Objective lens for endoscope

[0009] This application claims the priority of Japanese Patent Application No. 11-266242 filed on Sep. 20, 1999, which is incorporated herein by reference.

[0010] 1. Field of the Invention

[0011] The present invention relates to an objective lens attached to a tip portion of an endoscope; and, in particular, to an endoscope objective lens which can be used in normal viewing and close-up magnified viewing states.

[0012] 2. Description of the Prior Art

[0013] For utilizing endoscopes mainly in the medical field, image transmission by use of solid-state imaging devices such as CCD and image guide fibers, and analysis and viewing of thus captured images have come into practice, whereby demands for viewing with a higher accuracy and more accurate analysis have been increasing in recent years. Therefore, as to objective lenses, it has been desired to develop an endoscope objective lens which can obtain favorable images over a wide viewing range.

[0014] When viewing an object such as a diseased part, it is preferable that not only overview but also detailed viewing of a part under magnification can be carried out. In response to such a demand, a lens with a great depth of field has conventionally been used in general so as to widen the viewing range between its far and close objects.

[0015] On the other hand, as an endoscope objective lens which is switchable between a normal viewing (wide) state suitable for overview and a close-up magnified viewing (telephoto) state suitable for partial viewing, one disclosed in Japanese Patent No. 2876252, for example, has been known. This endoscope objective lens comprises four groups consisting of negative, positive, negative, and positive lens groups successively from the object side, in which the third group is moved along the optical axis, whereby it can adjust its focusing onto an object to view at a position outside its depth of field. Depending on the position of this moving lens group, viewing can be carried out not only at both ends of the normal viewing (wide) state and close-up magnified viewing (telephoto) state, but also in the intermediate region therebetween.

[0016] Though this endoscope objective lens enables viewing from the normal viewing state to the close-up magnified viewing state, there is a limitation on the magnification in use of the objective lens in the intermediate region. Namely, though the objective lens is normally capable of taking any object distance ranging from the normal viewing state to magnified viewing state with respect to a given object to view, the magnification in use of the objective lens would be uniquely determined upon focusing onto this object distance by use of internal lens movement.

[0017] For carrying out viewing with a higher accuracy or more accurate analysis, there is a demand for an objective lens having a higher degree of freedom in its magnification in use even in the viewing state in the intermediate region between both ends.

[0018] In view of such circumstances, it is an object of the present invention to provide a compact endoscope objective lens which can arbitrarily change its magnification in use in an intermediate region between both ends of its normal viewing state and close-up magnified viewing state, while various kinds of aberration are favorably corrected therein.

[0019] The endoscope objective lens in accordance with the present invention comprises, successively from an object side, a first lens group having a negative refracting power, a second lens group having a positive refracting power, a third lens group having a negative refracting power, and a fourth lens group having a positive refracting power; wherein two lens groups consisting of the third lens group and one of the second and fourth lens groups are moved along an optical axis so as to change magnification and carry out focusing; the endoscope objective lens further satisfying the following conditional expressions (1) and (2):

[0020] 2.0<β_1T/β_1W<10.0  (1)

[0021] 1.2<f_T/f_W  (2)

[0022] where

[0023] β_1Tis the magnification of the first lens group at the telephoto end;

[0024] f_1Wis the magnification of the first lens group at the wide end;

[0025] f_Tis the focal length of the whole lens at the telephoto end; and

[0026] f_Wis the focal length of the whole lens at the wide end.

[0027] Preferably, in the endoscope objective lens in accordance with the present invention, two lens groups consisting of the second and third lens groups are moved along the optical axis so as to change magnification and carry out focusing; the endoscope objective lens further satisfying the following conditional expressions (3), (6), and (7):

[0028] 0.1<|d₂|/f₂<1.0  (3)

[0029] 0.7<f₂/f_W<4.0  (6)

[0030] −3.0<f₃/f_W<−0.5  (7)

[0031] where

[0032] d₂is the moving distance of the second lens group from the wide end to the telephoto end;

[0033] f₂is the focal length of the second lens group;

[0034] f₃is the focal length of the third lens group; and

[0035] f_Wis the focal length of the whole lens at the wide end.

[0036] Preferably, in the endoscope objective lens in accordance with the present invention, two lens groups consisting of the third and fourth lens groups are moved along the optical axis so as to change magnification and carry out focusing; the endoscope objective lens further satisfying the following conditional expressions (4), (7), and (8):

[0037] 0.1<|d₄|/f₄<0.8  (4)

[0038] −3.0<f₃/f_W<−0.5  (7)

[0039] 1.0<f₄/f_W<2.5  (8)

[0040] where

[0041] d₄is the moving distance of the fourth lens group from the wide end to the telephoto end;

[0042] f₃is the focal length of the third lens group;

[0043] f₄is the focal length of the fourth lens group; and

[0044] f_Wis the focal length of the whole lens at the wide end.

[0045] Preferably, in the endoscope objective lens in accordance with the present invention, a stop is disposed between the second and third lens groups; the endoscope objective lens further satisfying the following conditional expression (5):

[0046] 0.4<β_3.4T/β_3.4W<2.5  (5)

[0047] where

[0048] β_3.4Tis the composite magnification of the third and fourth lens groups at the telephoto end; and

[0049] β_3.4Wis the composite magnification of the third and fourth lens groups at the wide end.

[0050] FIGS. 1A and 1B are views showing the configuration of the endoscope objective lens in accordance with Example 1 of the present invention;

[0051] FIGS. 2A and 2B are views showing the configuration of the endoscope objective lens in accordance with Example 2 of the present invention;

[0052] FIGS. 3A and 3B are views showing the configuration of the endoscope objective lens in accordance with Example 3 of the present invention;

[0053] FIGS. 4A and 4B are views showing the configuration of the endoscope objective lens in accordance with Example 4 of the present invention;

[0054] FIGS. 5A and 5B are views showing the configuration of the endoscope objective lens in accordance with Example 5 of the present invention;

[0055] FIGS. 6A to 6H are aberration charts of the endoscope objective lens in accordance with Example 1 of the present invention;

[0056] FIGS. 7A to 7H are aberration charts of the endoscope objective lens in accordance with Example 2 of the present invention;

[0057] FIGS. 8A to 8H are aberration charts of the endoscope objective lens in accordance with Example 3 of the present invention;

[0058] FIGS. 9A to 9H are aberration charts of the endoscope objective lens in accordance with Example 4 of the present invention; and

[0059] FIGS. 10A to 10H are aberration charts of the endoscope objective lens in accordance with Example 5 of the present invention;

[0060] In the following, endoscope objective lenses in accordance with two embodiments of the present invention will be explained with reference to the drawings.

[0061] To begin with, the endoscope objective lens in accordance with the first embodiment will be explained with reference to FIGS. 1A and 1B. These drawings show a basic configuration of the lens in accordance with Example 1.

[0062] As shown in FIGS. 1A and 1B, this endoscope objective lens comprises, successively from the object side, a first lens group G₁having a negative refracting power; a second lens group G₂, movable along the optical axis X, having a positive refracting power; a third lens group G₃, movable along the optical axis X, having a negative refracting power; and a fourth lens group G₄having a positive refracting power.

[0063] As shown in FIGS. 1A and 1B, this endoscope objective lens is configured such that, from its normal viewing (wide) state to close-up magnified (telephoto) state, the second lens group G₂moves toward the object along the optical axis, while the third lens group G₃moves toward the image along the optical axis at the same time, so as to make it possible to change magnification and carry out focusing.

[0064] Here, the normal viewing (wide) state refers to a state having a longer object distance and a wider angle of view, thus being suitable for viewing over a wide area. On the other hand, the close-up magnified (telephoto) state refers to a state having a shorter object distance, thus being suitable for partly-enlarged viewing. The wide end shown in FIG. 1A refers to a state where the whole system attains its lowest magnification in the normal viewing state as the movable lens groups are moved to their predetermined positions. On the other hand, the telephoto end shown in FIG. 1B refers to a state where the whole system attains its highest magnification in the close-up magnified state as the movable lens groups are moved to their predetermined positions.

[0065] Further, in this objective lens, a stop 1 is disposed between the second lens group G₂and the third lens group G₃; an optical path changing prism 2 is disposed on the image side from the fourth lens group G₄; and, though not depicted, a CCD device or image guide fiber is disposed on the image side from the prism 2, so that image information is transmitted thereto. In FIGS. 1A and 1B, the stop 1 is disposed so as be in contact with the surface of third lens group G₃closest to the object on the optical axis.

[0066] Also, this objective lens is configured so as to satisfy the following conditional expressions (1) to (3) and (5) to (7):

[0067] 2.0<β_1T/β_1W<10.0  (1)

[0068] 1.2<f_T/f_W  (2)

[0069] 0.1<|d₂|f₂<1.0  (3)

[0070] 0.4<β_3.4T/β_3.4W<2.5  (5)

[0071] 0.7<f₂/f_W<4.0  (6)

[0072] −3.0<f₃/f_W<−0.5  (7)

[0073] where

[0074] β_1Tis the magnification of the first lens group G₁at the telephoto end;

[0075] β_1Wis the magnification of the first lens group G₁at the wide end;

[0076] f_Tis the focal length of the whole lens at the telephoto end;

[0077] f_Wis the focal length of the whole lens at the wide end;

[0078] d₂is the moving distance of the second lens group G₂from the wide end to the telephoto end;

[0079] f₂is the focal length of the second lens group G₂;

[0080] f₃is the focal length of the third lens group G₃;

[0081] β_3.4Tis the composite magnification of the third lens group G₃and fourth lens group G₄at the telephoto end; and

[0082] β_3.4Wis the composite magnification of the third lens group G₃and fourth lens group G₄at the wide end.

[0083] Next, the endoscope objective lens in accordance with the second embodiment will be explained with reference to FIGS. 5A and 5B. These drawings show a basic configuration of the lens in accordance with Example 5.

[0084] As shown in FIGS. 5A and 5B, this endoscope objective lens comprises, successively from the object side, a first lens group G, having a negative refracting power; a second lens group G₂having a positive refracting power; a third lens group G₃, movable along the optical axis X, having a negative refracting power; and a fourth lens group G₄, movable along the optical axis X, having a positive refracting power.

[0085] As shown in FIGS. 5A and 5B, this endoscope objective lens is configured such that, from its normal viewing (wide) state to close-up magnified (telephoto) state, the third lens group G₃moves toward the image along the optical axis, while the fourth lens group G₄moves toward the object along the optical axis at the same time, so as to make it possible to change magnification and carry out focusing.

[0086] Further, this objective lens is provided with a stop 1 and a CCD cover glass 2 as in the first embodiment; and, though not depicted, a CCD device or image guide fiber is disposed on the image side from the cover glass 2, so that image information is transmitted thereto. In FIGS. 5A and 5B, the stop 1 is disposed so as be in contact with the surface of third lens group G₃closest to the object on the optical axis.

[0087] Also, this objective lens is configured so as to satisfy the following conditional expressions (1), (2), (4), (5), (7), and (8):

[0088] 2.0<β_1T/β_1W<10.0  (1)

[0089] 1.2<f_T/f_W  (2)

[0090] 0.1<|d₄|/f₄<0.8  (4)

[0091] 0.4<β_3.4T/β_3.4W<2.5  (5)

[0092] −3.0<f₃/f_W<−0.5  (7)

[0093] 1.0<f₄/f_W<2.5  (8)

[0094] where

[0095] β_1Tis the magnification of the first lens group G₁at the telephoto end;

[0096] β_1Wis the magnification of the first lens group G₁at the wide end;

[0097] f_Tis the focal length of the whole lens at the telephoto end;

[0098] f_Wis the focal length of the whole lens at the wide end;

[0099] d₄is the moving distance of the fourth lens group G₄from the wide end to the telephoto end;

[0100] f₃is the focal length of the third lens group G₃;

[0101] f₄is the focal length of the fourth lens group G₄;

[0102] β_3.4Tis the composite magnification of the third lens group G₃and fourth lens group G₄at the telephoto end; and

[0103] β_3.4Wis the composite magnification of the third lens group G₃and fourth lens group G₄at the wide end.

[0104] In an endoscope objective lens having a configuration such as those of the first and second embodiments, the magnification in use of the objective lens can be changed arbitrarily while carrying out focusing not only in the normal viewing and close-up magnified states but also in their intermediate region, so as to yield a magnification suitable for the object to view and its circumstances of use.

[0105] As will be explained in the following, the changes in magnification in these embodiments are obtained by an interaction between those generated upon the change in object distance shown in conditional expression (1) and those generated by appropriately moving the two movable groups as shown in conditional expression (2).

[0106] Conditional expression (1) defines the telephoto/wide magnification ratio in the first lens group G₁. While β_1T/β_1W=1 in a zoom in general, conditional expression (1) indicates that the objective lens changes its magnification by utilizing the fact that the object to view looks greater when the object distance becomes shorter as the objective lens approaches the telephoto end when shifting from the wide state to the telephoto state. If the lower limit of this expression is not satisfied, then the magnification becomes insufficient on the telephoto side. If the upper limit of this expression is exceeded, then the object distance on the telephoto side becomes too short, whereby parts not sufficiently irradiated with illumination light from a light guide may be viewed.

[0107] Conditional expression (2) shows a value corresponding to the zoom ratio in zoom lenses in general. It indicates that two lens groups consisting of the second lens group G₂and third lens group G₃or the third lens group G₃and fourth lens group G₄are moved, so as to change the focal length, thereby changing the magnification. If the ratio in focal length between the telephoto and wide ends is smaller than the lower limit of this expression, then the object distance becomes too short when the same magnification is to be attained on the telephoto end, whereby parts not sufficiently irradiated with illumination light from a light guide may be viewed.

[0108] Since two lens groups are movable as shown in conditional expression (2), the magnification in use can be changed arbitrarily while carrying out focusing with respect to a single object distance.

[0109] For example, in the case where the object distance is fixed at 2.2032 in Example 1 which will be explained later, the positions of two movable groups, i.e., positions of the second lens group G₂and third lens group G₃, can be set to the following types 1 and 2, both of which are in focus. Here, the lens group positions at this object distance are not restricted to these two types. In the process of moving from the wide end to telephoto end, combinations are possible at any position in their intermediate region, whereby the magnification in use can arbitrarily be changed in the intermediate region from the wide end to telephoto end according to the arrangement of two movable groups.

[0110] Type 1

[0111] variable value 1 (lens space between the first lens group G₁and second lens group G₂): 0.8000

[0112] variable value 2 (lens space between the second lens group G₂and third lens group G₃): 0.6615

[0113] variable value 3 (lens space between the third lens group G₃and fourth lens group G₄): 0.3827

[0114] Type 2

[0115] variable value 1 (lens space between the first lens group G₁and second lens group G₂): 0.2272

[0116] variable value 2 (lens space between the second lens group G₂and third lens group G₃): 0.9343

[0117] variable value 3 (lens space between the third lens group G₃and fourth lens group G₄): 0.6827

[0118] Here, the magnification is −0.4389 in type 1, and −0.4887 in type 2.

[0119] Conditional expressions (3) and (4) define the moving distance of the movable group having a positive refracting power, i.e., the moving distance of the second lens group G₂or fourth lens group G₄. If the moving distance of these movable groups becomes shorter than the respective lower limits of their corresponding expressions, then the magnification may not fully be changed in the intermediate region. If their upper limits are exceeded, on the other hand, then the moving distance becomes so long that the whole length of the tip-constituting part of the endoscope becomes too long, which is unfavorable as the endoscope objective lens. In particular, if the upper limit of conditional expression (4) is exceeded, then the exit pupil may fluctuate in excess.

[0120] Conditional expression (5) is concerned with the exit pupil in the case where the stop 1 is disposed. On the outside of the upper and lower limit, the exit pupil may fluctuate in excess, whereby high-quality images may not be obtained.

[0121] Conditional expressions (6) to (8) define power distributions of movable groups. If the lower limit of conditional expression (6) or (8) is not satisfied, then the power of the movable group having a positive refracting power, i.e., the power of the second lens group G₂or fourth lens group G₄, becomes so strong that aberration is hard to correct. Similarly, if the upper limit of conditional expression (7) is exceeded, then the power of the movable group having a negative refracting power, i.e., the power of the third lens G₃, becomes so strong that aberration is hard to correct. On the other hand, if the upper limit of conditional expression (6) or (8) is exceeded, then the moving distance of the movable group having a positive refracting power, i.e., the moving distance of the second lens group G₂or fourth lens group G₄, becomes so long that the lens may lose its compactness. Similarly, if the lower limit of conditional expression (7) is exceeded, then the moving distance of the movable group having a negative refracting power, i.e., the moving distance of the third lens group G₃, becomes so long that the lens may lose its compactness.

[0122] In the following, Examples 1 to 5 of the present invention will be explained specifically.

[0123] FIGS. 1A and 1B show the basic configuration of the endoscope objective lens in accordance with Example 1 at the wide and telephoto ends, respectively.

[0124] As explained as the first embodiment, the lens in accordance with Example 1 comprises, successively from the object side, a first lens group G₁having a negative refracting power; a second lens group G₂, movable along the optical axis X, having a positive refracting power; a third lens group G₃, movable along the optical axis X, having a negative refracting power; and a fourth lens group G₄having a positive refracting power.

[0125] In this Example, as shown in FIGS. 1A and 1B and the following Table 1, a stop 1 is disposed at a position identical to the object-side surface of the fifth lens L₅on the optical axis. In FIGS. 1A and 1B and the following Table 1, the radius of curvature of the stop 1 is expressed as R₈.

[0126] The upper part of Table 1 shows the radius of curvature R of each lens surface, axial surface space of each lens (center thickness of each lens and air space between each pair of neighboring lenses) D, and refractive index N and Abbe number ν of each lens at d-line in Example 1. In Table 1 and its subsequent Tables 2 to 5, the radius of curvature R and axial surface space D are values normalized with the focal length at the wide end being taken as 1.0, and the numbers referring to the symbols successively increase from the object side.

[0127] Also, the lower part of Table 1 shows values of object distance (normalized as with the axial surface space D; also in the following Tables 2 to 5), magnification, and variable group distances 1 to 3 of axial surface distance D at the wide and telephoto ends in Example 1.

[0128] FIGS. 2A and 2B show the basic configuration of the endoscope objective lens in accordance with Example 2 at the wide and telephoto ends, respectively.

[0129] The lens in accordance with Example 2 has a configuration substantially the same as that of Example 1.

[0130] In this Example, as shown in FIGS. 2A and 2B and the following Table 2, the stop 1 is disposed at a position identical to the image-side surface of the third lens L₃on the optical axis. In FIGS. 2A and 2B and the following Table 2, the radius of curvature of the stop 1 is expressed as R₇.

[0131] The upper part of Table 2 shows the radius of curvature R of each lens surface, axial surface space of each lens (center thickness of each lens and air space between each pair of neighboring lenses) D, and refractive index N and Abbe number ν of each lens at d-line in Example 2.

[0132] Also, the lower part of Table 2 shows values of object distance, magnification, and variable group distances 1 to 3 of axial surface distance D at the wide and telephoto ends in Example 2.

[0133] FIGS. 3A and 3B show the basic configuration of the endoscope objective lens in accordance with Example 3 at the wide and telephoto ends, respectively.

[0134] The lens in accordance with Example 3 has a configuration substantially the same as that of Example 1.

[0135] In this Example, as shown in FIGS. 3A and 3B and the following Table 3, the stop 1 is disposed between the second lens group G₂and third lens group G₃. In FIGS. 3A and 3B and the following Table 3, the radius of curvature of the stop 1 is expressed as R₈.

[0136] The upper part of Table 3 shows the radius of curvature R of each lens surface, axial surface space of each lens (center thickness of each lens and air space between each pair of neighboring lenses) D, and refractive index N and Abbe number ν of each lens at d-line in Example 3.

[0137] Also, the lower part of Table 3 shows values of object distance (normalized as with the axial surface space D), magnification, and variable group distances 1 to 4 of axial surface distance D at the wide and telephoto ends in Example 3.

[0138] FIGS. 4A and 4B show the basic configuration of the endoscope objective lens in accordance with Example 4 at the wide and telephoto ends, respectively.

[0139] The lens in accordance with Example 4 has a configuration substantially the same as that of Example 3. In FIGS. 4A and 4B and the following Table 4, the radius of curvature of the stop 1 is expressed as R₈.

[0140] The upper part of Table 4 shows the radius of curvature R of each lens surface, axial surface space of each lens (center thickness of each lens and air space between each pair of neighboring lenses) D, and refractive index N and Abbe number ν of each lens at d-line in Example 4.

[0141] Also, the lower part of Table 4 shows values of object distance, magnification, and variable group distances 1 to 4 of axial surface distance D at the wide and telephoto ends in Example 4.

[0142] FIGS. 5A and 5B show the basic configuration of the endoscope objective lens in accordance with Example 5 at the wide and telephoto ends, respectively.

[0143] As explained as the second embodiment, the lens in accordance with Example 5 comprises, successively from the object side, a first lens group G₁having a negative refracting power; a second lens group G₂having a positive refracting power; a third lens group G₃, movable along the optical axis X, having a negative refracting power; and a fourth lens group G₄, movable along the optical axis X, having a positive refracting power.

[0144] In this Example, as shown in FIGS. 5A and 5B and the following Table 5, a stop 1 is disposed at a position identical to the object-side surface of the fourth lens L₄on the optical axis. In FIGS. 5A and 5B and the following Table 5, the radius of curvature of the stop 1 is expressed as R₇.

[0145] The upper part of Table 5 shows the radius of curvature R of each lens surface, axial surface space of each lens (center thickness of each lens and air space between each pair of neighboring lenses) D, and refractive index N and Abbe number ν of each lens at d-line in Example 5.

[0146] Also, the lower part of Table 5 shows values of object distance, magnification, and variable group distances 1 to 3 of axial surface distance D at the wide and telephoto ends in Example 5.

[0147] The following Table 6 shows the values corresponding to conditional expressions (1) to (8) in Examples 1 to 5. Examples 1 to 5 satisfy all of their corresponding conditional expressions (1) to (8).

[0148] FIGS. 6A to 10H show various kinds of aberration (spherical aberration, astigmatism, distortion, and chromatic aberration in magnification) of Examples 1 to 5 at their wide and telephoto ends. In these aberration charts, ω indicates the half angle of view. As shown in FIGS. 6A to 10H, Examples 1 to 5 can make each of the above-mentioned kinds of aberration favorable.

[0149] Without being restricted to those of the above-mentioned Examples, the endoscope objective lens of the present invention can be modified in various manners. For example, the radius of curvature and lens space (lens thickness D) of each lens can be changed as appropriate.

[0150] Also, aspheric surfaces, GRIN lenses, and diffraction optical elements may be used in addition to or in place of elements in the endoscope objective lens in accordance with the above-mentioned embodiments, so as to correct chromatic aberration and other kinds of aberration.

[0151] As explained in the foregoing, the endoscope objective lens in accordance with the present invention can yield a compact endoscope objective lens in which various kinds of aberration are favorably corrected, and the magnification in use of the objective lens can arbitrarily be changed while carrying out focusing not only in the normal viewing (wide) state and close-up magnified viewing (telephoto) state but also in their intermediate region by moving predetermined two lens groups in four lens groups.