JPH0585884B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to an objective lens for use in endoscopes and rigid endoscopes, and particularly to an objective lens that satisfactorily corrects distortion aberration. BACKGROUND ART Conventionally, a retrofocus type objective lens as shown in FIG. 1 has been known as an objective lens for an endoscope (for example, see Japanese Patent Laid-Open No. 121547/1983). This retrofocus type objective lens has an aperture of S
A concave lens group is placed on the object side, and a convex lens group is placed on the image side. This objective lens uses a concave lens group placed in front of the aperture S to strongly bend the principal ray P, making it possible to widen the angle of view.
Further, a convex lens group behind the diaphragm S makes the principal ray P incident on the image plane parallel to the optical axis, so that the light beam enters the image guide G perpendicularly. By making the light beam enter the image guide G perpendicularly in this manner, the loss of light within the image guide G can be reduced.
Also, when using this objective lens with a rigid scope,
As shown in FIG. 2, in order to reduce the loss of light in the relay system R, the principal ray P needs to be perpendicular to the image plane. In this way, conventional retrofocus type endoscopic objective lenses meet the two requirements of endoscopic objective lenses: wide angle and the principal ray incident on the image plane being perpendicular to it. It satisfies two requirements. However, it has the disadvantage of large negative distortion. For example, in the endoscope objective lens shown in Fig. 1, the distortion at a half angle of view ω = 37° is:
-21%. In addition, the relationship between the angle of view and distortion of other conventional retrofocus type endoscope objective lenses is as follows, and the negative distortion is similarly large. Half angle of view (ω) 30° 40° 50° 60° Distortion -13.5% -23% -36% -50% Purpose The present invention was made in view of the above circumstances, and it An endoscope in which negative distortion is reduced by configuring the front group on the object side to include at least a convex lens in order from the object side and a negative meniscus lens with at least one convex surface facing the object side. Its purpose is to provide an objective lens. Overview As shown in Fig. 3, the endoscope objective lens of the present invention has a front group arranged in front of the aperture diaphragm, which has, in order from the object side, a positive lens convex to the object side and a negative meniscus lens convex to the object side. a negative lens;
In the rear group behind the diaphragm, a refractive surface convex to the image side, a positive/negative cemented positive lens convex to the object side, and a cemented meniscus lens convex to the image side are arranged. Then, the front focus of the rear group is located near the aperture,
The principal ray is made to be incident almost perpendicularly to the image plane. In general, distortion generated on one surface of a lens system is approximately proportional to the cube of the ray height of the principal ray. Therefore, if the distortion is corrected in a lens far from the aperture where the principal ray height is high in the lens system, the negative distortion of the entire endoscope lens system can be reduced. For this purpose, the lens closest to the object side may be a convex lens, or the lens closest to the image side may be a concave lens. In the latter method, the chief ray is no longer perpendicular to the image plane, so it is not suitable for use in endoscopes. For this reason, in the present invention, as described above, the lens closest to the object side is a convex lens. In addition, in order to increase the angle of view, it is necessary to place a concave lens in the front group in front of the aperture to expand the principal ray, but in this case, in order to correct the negative distortion that occurs with the concave lens, It should be refracted gradually. To achieve this, the concave lens should be shaped like a concave meniscus lens with a convex surface facing the object side. Furthermore, in order to better correct distortion aberration, it is sufficient to use two or more concave meniscus lenses. The shape of the lens closest to the object side is preferably a meniscus convex lens with its convex surface facing the object side, a convex plano lens, or a biconvex lens with a strong curvature of the object side surface. When a biconvex lens with a strong curvature on the image side surface is used, it is not preferable because astigmatism and coma aberration occur significantly. As mentioned above, in the objective lens of an endoscope, the principal ray is incident perpendicularly to the image plane, so the rear group behind the diaphragm must have a strong convex effect as a whole, resulting in strong distortion. Occur. This must be corrected almost exclusively in the front group before the aperture. In the case of a general camera lens, the chief ray does not need to be perpendicular to the image plane, so it is possible to reduce distortion generated in the lens behind the aperture. For the above reasons, in the case of an endoscope objective lens, compared to a general camera lens having the same angle of view, negative distortion cannot be sufficiently corrected unless the correction amount of negative distortion in the front group is significantly larger. The endoscope objective lens of the present invention has a lens configuration as already described (especially the lens configuration of the front group), so that the negative distortion aberration correction effect in the front group is strengthened, and the distortion is improved. However, in order to further strengthen the correction effect, it is desirable to do the following. That is, it is preferable to increase the ray height of the principal ray in the convex lens of the front group to strongly generate fifth-order positive distortion. In this way, in order to increase the ray height of the principal ray in a convex lens and maintain the same angle of view, the power of the convex lens, 1/f _T (f _T is the focal length of the convex lens), is increased by the front lens group placed in front of the aperture. The power of the concave lens must be less than a certain degree, and the following formula must be satisfied.

[Formula] When K≦4/ω In the above formula, f _i is calculated by counting only the concave lenses in the front group (the cemented concave lenses are considered as one concave lens as a whole and the total number is N) in order from the object side. The focal length of the i-th lens, ω, is the half angle of view of the objective lens. The reason why in the above equation, as ω increases, the right side becomes smaller and the upper limit value of the value K decreases is because the distortion becomes stronger as the angle of view becomes larger. As an example of a general camera lens,
In the lens system described in No. 54764, K at ω = 46°
= 0.129, 4/ω = 0.0869, and K>4/ω, which does not satisfy K≦4/ω in the above equation. In the present invention, since the entire system is telecentric, the power of the rear group after the diaphragm is much stronger than that of a camera lens, and large aberrations also occur in this part. Therefore, by configuring the rear group with a convex surface facing the image side, a cemented positive lens convex toward the object side, and a cemented meniscus lens convex toward the image side, various aberrations can be effectively corrected with a relatively small number of lenses. I have to. Examples Examples of the endoscope objective lens of the present invention described in detail above are shown below. Example 1

[Table] ω
Example 2

【table】

[Table] ω
However, r ₁ , r ₂ , ... are the curvature radius of each lens surface,
d ₁ , d ₂ , ... are the wall thickness and air spacing of each lens,
n ₁ , n ₂ , ... are the refractive indices of each lens, υ ₁ , υ ₂ , ...
is the Abbe number of each lens, and f is the focal length of the entire system. Among the above embodiments, Embodiment 1 has a lens configuration as shown in FIG. 3, and Embodiment 2 has a lens configuration as shown in FIG. It includes a negative meniscus lens. When the present invention is applied to the objective lens of a rigid scope, the actual diaphragm is often arranged at a position inside the relay lens system that is conjugate with the position where the diaphragm should be provided in the objective lens. Effects of the Invention In the endoscope objective lens of the present invention described above, various aberrations and distortion are well corrected as shown in the aberration curve diagrams of each embodiment shown in FIGS. 5 and 6.
This becomes even clearer when compared with the aberrations of the conventional example (see FIG. 7), which has the following data with the lens configuration shown in FIG. Conventional example

【table】 [Brief explanation of the drawing]

Fig. 1 shows the structure of a conventional endoscope objective lens, Fig. 2 shows the structure of a conventional rigid endoscope objective lens, and Figs. 3 and 4 show the lens structure of an embodiment of the present invention. 5 and 6 are aberration curve diagrams of each example, and FIG. 7 is an aberration curve diagram of a conventional endoscope objective lens.

Claims (1)

[Claims] 1. A front group that includes, in order from the object side, a positive lens with a convex surface facing the object side, a negative menicus lens with a convex surface facing the object side, and a negative lens and has negative refractive power as a whole. , an aperture diaphragm, a refractive surface convex to the image side disposed after the aperture diaphragm, a positive cemented meniscus lens with a convex surface on the object side which is made by cementing a positive lens and a negative lens, and a negative lens and a positive lens. a cemented meniscus lens having a convex surface on the image side, and a rear group having a positive refractive power as a whole, the aperture is disposed at the front focal position of the rear group, and the following conditions are met. An endoscope objective lens with a satisfying wide angle and low distortion. K≦4/ω K=(1/f _T )/ _N 〓 ⁱ⁼¹ | 1/f _i | However, f _T is the focal length of the positive lens in the front group, f _i
is the focal length of the i-th lens when counting the negative lens of the front group from the object side, and ω is the half angle of view of the objective lens.