JP2501861B2 - Converter lens for bifocal camera - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a converter lens for a bifocal camera that is inserted behind a front lens group during telephoto photography.

[Conventional technology]

In order to meet the demand for taking a large image of a distant subject, there is known a camera capable of switching focal lengths for telephoto and wide-angle.

In most of these bifocal cameras, by inserting a negative power converter lens behind the front lens in the photographic optical system, the focal length is increased by 1.5 to 2 times to enable telephoto photography. ing. A conventional converter lens is composed of a plurality of lenses in order to remove various aberrations such as spherical aberration. This allows
The image is properly corrected for aberrations in both wide-angle and telephoto conditions.

[Problems to be Solved by the Invention]

However, in the conventional bifocal camera equipped with the above-described photographic optical system, the converter lens is composed of a plurality of lenses, so that it is unavoidable that the cost becomes considerably high. For example, the bifocal camera described in JP-A-60-179712 is equipped with a converter lens composed of 3 to 4 constituent lenses in order to increase the magnification from 1.6 to 2 times. JP-A-61-144618,
The bifocal camera described in Japanese Patent Laid-Open No. 61-147214 and the like requires as many as seven lenses as compared with the three to four lenses of a monofocal camera. Therefore, it is unavoidable that the cost of the lens is more than doubled as compared with the conventional single focus camera.

[Object of the Invention]

The present invention has been made to solve the above-mentioned problems, and forms a subject image in a state in which aberrations are well corrected in both cases of a wide-angle focal length and a telephoto focal length, even though it has a single lens configuration. It is an object of the present invention to provide a low-cost converter lens for a bifocal camera that can be manufactured.

[Means for solving the problem]

In order to achieve the above-mentioned object, the present invention is designed for wide-angle shooting.
Only the front lens group having the power of P _f is used, and the power P is obtained by inserting the meniscus single lens having the power P _c as the converter lens at the time of telephoto shooting. A triplet or tesser type lens can be used as the front lens group.

Here, when the principal point distance between the front lens group and the converter lens is d and B '= P / P ₁ , the following equation is generally established.

The relationship in the above equation can be diagrammed as shown in FIG. 5, but since the converter lens is required to extend the focal length of the front lens group, the power P _c of the converter lens is
The solution will be in the negative region in the figure.

In order to suppress the generation of aberration by the converter lens to a small extent, it is necessary to weaken the power of the converter lens, and the principal point interval d becomes a range smaller than 1 / P ₁ , and the minimum value of the principal point interval d is set to “0”. , Then P _c = (B′−1) * P ₁ .

Therefore, in order to keep the power of the converter lens small, it is desirable that the principal point spacing d be small. Also, in order to suppress the occurrence of spherical aberration, it is required that the converter lens be a meniscus lens with a convex surface facing the object side, and in order to suppress the occurrence of off-axis aberration a meniscus lens with a concave surface facing the object is required. It is necessary to have it. Spherical aberration is 4 when the incident height h of the ray on the lens surface is
Since it is proportional to the power, it is possible to suppress the occurrence of spherical aberration when the incident height h is small, even if the object-side surface of the converter lens is a concave surface. In this case, it is necessary that the principal point distance d between the front lens group and the converter lens be large.

Considering such a contradictory condition, it is extremely difficult to correct aberrations well, but in the present invention, the surface distance between the rear surface of the front lens group and the front surface of the rear lens group when the converter lens is inserted is D ₆ , P'is the back focus when the converter lens is inserted, the power of the converter lens, the refractive index, the Abbe's number, the curvature radii on the object side and the image side are P _c , N _c , ν _c , R ₁ , R respectively. ₂ ) i) 1.2 * D ₆ <P ′ <4.0 * D ₆ ii) P _c <0.00 iii) R ₁ <0.000 iv) R ₂ <0.000 v) 1.55 <N _c <1.78 vi) ν _c ＞ By satisfying the condition of 45.0, good aberration correction is possible.

The above conditional expression i) is a condition necessary to balance axial and off-axis aberrations, and conditional expression ii) is a condition for limiting the power of the converter lens to a negative value.
iii) and iv) condition that the two curved surfaces of the converter lens both have a negative curvature on the object side,
Conditional expressions v) and vi) are expressions for limiting the refractive index and dispersion of the converter lens.

[Action]

According to the above configuration, even if a negative power meniscus single lens is used as the converter lens inserted behind the front lens group to extend the focal length during telephoto shooting,
The occurrence of aberration can be suppressed well.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment] FIG. 1 shows a first embodiment of the present invention, in which the upper side of the optical axis 5 is the wide-angle shooting state, and the lower side of the optical axis 5 is the telephoto shooting state. . As shown in the figure, during wide-angle shooting, shooting is performed by a triplet type master lens 13 including a convex lens 10, a concave lens 11, and a convex lens 12. Also, when shooting a telephoto image, the master lens
Behind 13 is combined a negative power converter lens 14 with the concave surface facing the object side.

The lens data of the first example is as shown in Table 1 below.

In Table 1, i is the lens number attached to each surface in order from the object side, R _i (mm) is the radius of curvature of each surface, and D
_i (mm) is the surface distance to the next surface, and N and ν are the refractive index and Abbe number of the glass material.

In the optical system of the first embodiment, F _W = 36.0 F _T = 53.4 is obtained as the focal lengths F _W (mm) and F _T (mm) at wide angle and at telephoto, and a telephoto ratio of about 1.5 times is obtained. Will be able to. Also, the F-number at wide-angle and telephoto is respectively
It is F5, F7.5. As the value of each of the back focus _{BF W (mm), BF T} (mm), BF W = 29.9 BF T = 30.0 (= P ') and are turned, the condition i described above), ii), iii), iv),
It is confirmed that v) and vi) are satisfied. The aberrations at wide angle and at telephoto are as shown in FIGS. 2 (A) and 2 (B), respectively, and there is practically no problem in terms of aberrations.

[Second Embodiment] FIG. 3 shows a second embodiment of the present invention, and those functionally common to the first embodiment are designated by the same reference numerals. The lens data of the second example are as shown in Table 2 below.

In the second embodiment, F _W = 36.0 (F5) F _T = 53.4 (F7.5) B F _W = 32.8 B F _T = 30.7 (= P '), and the conditional expressions i), ii), iii), iv), v), vi)
Is satisfied, and the aberrations at wide angle and at telephoto are as shown in FIGS. 4 (A) and 4 (B).

〔The invention's effect〕

As described above in detail, in the present invention, the focal length is switched by inserting / removing the meniscus single lens, which is manufactured so as to satisfy the six conditional expressions for aberration correction, to switch the focal length. The focal lengths for shooting and telephoto shooting were acquired. Therefore, it becomes possible to significantly reduce the cost as compared with the converter lens used in the conventional photographing optical system for the bifocal camera while maintaining good aberration correction.

[Brief description of drawings]

FIG. 1 is an explanatory view showing the outline of the first embodiment of the present invention. FIG. 2 shows aberrations of the first embodiment at wide angle and at telephoto. FIG. 3 is an explanatory view showing the outline of the second embodiment of the present invention. FIG. 4 shows aberrations of the second embodiment at wide angle and at telephoto. FIG. 5 is a diagram showing the relationship between the power of the converter lens and the principal point distance between the converter lens and the master lens. 5 ... Optical axis 10 ... Convex lens 11, 12 ... Concave lens 13 ... Master lens 14 ... Converter lens.

Claims (1)

(57) [Claims] 1. A converter lens for a bifocal camera, which is inserted behind a front lens group having a positive power to extend the focal length of the entire system, wherein the converter lens has a negative power with a convex surface facing the object side. It consists of a meniscus single lens, and the surface distance from the rear surface of the front lens group when the converter lens is inserted is D ₆ , the back focus when the converter lens is inserted is P ′, the power, refractive index, and Abbe of the converter lens. Number, radius of curvature on the object side and image side
When P _c , N _c , ν _c , R ₁ , R ₂ are set, i) 1.2 * D ₆ <P '<4.0 * D ₆ ii) P _c <0.00 iii) R ₁ <0.000 iv) R ₂ <0.000 v) A converter lens for a bifocal camera characterized by satisfying the condition of 1.55 <N _c <1.78 vi) ν _c > 45.0.