JP2518055B2 - Catadioptric optical system - Google Patents

Description

TECHNICAL FIELD The present invention relates to a catadioptric optical system using a reflecting system and a refracting system, and particularly, an F number of 3.5 while maintaining good aberration correction.
The present invention also relates to a catadioptric optical system having a large aperture ratio, which is suitable for a photographic camera, a video camera, etc., which has a large aperture ratio and a compact lens system.

(Prior Art) Conventionally, a catadioptric optical system has an extremely small chromatic aberration by using a reflecting surface, and it is easy to correct coma aberration generated on the reflecting surface by using a refracting system. Since it is possible to reduce the ratio of the distance from the first lens surface to the image surface to the distance), it is often used in optical systems with a long focal length.

These catadioptric optical systems are disclosed, for example, in Japanese Patent Publication No. 40-18354.
JP-B No. 43-5951, JP-B No. 60-18045, JP-A No. 60-184223, and the like.

However, catadioptric optical systems generally have a large aperture ratio and it is difficult to downsize the entire lens system,
In the past, in many cases, the overall lens system was downsized at the expense of the aperture ratio. For example, Japanese Examined Patent Publication No. Sho 60-32853 and Japanese Unexamined Patent Publication No. Sho 6
Japanese Laid-Open Patent Publication No. 2-184425 proposes a catadioptric optical system in which the overall lens length is shortened at the expense of the aperture ratio.

(Problems to be Solved by the Invention) Generally, in order to increase the aperture ratio in a catadioptric optical system, it is necessary to particularly favorably correct spherical aberration. In this case, it is preferable to widen the distance between the primary mirror and the secondary mirror in order to satisfactorily correct spherical aberration. However, for example
As proposed in Japanese Patent Laid-Open No. 18354, when such a method is adopted, the total lens length is longer (the tele ratio is larger) than the focal length of the entire system, the entire lens system becomes large, and the primary mirror As compared with the above, there arises a problem that the refracting power of the secondary mirror becomes too small and the back focus becomes short.

In addition, Japanese Patent Publication No. 40-18354 and Japanese Patent Laid-Open No. 60-184223
In the publication, a hole is formed in the center of the primary mirror and the lens system is supported in this hole. However, it is very difficult to form a hole in the primary mirror in terms of processing, and it is difficult to hold the lens system. was there.

Further, in many cases, the catadioptric optical system has a problem in that the secondary mirror causes a large amount of on-axis and off-axis light beams, and the vignetting light amount ratio decreases.

The present invention downsizes the entire lens system while maintaining the F-number 3.5, a large aperture ratio as a catadioptric optical system, and a predetermined amount of back focus by appropriately setting each lens element of the reflection system and the refraction system. In addition, it is an object of the present invention to provide a catadioptric optical system having high optical performance over the entire screen by reducing vignetting of a light beam by a secondary mirror.

(Means for Solving Problems) The present invention is a catadioptric optical system, in which a light flux from an object is sequentially passed through a positive lens L1 and a meniscus image surface side lens surface with a concave surface facing the object side. By refracting and reflecting to the object side with a refracting concave reflecting mirror L2 having a peripheral surface as a reflecting surface M1, through the lens L1,
Refractive reflecting mirror L3 whose lens surface on the image side is cemented to the lens surface on the object side of the lens L1 and whose lens surface on the object side is the reflecting surface M2
The lens L1 and both lens surfaces are convex, and the lens surface on the image side is a positive lens L4 in which the lens surface on the image side is joined to the lens surface on the object side of the refracting concave reflecting mirror. Reflector
After passing through the central transmission part of L2, when the light is guided to the image surface through a meniscus negative lens L5 having a concave surface facing the image surface side, the focal length of the entire system is f, and the refractive concave reflection mirror F2, the focal length of the system composed of the lens L1 and the refracting reflector L3 is f3, the radius of curvature of the object side lens surface of the lens L4 is R11, the lens L1 and the refracting concave reflector When the distance of L2 is D2, 2.0 <| f2 / f3 | <2.7 (1) 0.1 <D2 / f <0.2 (2) 0.01 <R11 / f <0.3 (3) It has a feature.

(Embodiment) FIGS. 1 and 2 are lens cross-sectional views of Numerical Embodiments 1 and 2 of the present invention, respectively. In the figure, each optical element of the catadioptric optical system of the present embodiment will be described in the order in which the light flux travels.

L1 is a positive lens whose both lens surfaces are convex, L2 is a meniscus shape with the concave surface facing the object side, and the peripheral part of the image side lens surface is a reflecting surface M1, the central part is a transmitting surface and is a refraction used as a primary mirror. Concave reflecting mirror, L3 cements the image side lens surface to the object side lens surface of lens L1 and also the object side lens surface (convex surface)
Is a refracting mirror which is used as a secondary mirror with M2 as a reflecting surface, and L4 is a positive lens whose both lens surfaces are convex, and the image side lens surface is cemented with the object side lens surface of the refracting concave reflecting mirror L2. There is. L5 is a negative meniscus lens with a concave surface facing the image side.

Incidentally, G1 is a removable filter, for example, an ND filter having a different density is exchangeably inserted instead of the diaphragm. G2 is an optical member such as a low pass filter and an infrared cut filter used in a video camera or the like. Q is the image plane.

In this embodiment, as shown in the figure, a light beam from a body (not shown) is condensed by a lens L1 and refracted and reflected toward the object side by a refracting surface of a refracting concave reflecting mirror L2 and a reflecting surface M1. Next, the light is refracted by the lens L1, reflected by the reflecting surface M2 of the refracting mirror L3 toward the object side, and then refracted and emitted through the lens L1. Then, the lens L4, the transmission surface in the central portion of the refracting concave reflecting mirror L2, and the lens L5 are sequentially refracted and passed, and then condensed on the image forming surface Q to form an object image.

In this embodiment, by setting each optical element as described above and satisfying the above-mentioned conditional expressions (1) to (3) for each optical element, a large aperture ratio and a miniaturization of the entire lens system are achieved, and We have obtained a catadioptric optical system that has a high optical performance with less vignetting of the light flux by the secondary mirror.

 Next, the technical meanings of the above conditional expressions will be described.

Conditional expressions (1) and (2) are for satisfactorily correcting spherical aberration, which is the most important factor in achieving a large aperture ratio, while achieving downsizing of the entire lens system. That is, as in conditional expression (2), the primary mirror M1 and the secondary mirror M2 are spaced to some extent to correct spherical aberration well, and the refractive power of the primary mirror system and the secondary mirror system is improved.
Is maintained in a well-balanced manner as in the conditional expression (1), and a predetermined back focus is secured while effectively shortening the total lens length.

If the upper limit of conditional expression (1) is exceeded, the back focus will increase, but the refracting power of the secondary mirror will become too strong, spherical aberration will be overcorrected, and negative Petzval sum will increase, resulting in good correction of field curvature. Becomes difficult. If the lower limit is exceeded, it will be difficult to obtain a predetermined amount of back focus while maintaining a large aperture ratio.

If the upper limit of conditional expression (2) is exceeded, it will be difficult to shorten the total lens length, and if the lower limit is exceeded, high-order spherical aberration will increase, making it difficult to achieve a large aperture ratio. .

Conditional expression (3) is for appropriately setting the radius of curvature of the object-side lens surface of the lens L4 cemented to the main mirror L2 of the rear refractive lens system for controlling the light flux after passing through the sub-mirror L3. Is. That is, since the convergent light reflected by the secondary mirror L3 is corrected for spherical aberration and coma in a well-balanced manner, when the central portion of the primary mirror is used as a transmissive refraction optical system, strong refraction of the first lens surface of the primary mirror Due to the concave surface of the force, spherical aberration is overcorrected. Therefore, in this embodiment, a positive lens L4 that satisfies the conditional expression (3) is arranged on the object side of the primary mirror L2, and the overcorrected spherical aberration at this time is satisfactorily corrected.

If the value of conditional expression (3) is exceeded, the light beam reflected by the secondary mirror will enter the first lens surface of the lens L4 at a large incident angle, resulting in overcorrection of spherical aberration. Is too short to be good.

In this embodiment, by satisfying each of these conditional expressions, a catadioptric optical system having a large aperture with good aberration correction and a short overall lens length is achieved. By arranging a negative meniscus lens-shaped lens L5 with a concave surface having a strong refractive power facing the image plane side, astigmatism is corrected particularly well and a field of view is expanded.

In this embodiment, the lens L3 and the lens that characterize the secondary mirror M2
Although L1 and the lens L4 forming the rear refracting lens system and the lens L2 forming the main mirror M1 are cemented, the object of the present invention can be achieved by supporting them separately. Further, in the present embodiment, it is preferable to perform focusing by moving both the positive lens L1 and the cemented lens of the secondary mirror M2 for aberration correction, but the entire lens system may be moved for focusing.

Next, numerical examples of the present invention will be shown. In the numerical example
Ri is the radius of curvature of the i-th lens surface in the order of light travel, Di is the i-th lens thickness and air gap in the order of light travel, Ni and ν
i is the refractive index and Abbe number of the glass of the i-th lens in the order in which light travels. However, Di is the length measured from the left to the right in the traveling direction of light, and the opposite is negative.

Conditional expression (1) = 2.2973 (2) = 1.1545 (3) = 0.1400 Conditional expression (1) = 2.368 (2) = 0.1745 (3) = 0.1181 (Effect of the invention) According to the present invention, by setting each lens element of the reflection system and the refraction system as described above, an F number of about 3.5 is obtained. It is possible to achieve a catadioptric optical system having high optical performance over the entire screen while achieving a large aperture ratio and downsizing of the entire lens system.

[Brief description of drawings]

FIGS. 1 and 2 are lens sectional views of Numerical Examples 1 and 2 of the present invention.
3 and 4 are graphs showing various aberrations of Numerical Examples 1 and 2 of the present invention. In the aberration diagrams (A) and (B), the imaging magnification β is β =
0, β = −0.064. In the figure, d is the d line, g is the g line, ΔS is the sagittal image plane, and ΔM.
Is the meridional image plane, and Q is the image plane.

Claims (1)

(57) [Claims] 1. A refracting concave reflecting mirror L2 in which a light flux from an object is sequentially passed through a positive lens L1, and a peripheral portion of a meniscus-shaped image side lens surface having a concave surface facing the object side is a reflecting surface M1. Refraction reflection to the image side, and through the lens L1, the image-side lens surface is joined to the object-side lens surface of the lens L1 and the object-side lens surface is defined as the reflecting surface M2 by the refracting / reflecting mirror L3. To the lens L1 and both lens surfaces are convex surfaces, and the image side lens surface is joined to the object side lens surface of the refracting concave reflecting mirror through a positive lens L4, and the refracting concave reflecting mirror L2 After passing through the central transmission part, when the light is guided to the image surface through the meniscus negative lens L5 with the concave surface facing the image surface side, the focal length of the whole system is f, and the focal point of the refracting concave reflecting mirror. Distance f2, the lens L1
, F3 is the focal length of the system composed of the refracting mirror L3, R11 is the radius of curvature of the lens surface of the lens L4 on the object side, and
Catadioptric, characterized by satisfying the condition of 2.0 <| f2 / f3 | <2.7 0.1 <D2 / f <0.2 0.01 <R11 / f <0.3, where D2 is the distance between the refracting concave reflecting mirror L2 and 1 Optical system.