JPS62231919A - Converter lens - Google Patents

Abstract

PURPOSE:To reduce the size of the whole system body and to obtain the excellent optical performance of a semitelephoto type which is about 25-30 deg. in photographic field angle when a converter lens is mounted by employing a rear converter system and specifying the lens constitution. CONSTITUTION:The lens surface of the 1st lens on an image plane side is made convex and the lens surface of the 2nd lens on an object side is brought to negative refracting power to set the shapes of both lenses properly, thereby compensating the spherical aberration. A converter lens is composed of three lenses having a positive, a negative, and a positive refractive index and an increase in Petzval sum is reduced as much as possible to prevent curvature of field from increasing. Meniscus lenses which have convex surfaces on the image plane side are used as the 1st and the 2nd lenses to reduce the generation of the comatic aberration of the whole image plane. The negative distortion when the converter lens is mounted is compensated by making the image-side lens surface of the 3rd lens convex.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a converter lens, and in particular to a compact converter lens that is detachably attached to the image field side of a main lens system and changes the focal length of the entire system in a particularly short direction. Moreover, the present invention relates to a converter lens suitable for high-performance photographic cameras, video cameras, and the like.

(Prior Art) Various converter lenses have been proposed that are attached to the object world side or the image field side of the main lens system and change the focal length of the entire system while maintaining the focal plane of the entire system at a constant position. . Among these methods, a so-called front converter method in which a converter lens is mounted on the object side of the main lens system tends to increase the lens diameter of the converter lens, making it difficult to miniaturize the entire system. On the other hand, a so-called rear converter method in which a converter lens is mounted on the image field side of the main lens system is advantageous in reducing the lens diameter compared to the front converter method, but it becomes difficult to correct aberrations and requires an increase in the number of lenses. However, the overall length of the lens tended to be longer.

Japanese Patent Application Laid-Open No. 54-97423 proposes a rear converter method in which a converter lens is attached to the space after the main lens system is extended to change the focal length of the entire prefecture, but the number of lenses is 6. Because of the large number of lenses, the overall length of the lens tended to become longer.

JP-A-59-29214 proposes a wide-angle converter lens that employs a rear converter method in which a lens with positive refractive power is attached to the rear of the main lens system to shorten the focal length of the entire system. are doing.

However, the converter lens proposed in the same publication had a tendency to increase in size as the total length of the lens increases when the converter lens is attached.

(Problems to be Solved by the Invention) The present invention further improves the converter lens previously proposed by the present applicant in Japanese Patent Laid-Open No. 56-111825, and fixes the main lens system so that the main lens system and the image plane The purpose of the present invention is to provide a converter lens that further miniaturizes the entire system and performs good aberration correction when a converter lens is attached between the two and the focal length of the entire system is shortened to expand the shooting angle of view. .

(Means for solving the problem) A converter lens is attached to the image plane side of the main lens system to change the focal length of the entire system, and the converter lens is attached to the image plane side in order from the object side. A meniscus-shaped first lens with a convex surface facing, a meniscus-shaped second lens with negative refractive power with a convex surface facing the image side, and a third lens with positive refractive power whose lens surface on the image side is convex. It is something that we have.

Other features of the invention are described in the Examples.

(Example) Fig. yJ1 and Fig. 5 are lens sectional views of a main lens system equipped with a converter lens according to the present invention.

Figure 2 is a cross-sectional view of the lens when the converter lens of Numerical Example 1 of the present invention is attached to the main lens system of Figure 1, and Figure 6 is a numerical example 2 of the present invention in the main lens system of Figure 5. FIG. 3 is a cross-sectional view of the lens when the converter lens is attached.

In the lens sectional view, M is a fixed main lens system, and C is a detachable converter lens.

Generally, the main lens system performs photographing by itself, so aberrations are well corrected by the main lens system alone. Therefore, in order to obtain good overall optical performance when a converter lens is attached, it is necessary to achieve good aberration correction with the converter lens alone.

However, when a converter lens with positive refractive power is attached to the image plane side of the main lens system and the focal length is changed to a shorter one, spherical aberration tends to be under-corrected in many cases, and the Petzval sum becomes positive. The field curvature becomes larger.

Therefore, in this embodiment, the converter lens is composed of three lenses having the above-mentioned lens shape, and overall good aberration correction is achieved.

Among these, the lens surface on the image side of the first lens has a shape with a convex surface facing the image side, and the lens surface on the object side of the second lens has a negative refractive power, and the shapes of both lenses are set appropriately. However, spherical aberration is well corrected.

Furthermore, the converter lens is composed of three lenses with positive, negative, and positive refractive powers to minimize the increase in the Petzval sum in the positive direction and prevent the increase in curvature of field.

': Both the 5 lenses and the second lens are composed of meniscus-shaped lenses with convex surfaces facing the image plane side, thereby reducing the occurrence of coma aberration on the entire screen.

Negative distortion when the converter lens is attached is corrected by making the lens surface on the image plane side of the third lens a convex surface.

The converter lens according to the present invention can obtain good optical performance by adopting the above configuration, but more preferably, the radius of curvature of the i-th lens surface from the object side of the converter lens is R1, and the radius of curvature of the i-th lens surface from the object side of the converter lens is set to R1, Ni for refractive index and dispersion, respectively.

When υi is the focal length of the i-th lens, fi is the focal length of the entire system when the converter lens is attached to the main lens system, then 1.55< (N l + N3 ) / 2 .
・・・・・・ (5) 55<ν3 ・・・・・・・・・
...The following condition (6) must be satisfied.

Conditional expression (1) is for appropriately setting the refractive power of the image-side lens surface of the first lens and the object-side lens surface of the second lens to properly correct spherical aberration, and the lower limit value Exceeding the upper limit will result in under-compensation, and conversely, exceeding the upper limit will result in over-compensation.

Conditional expression (2) relates to the refractive power of both lens surfaces of the first lens, and is mainly used to properly correct coma aberration. Exceeding the lower limit value causes extroverted coma, and exceeding the upper limit value. Many introverted frames appear.

Conditional expression (3) relates to the refractive power of the third lens, and is mainly used to reduce distortion. If the lower limit value is exceeded, the distortion aberration increases in the negative direction, and if the upper limit value is exceeded, the entire converter lens becomes When the refractive power becomes too small and a converter lens is attached, y,! This is not preferable because the amount of change in point distance decreases.

Conditional expression (4) is the refractive index of the glass of the second lens with negative refractive power, and conditional expression (5) is the refractive index of the glass of the third lens with positive refractive power as well as the first lens with positive refractive power. Regarding the average value, the aim is to minimize the Petzval sum of the converter lens and to properly correct the field curvature when attached to the main lens system, and if conditional expression (4) or conditional expression (5) is not satisfied, In either case, the Petzval sum increases and the image plane characteristics deteriorate, which is not good.

Conditional expression (6) is intended to satisfactorily correct chromatic aberration, especially chromatic aberration of magnification, when a converter lens is attached regarding the dispersion of the glass of the third lens.If conditional expression (6) is not satisfied, chromatic aberration of magnification at the periphery of the screen will be reduced. This is not desirable as it increases.

In this embodiment, when the focus is equipped with a converter lens, only the main lens system or the converter lens may be moved, or both may be moved integrally or separately at different speeds.

In this embodiment, the main lens system is not limited to the Ernostar type, but may be of any type.

Further, in this embodiment, the case where the first lens has a positive refractive power is shown, but it may be a lens with a negative refractive power.

Next, numerical examples will be shown in which the b-inverter lens of the present invention is attached to the main lens system. In the numerical examples, Ri is the radius of curvature of the i-th lens surface from the object side, Di is the thickness and air distance of the i-th lens from the object side, Ni and ν
i is the refractive index and Abbe number of the glass of the i-th lens in order from the object side. C is the converter lens, M is the main lens, and the angle of view is shown at the same time. Note that R17 and R18 are glass materials such as filters.

Numerical Example 1 Main lens system and converter lens F=18 FNO=I: 2.7 2ω=2
5. BiR10-Aperture 010-1.00 Old 7-oo 017-5.50 N 9-1
．． 516:l:l v 9-64.1 Main lens system only:
F=24.48, FNO=1 + 4゜2ω
= 18.6° Numerical Example 2 Main lens system and converter lens F=14 FNO= l:2.2 2ω=31.
9゜RIO-Aperture DIO-1,00 fl17-oo 017m5.50
N9II1.5163: l v 9 64. l
R18-00 Main lens system only: F=21 FNO=l: :11
．． 32ω = 21.6° (Effects of the Invention) According to the present invention, by adopting a single-type rear converter and specifying the lens configuration as described above, the entire lens system can be miniaturized, and it is possible to take pictures when the converter lens is attached. It is possible to achieve a converter lens having good optical performance with a semi-telephoto angle of view of about 25° to 30°.

[Brief explanation of drawings]

1 and 5 are lens sectional views of the main lens system equipped with the converter lens of the present invention, and FIGS. 2 and 4 are lens sectional views of the main lens system of FIG. Lens cross-sectional view and aberration diagram when a converter lens is attached,
Figures 6 and 8 are lens cross-sectional views and aberration diagrams when the converter lens of Numerical Example 2 of the present invention is attached to the main lens system of Figure 2, respectively, and Figure 3 is the main lens system of Figure 1. FIG. 7 is an aberration diagram of only the main lens system in FIG. In the figure, C is a converter lens, and M is a main lens system. Figure 83 F=Doo-01J=! 3.3' vJ=9.3°
IIJ = 9.3' Print 4 (2) No. 7, No. 7, No. 8, No. 8

Claims (1)

[Scope of Claims] (1) A converter lens that is attached to the image plane side of a main lens system to change the focal length of the entire system, the converter lens having a convex surface on the image plane side in order from the object side. a meniscus-shaped first lens with a convex surface facing the image plane, a meniscus-shaped second lens with a negative refractive power with a convex surface facing the image plane, and a third lens with a positive refractive power whose lens surface on the image plane side is convex. A converter lens characterized by: (2) The radius of curvature of the i-th lens surface from the object side of the converter lens is Ri, the refractive index and dispersion of the glass of the i-th lens are Ni, υi, and the focal length of the i-th lens is f.
i, when the focal length of the entire system when the converter lens is attached to the main lens system is f, 1.1<R_2/R_3<2.1 1 <R_1/R_2<1.5 0.4<f_3/ The converter lens according to claim 1, which satisfies the following conditions: f<1.0 N_2<1.75 1.55<(N_1+N_3)/2 55<ν_3.