates having a large thickness, are the filter group 220.

<Example 5>
A lens cross-sectional view of Example 5 is shown in FIG. 8, and various aberration diagrams thereof are shown in FIG. In the numerical data of Example 5 to be described later, the fourth surface to the sixth surface are a negative lens group 110 including a cemented lens of a biconcave negative lens and a biconvex positive lens, and the seventh surface to the thirteenth surface are a biconvex positive lens. And a positive lens group 120 including a positive meniscus lens having a convex surface directed toward the object side, and a cemented lens of a biconvex positive lens and a biconcave negative lens, and is disposed in front of the cover glass 230 of the solid-state imaging device. The sixteenth to seventeenth surfaces are a negative power field lens 130 made of a concave flat negative lens, which realizes a negative oblique incidence characteristic. A diaphragm 100 for determining the diameter of the light beam is disposed on the third surface, and flare diaphragms 310a and 310b are disposed on the fifteenth surface and the eighteenth surface. The first surface to the second surface are the cover glass 210, and the nineteenth surface to the twentieth surface are the cover glass 230 of the solid-state imaging device. The fourteenth to fifteenth surfaces, which are parallel flat plates having a large thickness, are the filter group 220.

<Example 6>
A lens cross-sectional view of Example 6 is shown in FIG. 9, and various aberration diagrams thereof are shown in FIG. In the numerical data of Example 6 described later, the fourth to fifth surfaces are negative lens groups 110 from negative meniscus lenses having a concave surface facing the object side, and the sixth to thirteenth surfaces are biconcave negative lenses and biconvex. A positive lens group 120 including a cemented lens of a positive lens, a cemented lens of a biconvex positive lens, a negative meniscus lens having a concave surface facing the object side, and a positive meniscus lens having a convex surface facing the object side. The 16th surface to the 17th surface disposed in front of the cover glass 230 of the element are a negative power field lens 130 made of a biconcave negative lens, thereby realizing a negative oblique incidence characteristic. A diaphragm 100 for determining the diameter of the light beam is disposed on the third surface, and flare diaphragms 310a and 310b are disposed on the fifteenth surface and the eighteenth surface. The first surface to the second surface are the cover glass 210, and the nineteenth surface to the twentieth surface are the cover glass 230 of the solid-state imaging device. The fourteenth to fifteenth surfaces, which are parallel flat plates having a large thickness, are the filter group 220.

<Example 7>
A lens sectional view of Example 7 is shown in FIG. 10, and various aberration diagrams thereof are shown in FIG. In the numerical data of Example 7 to be described later, the fourth to sixth surfaces are a negative lens group 110 including a cemented lens of a negative meniscus lens having a concave surface facing the object side and a positive meniscus lens having a concave surface facing the object side, The seventh surface to the twelfth surface are a positive lens group 120 including a convex flat positive lens, a positive meniscus lens having a convex surface facing the object side, and a positive meniscus lens having a convex surface facing the object side. The fifteenth to sixteenth surfaces arranged in front of the cover glass 230 are a negative power field lens 130 made of a biconcave negative lens, thereby realizing a negative oblique incidence characteristic. A diaphragm 100 that determines the diameter of the light beam is disposed on the third surface, and flare diaphragms 310a and 310b are disposed on the fourteenth and seventeenth surfaces. The first surface to the second surface are the cover glass 210, and the eighteenth surface to the nineteenth surface are the cover glass 230 of the solid-state imaging device. The thirteenth to fourteenth surfaces, which are parallel flat plates having a large thickness, are the filter group 220.
<Example 8>
FIG. 11 shows a lens cross-sectional view of Example 8, and FIG. 20 shows various aberration diagrams thereof. In the numerical data of Example 8 to be described later, the fourth surface to the fifth surface are a negative lens group 110 including negative meniscus lenses having a concave surface facing the object side, and the sixth to twelfth surfaces are biconvex positive lenses. A positive lens group 120 including a cemented lens of a negative meniscus lens having a concave surface facing the object side, a positive meniscus lens having a concave surface facing the object side, and a positive meniscus lens having a convex surface facing the object side. The fifteenth to sixteenth surfaces arranged in front of the cover glass 230 are negative power field lenses 130 made of plano-concave negative lenses, thereby realizing negative oblique incidence characteristics. A diaphragm 100 that determines the diameter of the light beam is disposed on the third surface, and flare diaphragms 310a and 310b are disposed on the fourteenth and seventeenth surfaces. The first surface to the second surface are the cover glass 210, and the eighteenth surface to the nineteenth surface are the cover glass 230 of the solid-state imaging device. The thirteenth to fourteenth surfaces, which are parallel flat plates having a large thickness, are the filter group 220.

<Example 9>
A lens sectional view of Example 9 is shown in FIG. 12, and various aberration diagrams thereof are shown in FIG. In numerical data of Example 9 to be described later, the fourth to sixth surfaces are a negative lens group 110 from a cemented lens of a concave plano negative lens and a planoconvex positive lens, and the seventh to tenth surfaces are biconvex positive lenses. The positive lens group 120 includes a positive meniscus lens having a convex surface directed toward the object side, and the thirteenth to fourteenth surfaces disposed in front of the cover glass 230 of the solid-state image sensor are negative negative lenses formed of biconcave negative lenses. This is a field lens 130 having a negative power, and thus a negative oblique incidence characteristic is realized. A diaphragm 100 for determining the diameter of the light beam is disposed on the third surface, and flare diaphragms 310a and 310b are disposed on the twelfth surface and the fifteenth surface. The first surface to the second surface are the cover glass 210, and the sixteenth surface to the seventeenth surface are the cover glass 230 of the solid-state imaging device. The eleventh to twelfth surfaces, which are parallel flat plates having a large thickness, are the filter group 220.

Below, the numerical data of the said Examples 1-9 are shown below. In each table, in addition to the above, “Fno” is the F value, “TL” is the total length from the first surface to the final surface, and “f”, “tw”, and “h” are as described above.

Example 1
Fno = 2.286 TL = 20.538
f = 7.000 tw = -6.282 h = 0.743
Nor d nd vd
1 ∞ 1.0446 1.51633 64.15
2 ∞ 1.9661
3 (Aperture) ∞ 0.4954
4 -4.7815 1.3032 1.88300 40.76
5 10.6356 2.7926 1.78800 47.37
6 -6.6194 0.4654
7 11.1207 2.0686 1.72916 54.68
8 ∞ 0.4137
9 8.0385 2.9994 1.78800 47.37
10 -6.3247 0.9619 1.84666 23.78
11 -29.1193 1.3032
12 ∞ 1.9229 1.54869 45.56
13 ∞ 0.2068
14 -4.5902 0.9515 1.69895 30.13
15 4.5902 0.1758
16 ∞ 0.3020
17 ∞ 1.1643 1.51633 64.14
18 ∞.

Example 2
Fno = 2.062 TL = 21.298
f = 7.000 tw = -8.829 h = 0.749
Nor d nd vd
1 ∞ 1.0515 1.51633 64.15
2 ∞ 1.9793
3 (Aperture) ∞ 0.4985
4 -3.5752 2.1562 1.88300 40.76
5 -7.1040 0.8329
6 -42.5677 1.7640 1.72916 54.68
7 -6.4995 0.5205
8 11.9109 2.0822 1.51633 64.14
9 -6.5107 1.1981 1.84666 23.78
10 -15.7368 0.5205
11 7.0151 1.7883 1.88300 40.76
12 10.4709 1.4575
13 ∞ 2.5704 1.54869 45.56
14 ∞ 0.2082
15 ∞ 1.0186 1.80518 25.42
16 3.3613 0.1770
17 ∞ 0.3019
18 ∞ 1.1719 1.51633 64.14
19 ∞.

Example 3
Fno = 2.084 TL = 18.164
f = 7.000 tw = -9.327 h = 0.744
Nor d nd vd
1 ∞ 1.0383 1.51633 64.15
2 ∞ 1.9541
3 (Aperture) ∞ 0.4926
4 -3.6085 2.1007 1.84666 23.78
5 -16.0285 1.4383 1.51633 64.14
6 -5.1216 0.0000
7 14.5096 1.7477 1.78800 47.37
8 -17.6939 0.4112
9 4.6014 1.5002 1.58913 61.14
10 17.3934 2.1819
11 ∞ 2.5382 1.54869 45.56
12 ∞ 0.2056
13 -2.9201 0.9252 1.92286 18.90
14 ∞ 0.1748
15 ∞ 0.2981
16 ∞ 1.1572 1.51633 64.14
17 ∞.

Example 4
Fno = 2.282 TL = 19.977
f = 7.000 tw = -10.072 h = 0.746
Nor d nd vd
1 ∞ 1.0463 1.51633 64.15
2 ∞ 1.9694
3 (Aperture) ∞ 0.4962
4 -4.4012 1.8647 1.84666 23.78
5 -28.9017 0.0000
6 25.4756 2.8293 1.80518 25.42
7 -9.0827 0.4558
8 7.7197 2.3021 1.77250 49.60
9 161.5536 0.0000
10 7.4096 2.6680 1.58913 61.14
11 -5.9039 1.0046 1.92286 18.90
12 -52.2869 0.9841
13 ∞ 1.9260 1.54869 45.56
14 ∞ 0.2279
15 -3.9020 0.8288 1.76182 26.52
16 12.3344 0.1772
17 ∞ 0.0300
18 ∞ 1.1662 1.51633 64.14
19 ∞.

Example 5
Fno = 2.323 TL = 18.757
f = 7.000 tw = -10.078 h = 0.736
Nor d nd vd
1 ∞ 1.0272 1.51633 64.15
2 ∞ 1.9329
3 (Aperture) ∞ 0.4877
4 -3.8800 1.1187 1.84666 23.78
5 16.1481 1.8307 1.58913 61.14
6 -12.9749 0.1526
7 134.6161 1.2713 1.80518 25.42
8 -6.9543 0.3946
9 6.5208 1.5256 1.77250 49.60
10 33.9136 0.1424
11 6.4313 2.4409 1.58913 61.14
12 -18.4720 0.9863 1.92286 18.90
13 11.6383 0.9662
14 ∞ 1.8909 1.54869 45.56
15 ∞ 0.2237
16 -2.6776 0.8136 1.76182 26.52
17 ∞ 0.1729
18 ∞ 0.2339
19 ∞ 1.1449 1.51633 64.14
20 ∞.

Example 6
Fno = 2.085 TL = 21.456
f = 7.000 tw = -9.532 h = 0.741
Nor d nd vd
1 ∞ 1.0376 1.51633 64.15
2 ∞ 1.9526
3 (Aperture) ∞ 0.4923
4 -3.1941 2.0291 1.88300 40.76
5 -5.9542 0.4109
6 -42.9254 0.9691 1.84666 23.78
7 43.0953 1.6708 1.51633 64.14
8 -5.7855 0.4995
9 14.4667 2.2659 1.71999 50.23
10 -13.6149 1.2327 1.92286 18.90
11 -18.6917 0.4109
12 4.9273 1.6033 1.72916 54.68
13 6.3516 1.4382
14 ∞ 2.5365 1.54869 45.56
15 ∞ 0.2054
16 -6.7710 1.0726 1.92286 18.90
17 5.4060 0.1746
18 ∞ 0.2979
19 ∞ 1.1564 1.51633 64.14
20 ∞.

Example 7
Fno = 2.297 TL = 18.710
f = 7.000 tw = -9.887 h = 0.747
Nor d nd vd
1 ∞ 1.0396 1.51633 64.15
2 ∞ 1.9566
3 (Aperture) ∞ 0.4932
4 -3.9290 1.1806 1.92286 18.90
5 -11.6470 2.0064 1.71999 50.23
6 -5.5873 0.8749
7 9.2798 1.4776 1.69680 55.53
8 ∞ 0.1441
9 6.3469 1.9042 1.58913 61.14
10 12.8757 0.5661
11 7.5052 0.9264 1.76182 26.52
12 17.3161 1.6057
13 ∞ 1.9136 1.54869 45.56
14 ∞ 0.2265
15 -3.1727 0.8235 1.76182 26.52
16 15.7734 0.1750
17 ∞ 0.2368
18 ∞ 1.1587 1.51633 64.14
19 ∞.

Example 8
Fno = 2.084 TL = 20.456
f = 7.000 tw = -7.843 h = 0.745
Nor d nd vd
1 ∞ 1.0416 1.51633 64.15
2 ∞ 1.9603
3 (Aperture) ∞ 0.8550
4 -2.6297 2.1657 1.88300 40.76
5 -3.8958 0.1238
6 21.3456 1.5953 1.75500 52.32
7 -8.2737 1.2197
8 -4.8226 0.5156 1.92286 18.90
9 -9.2814 1.1035 1.58913 61.14
10 -5.4769 1.0835
11 8.7735 1.8839 1.71999 50.23
12 82.3879 1.4438
13 ∞ 2.5464 1.54869 45.56
14 ∞ 0.2062
15 ∞ 1.0767 1.92286 18.90
16 4.2832 0.1753
17 ∞ 0.2990
18 ∞ 1.1610 1.51633 64.14
19 ∞.

Example 9
Fno = 2.285 TL = 19.090
f = 7.000 tw = -12.107 h = 0.747
Nor d nd vd
1 ∞ 1.0406 1.51633 64.15
2 ∞ 1.9585
3 (Aperture) ∞ 0.4936
4 -3.4333 1.1818 1.92286 18.90
5 ∞ 2.5100 1.75500 52.32
6 -5.2979 0.8964
7 10.6062 1.9885 1.76182 26.52
8 -36.0611 0.2576
9 4.9313 1.9061 1.58913 61.14
10 17.7581 2.3182
11 ∞ 1.9155 1.54869 45.56
12 ∞ 0.2267
13 -2.7237 0.8243 1.76182 26.52
14 5.8547 0.1752
15 ∞ 0.2370
16 ∞ 1.1598 1.51633 64.14
17 ∞.

The values relating to the conditions (1) to (3) in Examples 1 to 9 are shown below. The pixel pitch p in condition (2) was set to 0.01.

Example Condition (1) Left side Condition (2) Left side Condition (2) Right side | φ _n | / φ _p φ _p / φ _f
1 1.645 0.007 0.060 0.149 0.512
2 1.436 0.006 0.054 0.482 0.754
3 1.541 0.007 0.056 0.326 0.592
4 1.647 0.005 0.060 0.652 0.920
5 1.680 0.007 0.063 0.653 0.932
6 1.577 0.008 0.056 0.436 0.597
7 1.626 0.006 0.061 0.211 0.576
8 1.284 0.005 0.055 0.152 0.649
9 2.009 0.011 0.061 0.279 0.412
.

  Each of the above embodiments satisfies each conditional expression. In all the embodiments, the negative power field lens realizes the oblique incidence characteristic, and various aberrations are well corrected.

It is a schematic block diagram of the camera head optical system which attaches to an eyepiece part and images | photographs. It is a block diagram at the time of changing the position of an aperture stop in the camera head optical system of FIG. It is a schematic block diagram of the camera head optical system by this invention. It is lens sectional drawing of Example 1 of the camera head optical system by this invention. It is lens sectional drawing of Example 2 of the camera head optical system by this invention. It is lens sectional drawing of Example 3 of the camera head optical system by this invention. It is lens sectional drawing of Example 4 of the camera head optical system by this invention. It is lens sectional drawing of Example 5 of the camera head optical system by this invention. It is lens sectional drawing of Example 6 of the camera head optical system by this invention. It is lens sectional drawing of Example 7 of the camera head optical system by this invention. It is lens sectional drawing of Example 8 of the camera head optical system by this invention. It is lens sectional drawing of Example 9 of the camera head optical system by this invention. FIG. 6 is a diagram illustrating all aberrations of Example 1. FIG. 6 is a diagram illustrating all aberrations of Example 2. FIG. 6 is a diagram showing aberrations of Example 3. FIG. 6 is a diagram illustrating various aberrations of Example 4. FIG. 10 is a diagram illustrating all aberrations of Example 5. FIG. 10 is a diagram illustrating all aberrations of Example 6. FIG. 10 is a diagram illustrating all aberrations of Example 7. FIG. 10 is a diagram illustrating all aberrations of Example 8. FIG. 10 is a diagram illustrating all aberrations of Example 9.

Explanation of symbols

10 ... Fiberscope (image guide fiber)
DESCRIPTION OF SYMBOLS 20 ... Eyepiece lens 30 ... Camera head 31 ... Diaphragm 32 ... Imaging lens 33 ... Solid-state image sensor 34 ... Negative power field lens 35 ... Light ray at the center of light beam (principal ray)
DESCRIPTION OF SYMBOLS 100 ... Brightness diaphragm 110 ... Negative power lens group 120 ... Positive power lens group 130 ... Negative power field lens 140 ... Imaging surface 210 ... Cover glass 220 ... Filter group 230 ... Cover glass of solid-state image sensor 310a, 310b ... Flare stop

Claims (13)

A camera head optical system comprising a negative power field lens in an optical system of a camera head having an imaging lens attached to an eyepiece portion of an endoscope for photographing. 2. The camera head optical system according to claim 1, wherein the imaging lens satisfies the following conditional expression.
f · tan (−tw) / h> 1.0 (1)
Here, f is the focal length of the imaging lens, h is the maximum image height on the imaging surface, and tw is the angle at which the central ray of the oblique ray bundle enters the imaging surface at the maximum image height. 3. The camera head optical system according to claim 1, wherein when the negative power field lens is the k-th lens element counting from the object side, the imaging lens satisfies the following conditional expression: 4. .
_k
| Σ1 / n _i f _i | ≦ (2F · p) / (F ² p ² + h ² ) (2)
^{i = 1}
Where n _i is the refractive index of the i-th lens element, f _i is the focal length of the i-th lens element, F is the F value of the imaging lens, h is the maximum image height on the imaging surface, and p is the imaging surface The pitch of the image sensor at. 4. The camera head according to claim 1, wherein the camera head has a configuration of an aperture, a negative lens group, a positive lens group, and a negative power field lens from the object side to the image side. Optical system. When the respective powers of the negative lens group, the positive lens group, and the negative power field lens are φ _n , φ _p , φ _f ,
_{| Φ n | / φ p <} φ p / | φ f |, φ p / | φ f | <1 ··· (3)
The camera head optical system according to claim 4, wherein: is satisfied. 6. The camera head optical system according to claim 4, wherein, in the negative lens group, a lens element closest to the object side has a meniscus shape with a concave surface facing the object side. 7. The camera head optical system according to claim 4, wherein the positive lens group includes a cemented lens of at least one convex lens and a concave lens. 8. The camera head optical system according to claim 4, wherein a lens element closest to the image side in the positive lens group has a meniscus shape with a convex surface facing the object side. 9. 9. The camera head optical system according to claim 4, wherein a filter for removing moire is disposed in front of the negative power field lens. 10. The camera head optical system according to claim 9, wherein the filter for removing moire is a joint of three quartz plates. 10. The camera head optical system according to claim 9, wherein the filter for removing moire comprises at least two quartz plates, and a member having a λ / 4 plate effect is sandwiched between the quartz plates. 12. The camera head optical system according to claim 11, wherein the member having the effect of the λ / 4 plate is the same crystal as the filter for removing the moire. 12. The camera head optical system according to claim 11, wherein the member having the effect of the λ / 4 plate is a birefringent polymer.

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