JPS5832681B2 - Compact auxiliary lens system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an auxiliary lens system called a rear converter, which is installed synthetically on the image side optical axis of a photographic lens to expand the focal length. This invention relates to a compact and high-performance auxiliary lens system.

The method of expanding the focal length by installing an auxiliary lens system with negative refractive power on the image-side optical axis of the photographic lens has long been known as a method of inexpensively taking photographs with different focal lengths. It is well known that cameras with detachable lenses, especially cameras called reflex cameras, are currently widely used.

However, these auxiliary lens systems for eye reflex cameras require attachment/detachment operations before use, are inconvenient to carry, and, furthermore, few have been satisfactory in terms of performance.

The purpose of the present invention is to solve the above-mentioned drawbacks when using an auxiliary lens system, by providing an auxiliary lens system with unprecedented high performance and compactness, thereby making it possible to incorporate the auxiliary lens into the camera. This is what we are trying to do.

For example, patent application No. 15383 filed by the present applicant.
No. 3 (Japanese Unexamined Patent Publication No. 52-76919) and Patent Application No. 52-9
This is suitable for the sub lens system in No. 9352.

In order to incorporate an auxiliary lens system into the camera, it goes without saying that the outer diameter and overall length of the lens (the sum of the lens thickness and air gap) must be kept as small as possible. If you install an auxiliary lens system with negative refractive power on the image side of the taking lens with the focal point of the lens aligned with the film plane, the focal length will be expanded, but the position of the focal point of the combining system will be Because it moves toward the image side according to the image, it does not match the film surface.

Therefore, in order to take pictures with a composite system, it is necessary to bring out the entire composite system consisting of a photographic lens and an auxiliary lens system toward the front object side, and to align the focal point of the composite system with the film plane.

However, since there is a limit to the amount by which the synthetic system can be pulled out without eccentricity, it is desirable that the amount to be pulled out is small.

On the other hand, when expanding the focal length using an auxiliary lens system,
The smaller the distance between the front principal point of the auxiliary lens system and the rear principal point of the photographic lens, the smaller the absolute value of the refractive power of the auxiliary lens system to obtain the same composite focal length; The negative Petzval sum also becomes smaller, which is very advantageous for correcting aberrations in the synthesis system, but since the back focus of the synthesis system becomes very large, the amount of extraction also becomes quite large, making it difficult to incorporate an auxiliary lens system. is not suitable.

In order to keep the amount of withdrawal to an appropriate value, it is necessary to widen the distance between the principal points of the photographic lens and the auxiliary lens system to some extent.
If the auxiliary lens system is separated from the photographic lens, the oblique rays exiting the photographic lens will spread out, resulting in an increase in the outer diameter of the auxiliary lens system, and if the distance between the principal points increases, the auxiliary lens It is necessary to increase the absolute value of the refractive power of the system, which also increases the negative Petzval sum, making it difficult to correct aberrations.

In order to solve the above problems and realize a compact and high-performance auxiliary lens system, which is the objective of the present invention, the auxiliary lens system is divided into two groups, front and rear, and the front group has a positive refractive power and the rear group has a positive refractive power. It is necessary to provide negative refractive power, and by doing so, the position of the front principal point of the auxiliary lens system can be lowered to the rear of the image side without separating the auxiliary lens system from the photographing lens, so it is possible to improve the composition system. The adoption of the mata-to configuration not only lowers the principal point to the rear, but also allows the front group close to the photographic lens to have positive refractive power, allowing the auxiliary lens system to capture oblique rays of light. This is very effective in converging the lens and keeping the outer diameter of the lens small, and can fully satisfy one of the objectives of the present invention, which is compactness.

Furthermore, since the rear group, which is far away from the photographic lens, has a negative refractive power that considerably exceeds that of the front group in absolute value, it is quite possible to correct the negative Petzval of the rear group with the front group, and in this way. Excellent performance, which is another objective of the present invention, is also achieved.

Next, the configuration of the auxiliary lens system of the optical system of the present invention will be described in detail with reference to FIG.
For example, from the object side, the first lens is a meniscus negative lens with a concave side facing the object side, the second lens 2 is a biconvex lens, the third lens 3 is a biconcave lens, the fourth lens 4 is a biconvex lens, and the fifth lens is a biconvex lens.
The lens 5 is a negative meniscus lens with a concave surface facing the object side, the fourth lens 4 and the fifth lens 5 are cemented together to form a group, and have a composite positive refractive power, and the sixth lens 6 is a negative meniscus lens with a concave surface facing the object side. It is composed of 6 lenses in 5 groups consisting of meniscus negative lenses with concave surfaces facing the side, and the front group consists of lenses 1 to 5, which collectively have a positive refractive power, and the 6th lens 6 in the rear group has positive refractive power. It has negative refractive power.

In addition, if the diaphragm 7 is installed on the rear image side of the photographing lens I, and the auxiliary lens system (2) is installed immediately behind the diaphragm 7, the outer diameter of the lens can be reduced and it can be easily incorporated. In addition, f is the composite focal length of the auxiliary lens system ①, f (1 to 5) is the composite focal length of the front group with positive refractive power from the first lens 1 to the fifth lens 5, 'h t Fa
t f(+~5) and f6 are respectively the second lens 2,
Let d be the focal length of the front group composite lens of the third lens 3, the fourth lens 4, and the fifth lens 5, and the sixth lens 6 of the rear group, and d is the air distance between the fifth lens 5 and the sixth lens 6, rl *r4
tr5rto on the 1st, 4th, 5th, and 1st surfaces, respectively.
When the radius of curvature of the 0-plane is taken as, it satisfies each of the above conditions.

Positive refractive power is provided to the front group from lens 1 to lens 5,
Providing negative refractive power to the sixth lens 6 in the rear group is an essential condition for incorporating the auxiliary lens system (■) into the camera for the reasons mentioned above, and the above condition (1) is also the same as condition (2). Relatedly, by giving appropriate refractive power to the front and rear groups,
It is necessary to balance the outer diameter of the lens and aberration.
If the upper limit is exceeded, distortion aberration increases and the Petzval sum deteriorates, and if the lower limit is exceeded, the outer diameter of the auxiliary lens and the back focus of the composite group increase, making it unsuitable for use in a built-in camera.

Also, the air distance d9 between the front group and the rear group is also the same as the rear group sixth lens 6.
There is a relationship between the refractive power of condition (2) and the outer diameter of the lens.Exceeding the upper limit of condition (2) will lead to an increase in the outer diameter of the lens and worsening of distortion, while exceeding the lower limit will cause the back focus to become extremely large. I also don't like it.

Condition (3) is the second lens 2 and third lens 3 in the front group.
, the fourth of the joining. It expresses the relationship between the refractive powers of the fifth lenses 4 and 5, and is an important formula for improving the Petzval sum of the auxiliary lens system (■).If the lower limit is exceeded, the Petzval sum cannot be improved, and if the upper limit is exceeded. Conditions (4), (5), and (6) make it difficult to correct spherical aberration, and conditions (1) to (3) are satisfied for the auxiliary lens system (■) in which back focus and Petzval sum are corrected. It is necessary to correct spherical aberration, field curvature, coma aberration, etc. If the radius of curvature of the first surface r0 increases beyond the upper limit of condition (4), it is difficult to correct coma aberration, and If the lower limit is exceeded, it becomes difficult to correct field curvature.

Condition (5) expresses the relationship between the fourth surface r4 and the fifth surface r5, which have strong curvature, of the second lens 2 and the third lens 3, which have strong refractive power, and when the upper limit is exceeded, it is difficult to correct spherical aberration. If the lower limit is exceeded, the curvature of field worsens.

Regarding condition (6), if the radius of curvature of the tenth surface exceeds the upper limit, and if the curvature of field exceeds the lower limit, comatic aberration worsens.

Through the above steps, it is possible to obtain a compact and high-performance auxiliary lens system suitable for being built into a camera, which is the object of the present invention.

Of course, it goes without saying that it is possible to use it as a compact and high-performance auxiliary lens system even if it is used in a conventional manner other than using it built into a camera.

Next, a numerical example of the present invention will be shown, but before that, an example of a photographing lens I, which is installed in combination with this auxiliary lens system (2), will be shown for reference. In Fig. 1, from the object side, R1, R2,
・・・・・・R7 is the radius of curvature of each refracting surface, D□
, D2...D6 are sequentially determined by the lens thickness and air spacing, N1. N2. N3. N, the sequential lens (
7) Refractive index for d-line, "1 t V21 V3,
When V4 is sequentially taken as the Atsube number of each lens and Σp is the Petzval sum, then to show a numerical example of the auxiliary lens system ■, rl, r2...r1□ are sequentially refracted from the object side. The radius of curvature of the surface, do, d2... (lto is sequentially the lens thickness and air gap, n□1 R2...R
6 is the refractive index of each lens for the d-line, Vl, v2・
... Let v6 be the Atsube number of each lens, fT be the composite focal length when combined and installed on the photographing lens I of the numerical example, and d be the air distance between the photographic lens I and the auxiliary lens system (2).

This patent is characterized by the Petzval sum of synthetic yarns and is suitable for achieving the object of the present invention.

[Brief explanation of drawings]

Figure 1 of the drawings is a lens system configuration diagram when the auxiliary lens system (2) of the embodiment according to the present invention is installed in combination with the photographic lens (2), and Figure 2 is an aberration diagram of the photographic lens (2) in the case of the numerical embodiment. FIG. 3 is an aberration diagram in the case of a composite system of the auxiliary lens system (2) of the numerical example and the photographing lens (2) of FIG. The symbols ``■'' are a photographing lens, ``■'' is an auxiliary lens system, 1 is a lens, 2 is a second lens, 3 is a third lens, 4 is a fourth lens, 5 is a fifth lens, and 6 is a sixth lens.

Claims (1)

[Claims] 1. In order from the object side, the first lens is a meniscus negative lens with a concave surface facing the front object side, the second lens is a biconvex lens, the third lens is a biconcave lens, the fourth lens is a biconvex lens, and the fifth lens is a biconvex lens.
The lens is a negative meniscus lens with a concave surface facing the object side.The fourth and fifth lenses are cemented together and have a positive refractive power, and the sixth lens is a negative meniscus lens with a concave surface facing the object side. The auxiliary lens system is composed of 6 lenses in 5 groups, where f8 is the composite focal length of the auxiliary lens system, and f(1-5)2 is the composite focal length f2 from the first lens to the fifth lens. f
3. f (4 to o and f6 are the second and third lenses, respectively)
lens, a composite lens of the fourth lens and the fifth lens, and a sixth lens
The focal length of the lens, d, is the air distance between the fifth lens and the sixth lens, rl, r4, r5, , ]rto
A compact auxiliary lens system characterized by satisfying each of the above conditions, where are the radii of curvature of the first, fourth, fifth, and tenth surfaces, respectively.