JP2008152210A - Endoscope objective lens - Google Patents

Abstract

An objective lens for an endoscope has a long back focus as compared with a focal length and corrects chromatic aberration of magnification satisfactorily.
A front group G1 having a negative refractive power as a whole, an aperture stop S, and a rear group G2 having a positive refractive power as a whole are arranged in order from the object side. The second lens L2 is composed of a negative first lens L1 having a flat surface or a convex surface facing the object side, and the rear group G2 is a positive single lens having a surface with a larger absolute value of the radius of curvature facing the object side. And a first cemented lens L34 composed of a biconvex third lens L3 and a meniscus negative fourth lens L4, a biconvex fifth lens L5 and a meniscus negative sixth lens L6. The second cemented lens L56 is arranged in order from the object side.
[Selection] Figure 3

Description

  The present invention relates to an endoscope objective lens, and more particularly to an endoscope objective lens suitable for use by arranging a prism or the like between the endoscope objective lens and its imaging surface. is there.

  Conventionally, an endoscope is used when observing or treating a patient's body in a medical field or the like. An observation objective lens is disposed at the distal end of the insertion portion of the endoscope. As such an objective lens, for example, three lenses whose rear group converging lens system after the aperture has a positive refractive power are used. There are known those composed of a group (see Patent Document 1) and those having two sets of cemented lenses in the rear group converging lens system after the stop (see Patent Document 2).

  Further, as the endoscope, a direct-view type that uses a solid-state image sensor and observes the long axis direction of the insertion portion is often used. In many endoscopes of this type, the light receiving surface of a solid-state image sensor is arranged in parallel to the long axis direction of the insertion portion. In such a configuration, an optical path is generally provided between the objective lens and the solid-state image sensor. An optical path conversion prism for bending by 90 degrees is inserted and arranged. For this reason, it is necessary to sufficiently secure the distance from the final surface of the objective lens to the imaging position, that is, the back focus, where the optical path conversion prism is inserted and arranged.

The size of the optical path conversion prism is determined by the image size. In recent years, the image size has been reduced by downsizing the solid-state imaging device. However, if there is not enough room between the prism wall and the effective luminous flux, flare and ghosting will occur. It is difficult to reduce the size in proportion to the image size. For this reason, an objective lens having a long back focus compared to the focal length is required.
Japanese Examined Patent Publication No. 61-46807 JP 2006-39259 A

  As described above, there is a demand for an objective lens having a long back focus compared to the focal length. However, the one described in Patent Document 1 does not take into consideration the use of the optical path conversion prism as described above, and thus does not have a long back focus.

  In addition, the endoscope objective lens is required to have a long back focus, and to have chromatic aberration corrected well in order to perform an accurate diagnosis. As a method for correcting chromatic aberration, for example, it is conceivable to arrange a cemented lens closest to the image plane to balance chromatic aberration.

  However, in a system with a long back focus, even if the cemented lens is arranged closest to the image plane side, the distance between the cemented lens and the imaging surface is long, so that the height of the light beam at the cemented lens is low, and the screen periphery is The effect of correcting chromatic aberration is weakened, and correction of chromatic aberration of magnification is insufficient.

  In view of the above circumstances, an object of the present invention is to provide an endoscope objective lens that has a long back focus compared to the focal length and in which chromatic aberration of magnification is favorably corrected.

  An endoscope objective lens according to the present invention includes, in order from the object side, a front group having a negative refractive power (hereinafter also referred to as power) as a whole, an aperture stop, and a rear group having a positive refractive power as a whole. And the front group is composed of a negative first lens having a flat surface or a convex surface facing the object side, and the rear group is directed to the surface having the larger absolute value of the radius of curvature toward the object side. A second lens that is a positive single lens, a first cemented lens composed of a biconvex third lens and a meniscus negative fourth lens, a biconvex fifth lens and a meniscus negative first lens A second cemented lens composed of six lenses is arranged in order from the object side.

  The endoscope objective lens of the present invention having the above configuration includes a front group of a diverging lens system and a rear group of a converging lens system, and constitutes a retrofocus type power arrangement with a long back focus, and a rear group Two sets of cemented lenses effective for chromatic aberration correction are provided on the image surface side of the lens to improve the effect of correcting chromatic aberration of magnification.

  Note that the object-side surface shape of the second lens may be any of a flat surface, a concave surface, and a convex surface. In the first cemented lens and the second cemented lens, the order of arrangement of the biconvex lens and the meniscus negative lens is not limited, and the biconvex lens and the meniscus negative lens are not limited. Any of them may be located on the object side.

The endoscope objective lens according to the present invention is preferably configured to satisfy the following conditional expressions (1) to (3).
0.8 <| f ₁ /f|<1.1 (1)
2.0 <D ₁₂ /f<3.5 (2)
0.6 <f ₅₆ / f ₃₄ <1.2 (3)
Where f ₁ is the focal length of the front group, f is the focal length of the entire system, D ₁₂ is the distance from the rear principal point of the front group to the front principal point of the rear group, f ₃₄ is the focal length of the first cemented lens, f ₅₆ is the focal length of said second cemented lens.

Furthermore, the endoscope objective lens of the present invention is preferably configured to satisfy the following conditional expression (4).
n ₁ > 2.0 (4)
Here, n ₁ is the refractive index of the first lens.

  Furthermore, in the endoscope objective lens of the present invention, it is preferable that the first cemented lens and the second cemented lens have the same configuration.

  Note that the endoscope objective lens of the present invention has a “back focus longer than the focal length”, but this is preferably so that the following conditional expression (5) is satisfied. It has a back focus that is at least twice as long as the distance.

Bf / f ≧ 2.0 (5)
However, Bf is the back focus Bf (air equivalent length), f is the focal length of the entire system, and Bf / f is called the back focus ratio.

  The above refractive index, focal length, and distance between principal points relate to the d-line (wavelength 587.6 nm).

  According to the endoscope objective lens of the present invention, in order from the object side, a front group having a negative refractive power as a whole and a rear group having a positive refractive power as a whole are arranged, and the retrofocus type lens is arranged. In addition to constituting the power arrangement, two sets of cemented lenses effective for chromatic aberration correction are arranged on the image surface side of the rear group, so that it has a long back focus compared to the focal length and corrects chromatic aberration of magnification well. can do.

  Hereinafter, embodiments of an endoscope objective lens according to the present invention will be described in detail with reference to the drawings. The endoscope objective lens of the present embodiment is provided at the distal end portion of the endoscope insertion portion, and is used with an optical path conversion prism or the like disposed between the endoscope objective lens and the imaging plane. Is. FIG. 1 is a diagram showing a distal end surface of a distal end portion 1 where an endoscope objective lens according to the present embodiment is arranged, and FIG. 2 is a cross section including an optical axis of the endoscope objective lens shown in FIG. It is principal part sectional drawing of the front-end | tip part 1 in the -A cross section.

  As shown in FIG. 1, the distal end surface of the distal end portion 1 has an observation window 3 that is an outer surface of the endoscope objective lens 2, two illumination windows 4 arranged on both sides of the observation window 3, and a treatment instrument guide. An outlet 5 and an air / water supply nozzle 6 are formed.

  Further, as shown in FIG. 2, an endoscope objective lens 2 in which the optical axis is arranged in parallel to the major axis direction of the distal end portion 1 inside the distal end portion 1, and the endoscope objective lens 2. An optical path conversion prism 7 for bending the optical path on the image side by 90 degrees and a solid-state imaging device 8 bonded to the optical path conversion prism 7 so that the light receiving surface thereof is parallel to the major axis direction of the tip 1 are disposed. Yes. By arranging the solid-state imaging device 8 in this way, a direct-view type observation optical system is configured in the lower half of the distal end portion 1 shown in FIG. 2, and the treatment instrument is inserted in the upper half of the distal end portion 1 shown in FIG. A channel 9 is formed, and a large number of elements are arranged in the narrow insertion portion.

  The solid-state imaging device 8 has a cover glass for protecting the light-receiving surface, but in FIG. 1 and FIG. 2, the solid-state imaging device 8 including the cover glass is illustrated. In FIG. 2, the optical axis of the endoscope objective lens 2 is indicated by a one-dot chain line. The endoscope objective lens 2 shown in FIG. 2 does not show a lens shape but is conceptually illustrated. As can be seen from FIG. 2, the endoscope objective lens 2 requires a long back focus because the optical path conversion prism 7 is disposed between the endoscope objective lens 2 and the imaging plane.

  FIG. 3 shows a lens configuration diagram of the endoscope objective lens 2 according to the embodiment of the present invention. Note that the configuration example shown in FIG. 3 corresponds to the lens configuration of Example 1 described later. The endoscope objective lens of FIG. 3 has a four-group, six-element configuration, and in order from the object side, the front group G1 having a negative refractive power as a whole, an aperture stop S, and a positive refractive power as a whole. The rear group G2 is arranged, the front group G1 is composed of a negative first lens L1 having a plane or a convex surface facing the object side, and the rear group G2 has a larger absolute value of the radius of curvature on the object side. A second lens L2 that is a positive single lens facing the surface, a first cemented lens L34 that includes a biconvex third lens L3 and a meniscus negative fourth lens L4, and a biconvex second lens. A fifth lens L5 and a second cemented lens L56 including a meniscus negative sixth lens L6 are arranged in order from the object side.

  In FIG. 3, instead of the optical path conversion prism 7 and the cover glass for the solid-state image sensor 8 shown in FIG. 2, a prism P1 having an optical optical path length equivalent to the sum of these optical optical path lengths is used. The entire image forming plane coincides with the image-side surface of the prism P1.

  Below, the preferable aspect as an objective lens for endoscopes of this embodiment is demonstrated. Note that the refractive index, focal length, and distance between principal points described below relate to the d-line (wavelength 587.6 nm) unless otherwise specified.

The endoscope objective lens according to this embodiment is preferably configured to satisfy the following conditional expressions (1) to (3), and is configured as such in the example shown in FIG.
0.8 <| f ₁ /f|<1.1 (1)
2.0 <D ₁₂ /f<3.5 (2)
0.6 <f ₅₆ / f ₃₄ <1.2 (3)
However, f ₁ is the focal length of the front group G1, f is the focal length of the entire system, D ₁₂ is the distance to the front principal point of the rear group G2 from the side principal point of the front group G1, f ₃₄ is the focal length of the first cemented lens _{L34, f 56} is the focal length of the second cemented lens L56.

The endoscope objective lens according to the present embodiment is preferably configured to satisfy the following conditional expression (4).
n ₁ > 2.0 (4)
However, _{n 1} is the refractive index of the first lens L1.

  Furthermore, in the endoscope objective lens according to the present embodiment, it is preferable that the first cemented lens L34 and the second cemented lens L56 have the same configuration, and in the example shown in FIG. Has been.

Further, in the endoscope objective lens according to the present embodiment, the back focus which is longer than the focal length, specifically, is longer than the focal length so as to satisfy the following conditional expression (5). It is preferable that the back focus is secured.
Bf / f ≧ 2.0 (5)
However, Bf is the back focus Bf (air equivalent length), f is the focal length of the entire system, and Bf / f is called the back focus ratio.

  The operation and effect of the endoscope objective lens configured as described above will be described in detail. Conditional expressions (1) and (2) are expressions for obtaining a sufficiently long back focus while satisfactorily correcting the lateral chromatic aberration. The sufficient length is a length (air equivalent length) that is at least twice the focal length of the entire system. Generally, when a cemented lens having a thick center thickness is inserted as compared with a single lens, the back focus is shortened. However, by defining conditional expressions (1) and (2), the objective lens for an endoscope according to this embodiment is used. Therefore, a long back focus can be secured while having two sets of cemented lenses. Hereinafter, this point will be described.

For simplicity, a system of two thin lenses will be considered by replacing the front group G1 and the rear group G2 with thin lenses. Of these two thin lenses, if the focal length of one lens (front lens) is fa, the focal length of the other lens (back lens) is fb, and the distance between the two thin lenses is dx, The combined focal length f of the two thin lenses at this time is
1 / f = 1 / fa + 1 / fb−dx / (fa · fb) (i)
And the back focus Bf is
Bf = f (1-dx / fa) (ii)
It is represented by Each symbol includes not only the size but also the direction (symbol).

Here, Bf = 2f (iii) where the length of the required back focus Bf is twice the combined focal length f.
Then, formula (ii) is
2f = f (1-dx / fa)
And
dx = −fa (iv)
Is obtained.

Considering that the front group G1 of the embodiment described above has a negative refractive power as a whole and the rear group G2 has a positive refractive power as a whole, assuming that fa <0 and fb> 0,
| Dx / fa | ≧ 1 (v)
When,
Bf ≧ 2f (vi)
Is realized.

The above is the case of a thin lens. In the objective lens as shown in FIG. 3, the position of the rear principal point of the rear group G2 enters the rear group G2 (on the object side) substantially by the focal length of the entire system. For this reason, considering this as a correction amount, the sum of the back focus Bf having a length twice as long as the focal length f of the entire system and the amount f of the rear principal point position entering the rear group G2. A correction value considering a length of three times a certain focal length f is BF,
BF = 3f (vii)
Is used instead of Bf = 2f in equation (iii),
| Dx / fF | ≧ 2 (viii)
The following conditional expression is obtained.

  By satisfying this conditional expression, in the objective lens of the present embodiment, a long back focus that is at least twice the focal length of the entire system can be secured, and conditional expression (5) can be satisfied. Then, the subject image is placed on the solid-state image sensor in a state where the optical path conversion prism is inserted between the solid-state image sensor in which the light receiving surface is arranged in parallel to the long axis direction of the insertion portion of the endoscope and the objective lens. An image can be formed.

  Equation (viii) is the ratio of the distance between the principal points of the front group G1 and the rear group G2 and the focal length of the front group G1, and has a wide range of application, but it is a dimension that can be realized in an actual optical system. Is considered, the range is limited as in conditional expressions (1) and (2).

  Conditional expression (1) defines an appropriate range for the ratio of the focal length of the front group G1 to the focal length of the entire system. If the lower limit of conditional expression (1) is exceeded, the power of the front group G1, which is a divergent optical system, becomes too strong, and various aberrations, particularly astigmatism, deteriorate. If the upper limit of conditional expression (1) is exceeded, the power of the front group G1, which is a divergent optical system, is too weak to obtain sufficient back focus.

  Conditional expression (2) defines an appropriate range for the ratio of the distance from the rear principal point of the front group G1 to the front principal point of the rear group G2 and the focal length of the entire system. If the lower limit of conditional expression (2) is exceeded, astigmatism will deteriorate as in conditional expression (1). Exceeding the upper limit of conditional expression (2) means an increase in the distance between the front group G1 and the rear group G2, leading to an increase in the lens diameter of the front group G1, which is a diverging optical system, and is an endoscope that is desired to be downsized. This is an undesirable result for a mirror objective lens.

  Conditional expression (3) defines an appropriate range for the ratio of the focal lengths of the two cemented lenses included in the rear group G2. In order to suppress the generation of various aberrations, particularly field curvature and astigmatism, it is necessary to avoid that one power is extremely large compared to the other power, and it is desirable that the power is approximately the same. If the lower limit of conditional expression (3) is exceeded, the power of the second cemented lens L56 will increase, and the light beam toward the periphery will be greatly bent at a high position, so that field curvature and astigmatism will be increased. Increase. If the upper limit of conditional expression (3) is exceeded, the power of the first cemented lens L34 becomes too strong, which is not preferable for increasing the back focus.

  Conditional expression (4) defines an appropriate range for the refractive index of the first lens L1 of the front group G1. Conditional expression (4) shows a suitable condition for moving the image forming position by the rear group G2 of the object surface away from the image forming position of the entire system when the image side surface of the first lens L1 is an object surface. ing. Hereinafter, this point will be described in detail.

  The endoscope observes a distance in the advancing direction during insertion of the insertion portion, and observes the very vicinity of the distal end portion after insertion of the insertion portion, so that a sufficiently deep depth of field is required. For this reason, the endoscope objective lens is an optical system having a large F-number. Therefore, dust or the like attached to the surface of the lens closest to the object may be observed. In the present embodiment, the object-side surface of the first lens L1, which is the most object-side lens, is not a concave shape but a flat or convex shape so that dust does not collect easily. Further, since the object side surface of the first lens L1 is exposed to the outside, even if dust or the like adheres to this portion, it can be removed by the gas or liquid ejected from the air / water feeding nozzle 6. However, dust attached to the image side surface of the first lens L1 cannot be removed.

  Therefore, in the objective lens of the present embodiment, in order to prevent dust attached to the image side surface of the first lens L1 from being observed, the result of the rear group G2 when the image side surface is used as an object surface. It is effective to keep the image position and the imaging position of the entire system where the image sensor is arranged as far as possible.

  The distance between the front group G1 and the rear group G2 may be reduced in order to keep the two imaging positions away without changing the focal length of the lens system and the entire system of the rear group G2. This interval corresponds to the interval dx in the above equation (i). In order to reduce the distance between the front group G1 and the rear group G2 from the equation (i), the focal length of the first lens L1 in the front group G1 can be shortened, that is, the power of the first lens L1 in the front group G1 is increased. do it. For this purpose, the radius of curvature of the first lens L1 may be reduced or the refractive index of the glass material used may be increased. The endoscope objective lens originally has a small radius of curvature as a whole. Therefore, if the radius of curvature is further reduced, the workability deteriorates, which is not preferable. Therefore, it is preferable to use a glass material having a high refractive index. ) Is derived.

  In the objective lens according to the present embodiment, when the first cemented lens L34 and the second cemented lens L56 are made to have the same configuration, they can be made into common parts, reducing the production cost and producing. Can be improved.

  Next, specific numerical examples of the endoscope objective lens according to the present embodiment will be described.

<Example 1>
Table 1 shows the specification values of the endoscope objective lens according to Example 1, and FIG. In Table 1, Si indicates the i-th (i = 1 to 13) surface number that sequentially increases toward the image side with the surface of the component closest to the object side as the first. Ri represents the radius of curvature of the i-th surface, and Di represents the surface interval on the optical axis Z between the i-th surface and the i + 1-th surface. Ndj represents the refractive index with respect to the d-line (wavelength: 587.6 nm) of the j-th lens (j = 1 to 7), which gradually increases toward the image side with the most object-side lens as the first lens, and νdj Indicates the Abbe number for the d-line of the j-th lens or prism. In Table 1, the unit of the radius of curvature and the surface interval is mm, and the radius of curvature is positive when convex on the object side and negative when convex on the image side. The meanings of the symbols in the table are the same for Tables 2 to 6 and Table 9.

  Symbols Ri and Di in FIG. 3 correspond to Ri and Di in Table 1. In FIG. 3, the stop S in the drawing does not indicate the shape or size but indicates the position on the optical axis Z, and this also applies to other lens configuration diagrams. The reference numerals in Table 1 and FIG. 3 also include the stop S and the prism P1.

  FIG. 4 shows an imaging position PZ by the rear group G2 when the image side surface S2 of the first lens L1 in the first embodiment is an object surface. As described above in the description of Expression (4), the difference DZ in the distance between the imaging position of the entire system (the position on the image side surface of the prism P1) and the imaging position PZ is preferably larger. The DZ in 1 is 0.888 mm.

  In the first embodiment, the first cemented lens L34 and the second cemented lens L56 have the same configuration and are designed with high productivity.

<Example 2>
Table 2 shows the specification values of the endoscope objective lens according to Example 2, and FIG. In FIG. 5, symbols Ri and Di correspond to Ri and Di in Table 2.

  In Example 1 and Example 2, the rear group G2 is common, and only the focal length of the front group G1 and the interval between the front group G1 and the rear group G2 are different. More specifically, compared with Example 1, Example 2 uses a glass material of the first lens L1 having a high refractive index to increase the power of the first lens L1, thereby increasing the air in the front group G1 and the rear group G2. The interval is narrowed. By merely making these changes from Example 1, in Example 2, the focal length of the entire system is kept the same, and the image is formed by the rear group G2 when the image side surface of the first lens L1 is the object surface. It is possible to move the image position PZ away from the imaging position of the entire system. This means that in the second embodiment, the dust attached to the image side surface of the lens L1 is less likely to be focused and less noticeable than the first embodiment. Specifically, DZ in Example 2 is 1.019 mm, which is 0.131 mm longer than DZ in Example 1. This increment corresponds to about 26% of the focal length of the entire system.

<Example 3>
Table 3 shows the specification values of the endoscope objective lens according to Example 3, and FIG. In FIG. 6, symbols Ri and Di correspond to Ri and Di in Table 3. DZ in Example 3 is 0.974 mm.

<Example 4>
Table 4 shows specification values of the endoscope objective lens according to Example 4, and FIG. 7 shows a lens configuration diagram. In FIG. 7, symbols Ri and Di correspond to Ri and Di in Table 4.

  Similar to the relationship between the first embodiment and the second embodiment, the rear group G2 is common in the third embodiment and the fourth embodiment, and the focal length of the front group G1 and the front group G1 and the rear group G2 are the same. Only the interval is different. The DZ in Example 4 is 1.116 mm, which is 0.142 mm longer than the DZ in Example 3. This increment corresponds to about 28% of the focal length of the entire system.

<Example 5>
Table 5 shows the specification values of the endoscope objective lens according to Example 5, and FIG. In FIG. 8, symbols Ri and Di correspond to Ri and Di in Table 5. DZ in Example 5 is 1.067 mm.

<Example 6>
Table 6 shows specification values of the endoscope objective lens according to Example 6, and FIG. 9 shows a lens configuration diagram. In FIG. 9, symbols Ri and Di correspond to Ri and Di in Table 6.

  Similar to the relationship between Example 1 and Example 2 above, Example 5 and Example 6 share the rear group G2, and the focal length of the front group G1 and the front group G1 and the rear group G2 Only the interval is different. The DZ in Example 6 is 1.226 mm, which is 0.159 mm longer than the DZ in Example 5. This increment corresponds to about 32% of the focal length of the entire system.

  FIGS. 10 to 15 show aberration diagrams of spherical aberration, astigmatism, distortion (distortion), and lateral chromatic aberration of the objective lens for endoscope according to Examples 1 to 6, respectively. Each aberration diagram shows aberrations with the d-line as a reference wavelength, but the spherical aberration diagram and lateral chromatic aberration diagram also show aberrations for the F-line (wavelength 486.1 nm) and C-line (wavelength 656.3 nm). . In the astigmatism diagram, the solid line indicates the sagittal direction and the broken line indicates the tangential direction. F in the spherical aberration diagram is an F value, and ω in the other aberration diagrams is a half angle of view. As can be seen from FIG. 10 to FIG. 15, each of the aberrations in Examples 1 to 6 is corrected well.

The image size, the object distance, the angle of view, the focal length f of the entire system, the back focus Bf (air conversion length), the focal length f _{1 of} the front group G1, and the rear main of the front group G1 in the _first to sixth embodiments. The point position, the front principal point position of the rear group G2, the distance D ₁₂ from the rear principal point position of the front group G1 to the front principal point position of the rear group G2, the focal length f _{34 of} the first cemented lens L34, the second Table 7 shows differences in focal lengths f ₅₆ , DZ, and DZ of the cemented lens L56 (difference in DZ in the two examples in which the rear group G2 is common). In Table 7, the unit of angle of view is degrees, and all other units are mm.

  Table 8 shows the values of conditional expressions (1) to (3) and (5) in Examples 1 to 6. As is apparent from Table 8, Examples 1 to 6 satisfy the conditional expressions (1) to (3) and (5).

  Next, as a comparative example, an example in which scaling is performed on Example 1 of Patent Document 1 so as to have a focal length equivalent to that of the present example will be described. The specification values of the comparative example are shown in Table 9, and the lens configuration diagram is shown in FIG. FIG. 17 shows aberration diagrams of the spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the comparative example. The meanings of the symbols shown in FIG. 17 are the same as those in the above-described embodiment of the present invention. Table 10 shows the image size, object distance, angle of view, focal length f of the entire system, back focus Bf, and the value of conditional expression (5) (back focus ratio) in the comparative example. In Table 10, the unit of angle of view is degrees, and the other units excluding the back focus ratio are mm.

  As shown in FIG. 16, this comparative example has, in order from the object side, a four-group five-lens configuration including a negative lens, a positive lens, a positive lens, a biconvex positive lens, and a meniscus negative lens cemented lens. In this comparative example, the distance from the lens closest to the image side to the imaging plane P is short, and it is difficult to arrange a prism or the like.

  Comparing the example of the present invention and the comparative example, the value of the conditional expression (5) (back focus ratio) of the comparative example is less than 1 as shown in Table 10, and the back focus is short. The value of conditional expression (5) in the example is 2 or more and the back focus is long. In addition, each aberration, particularly the chromatic aberration of magnification, is corrected as well as or better than that of the comparative example.

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

The figure which shows the front end surface of the front-end | tip part by which the objective lens for endoscopes concerning embodiment of this invention is arrange | positioned. Sectional drawing of the principal part of the front-end | tip part in the AA sectional view of FIG. The figure which shows the lens structure of the objective lens for endoscopes concerning Example 1 of this invention. The figure for demonstrating the image formation position by a rear group when the image side surface of a 1st lens is made into an object surface. The figure which shows the lens structure of the objective lens for endoscopes concerning Example 2 of this invention. The figure which shows the lens structure of the objective lens for endoscopes concerning Example 3 of this invention. The figure which shows the lens structure of the objective lens for endoscopes concerning Example 4 of this invention. The figure which shows the lens structure of the objective lens for endoscopes concerning Example 5 of this invention. The figure which shows the lens structure of the objective lens for endoscopes concerning Example 6 of this invention. Each aberration diagram of the endoscope objective lens according to Example 1 of the present invention Each aberration diagram of the endoscope objective lens according to Example 2 of the present invention Each aberration diagram of the endoscope objective lens according to Example 3 of the present invention Each aberration diagram of the endoscope objective lens according to Example 4 of the present invention Each aberration diagram of the endoscope objective lens according to Example 5 of the present invention Each aberration diagram of the endoscope objective lens according to Example 6 of the present invention The figure which shows the lens structure of the objective lens for endoscopes concerning a comparative example Aberration diagram of endoscope objective lens according to comparative example

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Tip part 2 Endoscope objective lens 3 Observation window 4 Illumination window 5 Treatment tool outlet 6 Air supply / water supply nozzle 7 Optical path conversion prism 8 Solid imaging device 9 Treatment tool insertion channel Di (i = 1 to 12) i th G1 front group G2 rear group L1 first lens L2 second lens L3 third lens L34, L56 cemented lens L4 fourth lens L5 fifth lens L6 sixth lens P1 Prism Ri (i = 1 to 13) Radius of curvature of i-th surface S Aperture Z Optical axis

Claims (4)

In order from the object side, a front group having negative refractive power as a whole, an aperture stop, and a rear group having positive refractive power as a whole are arranged,
The front group includes a negative first lens having a flat surface or convex surface facing the object side,
The rear group includes a second lens that is a positive single lens having a surface with a larger absolute value of the radius of curvature toward the object side, a biconvex third lens, and a meniscus negative fourth lens. An endoscope objective comprising: a first cemented lens; and a second cemented lens including a biconvex fifth lens and a meniscus negative sixth lens arranged in order from the object side. lens. 2. The endoscope objective lens according to claim 1, wherein the objective lens is configured to satisfy the following conditional expressions (1) to (3).
0.8 <| f ₁ /f|<1.1 (1)
2.0 <D ₁₂ /f<3.5 (2)
0.6 <f ₅₆ / f ₃₄ <1.2 (3)
Where f ₁ : focal length of the front group f: focal length of the entire system D ₁₂ : distance from the rear principal point of the front group to the front principal point of the rear group f ₃₄ : of the first cemented lens Focal length f ₅₆ : Focal length of the second cemented lens The endoscope objective lens according to claim 1 or 2, characterized in that the following conditional expression (4) is satisfied.
n ₁ > 2.0 (4)
Where n _{1 is} the refractive index of the first lens.   The objective lens for an endoscope according to any one of claims 1 to 3, wherein the first cemented lens and the second cemented lens have the same configuration.

Images