JPH0359508A - Teleconverter lens - Google Patents

Abstract

PURPOSE:To almost completely correct especially both an axial chromatic aberration and a chromatic difference of magnification in various aberrations by providing one piece of negative lens and two pieces of positive lenses in a front group, using a positive lens as the nearest lens to an image side of the front group, and also, allowing the subject lens to satisfy prescribed conditions. CONSTITUTION:A teleconverter lens is installed in an object side of a main lens I and has a function for lengthening a focal distances of the main lens I, consists of a front group A having refracting power and a rear group B having negative refracting power, the front group A has at least one piece of negative lens and two pieces of positive lenses, and also, the nearest lens to an image side of the front group A is a positive lens, and also, this teleconverter lens is allowed to satisfy conditions of inequalities I - III. In the inequalities I - III, phin, (tn), nuP, and nuN denote refracting power of the negative lens in the front group A, core thickness of the negative lens, a composite dispersion of the front group, and a composite dispersion of the rear group, respectively. In such a manner, to say nothing of a chromatic aberration, the curvature of image and a spherical aberration can be corrected satisfactorily.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a teleconverter lens, and more particularly to a teleconverter lens that can be attached to a small, high-performance camera such as a video camera.

In recent years, technological progress regarding electronic components such as video cameras has been remarkable. For example, solid-state imaging devices with extremely high resolution have come to be used for consumer use. Along with this, the performance of optical systems is also required to be higher than before. However, when an auxiliary optical system such as a converter lens is installed, the performance of the entire optical system deteriorates.
The resolving power of the main lens and image sensor was not fully utilized.

Generally, when an auxiliary optical system such as a teleconverter lens is used to increase the focal length of the main lens, it is necessary to completely correct various aberrations within the auxiliary optical system. The amount of aberrations generated is proportional to the focal length, so if complete aberration correction is not performed within the auxiliary optical system, the amount of aberrations generated in the entire system including the auxiliary optical system, that is, the teleconverter lens, will be proportional to the focal length. becomes larger in proportion to the magnification. Therefore, aberrations are completely corrected within the auxiliary optical system,
In order to obtain the same level of performance as the main lens in the entire system, it is conceivable to increase the number of lenses in the auxiliary optical system. However, when the number of lenses is increased, especially in the front teleconverter lens, the effective diameter also increases significantly, and the increase in weight is therefore unavoidable.

Furthermore, in teleconverter lenses, correction of chromatic aberration is particularly problematic among various aberrations. For example, when a teleconverter lens is attached to a main lens such as a zoom lens, the amount of chromatic aberration generated in the main lens increases in proportion to the focal length, and at the same time, the chromatic aberration of the teleconverter lens also increases. At this time, the influence of the chromatic aberration of the teleconverter lens on the chromatic aberration of the entire system including the teleconverter lens is opposite for axial chromatic aberration and lateral chromatic aberration. For example, when axial chromatic aberration is under-corrected in a teleconverter lens, the axial chromatic aberration of the entire system is under-corrected, but the lateral chromatic aberration of the entire system is over-corrected. On the other hand, teleconverter
If the axial chromatic aberration of the lens is overcorrected, the axial chromatic aberration of the entire system will be overcorrected, but the lateral chromatic aberration of the entire system will be undercorrected.

On the other hand, in the front teleconverter lens, the absolute value of the refractive power of the front group is smaller than the absolute value of the refractive power of the rear group. Therefore, in order to correct longitudinal chromatic aberration, a material with low dispersion is required for the rear group, whereas in order to correct lateral chromatic aberration, a material with high dispersion is required in the rear group. However, in reality, materials with as low dispersion as possible are used for both the front and rear groups to balance lateral chromatic aberration and longitudinal chromatic aberration. For example, JP-A-62-
In Publication No. 87924, the amount of chromatic aberration generated is suppressed by using as low dispersion glass as possible for the positive lens, and the chromatic aberration in the converter lens is corrected for the negative lens with the low dispersion glass used for the positive lens. With this correction method, the overall chromatic aberration can be balanced, so neither the longitudinal chromatic aberration nor the lateral chromatic aberration will be noticeable, but the overall amount of aberration will increase. Therefore, when this teleconverter lens is attached to the main lens, the performance of the entire system deteriorates, and it is not possible to obtain the same level of resolution as the main lens. This is undesirable considering that it is attached to recent video zoom lenses that require high performance.

SUMMARY OF THE INVENTION An object of the present invention is to provide a teleconverter lens that can almost completely correct both longitudinal chromatic aberration and lateral chromatic aberration among various aberrations.

In order to achieve the above object, the teleconverter lens according to the present invention has a function of increasing the focal length of the main lens by being attached to the object side of the main lens, and has a front group with a positive refractive power and a front group with a negative refractive power. It consists of a powerful rear group, the front group has at least one negative lens, and the lens closest to the image side of the front group is a positive lens, and is characterized by satisfying the following conditions: be.

1φ■t, I < 0.08 (1) However, φ■: Refractive power of the negative lens in the front group, t: Core thickness of the negative lens.

For front teleconverter lenses, the light rays pass through a higher position from the optical axis in the front group with positive refractive power than in the rear group with negative refractive power, so chromatic aberration is more likely to occur in this area. . Also, teleconverter
When a lens is attached to the main lens, the entire system becomes a converging system, so the positive lens has greater refractive power in the entire system.
Therefore, the amount of chromatic aberration generated by the positive lens increases. Therefore, in a teleconverter lens, it is necessary to suppress the amount of chromatic aberration generated in the front group having positive refractive power to the minimum possible level, or to sufficiently correct it.

Here, in order to correct the chromatic aberration of the front group, at least one negative lens is required in the front group. Furthermore, the lens closest to the image in the front group is a positive lens. If an attempt is made to correct this inclination of the image plane, the spherical aberration will be greatly inclined to the over side.

Furthermore, since recent video cameras and the like have become smaller and lighter, the converter lens attached to them is also required to have a compact configuration. Therefore, the negative lens included in the front group needs to satisfy condition (1). If the upper limit of condition (1) is exceeded, the core thickness of the negative lens included in the front group will increase, and the overall length of the teleconverter lens and the diameter of the front lens will increase more than necessary. Therefore, a compact teleconverter lens cannot be realized, and the teleconverter lens ends up being larger than the main lens.

Furthermore, in order to achieve the above object, it is desirable that the front group having positive refractive power includes at least one cemented lens of a negative lens and a positive lens. becomes larger with a negative lens. Further, even if the cemented surface has a strong curvature, the error sensitivity does not increase, so it is possible to more easily construct a highly accurate converter lens.

Furthermore, it is desirable that the teleconverter lens of the present invention satisfies the following conditions.

νF>64 (2) ν,>50
(3) However, Shear: composite dispersion of the front group, ShiM: composite dispersion of the rear group, φP−Σψ□ (sum of refractive powers of each lens in the front group), φ8=
Σl'NI (sum of refractive power of each lens in the rear group), φPl: refractive power of each lens in the front group, φ. : Refractive power of each lens in the rear group, C, : Atsube number of each lens in the front group, and VY : Atsube number of each lens in the @ group.

The composite dispersions ν and .v8 of the front group and the rear group indicate the dispersion of the entire front group and the entire rear group, respectively. Similar to the Abbe number generally used for optical glasses, the larger the composite dispersion, the lower the dispersion and the better the chromatic aberration is corrected.

If the composite dispersion of the front group exceeds the lower limit of condition (2) and the front group becomes high dispersion, a configuration that satisfactorily corrects either axial chromatic aberration or lateral chromatic aberration, or a configuration that satisfactorily corrects only axial chromatic aberration or lateral chromatic aberration, or a configuration that satisfactorily corrects axial chromatic aberration and magnification within the axial chromatic aberration. It is not possible to completely correct both chromatic aberrations and obtain the same level of performance as the main lens as a whole. The condition (
As in case 2), it is not possible to completely correct both the Hosochromatic aberration and the lateral chromatic aberration within the converter lens and obtain the same level of performance as the main lens in the entire system.

Further, in the configuration of the teleconverter lens of the present invention, the front group having positive refractive power includes a negative lens in order from the object side, a negative lens in order from the object side or in order from the object side, and a negative lens in order from the object side. It is composed of a total of three lenses, and it is desirable that the rear group having negative refractive power be smaller than one negative lens.

As mentioned before, in a teleconverter lens, it is necessary to keep the amount of chromatic aberration generated in the front group to a small level and to sufficiently correct the aberration that occurs. Here, in order to suppress the amount of aberrations generated as much as possible, it is desirable that the front group includes two or more positive lenses. If an attempt is made to reduce the amount of aberrations generated using only one positive lens, the refractive power of the front group must be significantly weakened, making it impossible to realize a compact teleconverter lens. On the other hand, if the refractive power of the front group is strengthened in order to make the lens compact, the amount of aberrations generated will increase significantly.

Furthermore, in the rear group having negative refractive power, light rays pass through a lower position from the optical axis than in the front group having positive refractive power. Therefore, the amount of aberrations generated is relatively small, and aberrations can be corrected with one negative lens. Therefore, in order to have a compact configuration, it is desirable that the rear group consists of one negative lens. With the lens configuration as described above, not only chromatic aberration but also field curvature and spherical aberration can be well corrected.

By satisfying the above conditions, it is possible to realize a compact, high-performance teleconverter lens with small chromatic aberration.

Examples 1 to 6 of the present invention are shown in Tables 1 to 6, respectively. In the following description, f is the focal length ratio of the entire system, and ν1 is the Abbe number for the a-line of the second lens from the object side.

The first to seventh surfaces of Example 1 are the teleconverter lenses of the present invention, and are attached to the main lens I after the eighth surface. Example 2゜3.5 1st to 7th sides, Example 4
The first to eighth surfaces of the lens are teleconverter lenses of the present invention, respectively, and are attached to the main lens (2). The first to seventh surfaces of Example 6 are the teleconverter lenses of the present invention, which are attached to the main lens (2).

Table 7 shows the lens configuration of the main lens (2) used in Examples 2-5. Further, Table 8 shows the relationship between each example and conditions (1) to (3).

In addition, Sections 1.3, 5.7.9. FIG. 1 is a sectional view showing the lens structure of Examples 1 to 6 of the present invention, and FIG. 13 is a sectional view showing the lens structure of the main lens (2) used in Examples 2 to 5 of the invention. 2.4゜6.8, 10.12 shows the teleconverter lens according to Embodiment 1 of the present invention as the main lens ■, the teleconverter lenses according to Examples 2 to 5 as the main lens ■, and Embodiment 6 FIG. 3 is a diagram of each lens aberration when a teleconverter lens according to the above is attached to the main lens (2). However, F is the open F value, and ω is the half angle of view.

Table (Example 1) f-72, 6 ~ 292 ~ +7-7 + Ib rate radius ◆ Peak spacing Refractive index (Nd) Atsube number (Shi d) Table 2 (Example 2) f- (Company), 2 ~ 27.5~17.7 III skewer radius Refractive index (Nd) Atsbe number (νd) Table (Example 3) f-96,2 1111 rate radius ◆★Refractive index (Nd) Atsbe number d) Table (Example 4) f-ku, 2~46.4~27.6 Curvature '1! Diameter Axis spacing refractive index (Nd) Atsube number (νd) Table (Example 5) f-%, 2 Molecular radius Axis spacing refractive index (Nd) Atsube number (νd) Table (Example 6) f-95 ,7~794~66.0 Molecular radius axis upper surface distance Refractive index (Nd) Atsube number (νd) Table 7 (Main lens structure 11t of Examples 2 to 5) f-66,3~
19.0~&7 Radius of curvature axis upper surface interval Refractive index (Nd) Atsbe number (νd) Table 8

[Brief explanation of drawings]

FIG. 1 is an aberration diagram when a teleconverter lens according to a first embodiment of the present invention is attached. Figures 3.5 and 7.9 are cross-sectional views showing the lens configuration when the teleconverter lenses according to Examples 2 to 5 of the present invention are attached to the main lens ■, respectively, and Figures 4.6 and 8.10 are FIG. 6 is an aberration diagram when the teleconverter lenses according to Examples 2 to 5 are attached to the main lens (2). Fig. 11 is a sectional view showing the lens configuration when the teleconverter lens according to Example 6 of the present invention is attached to the main lens ■, and Fig. 12 is a cross-sectional view showing the lens configuration when the teleconverter lens according to Example 6 is attached to the main lens ■. FIG. FIG. 13 is a sectional view showing the lens structure of the main lens (2) to which the teleconverter lens according to the present invention can be attached. A: Front group B: Rear group 1, II, I [: Main lens

Claims (4)

[Claims] (1) A teleconverter lens that is attached to the object side of the main lens and has the function of increasing the focal length of the main lens, which is composed of a front group with positive refractive power and a rear group with negative refractive power, A teleconverter lens characterized in that the front group has at least one negative lens, the lens closest to the image side of the front group is a positive lens, and satisfies the following conditions: |φ_■t_■|< 0.08 However, φ_■: Refractive power of the negative lens in the front group t_■: Core thickness of the negative lens. (2) The front group includes at least one cemented lens of a negative lens and a positive lens.
Teleconverter lens described in section. (3) The teleconverter lens according to claim 1 or 2, which satisfies the following conditions: ν_P>64 ν_N>50, where ν_P: composite dispersion of the front group; ν_N: The composite dispersion of the rear group. (sum) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (Sum of the refractive power of each lens in the rear group) φ_P_i: Refractive power of each lens in the front group φ_N_j: Refractive power of each lens in the rear group ν_P_i: Each lens in the front group Abbe number ν_N_j of lens: Abbe number of each lens in the rear group. (4) The front group includes, in order from the object side, a negative lens, a positive lens,
Positive lens, or a total of 3 positive lenses, negative lenses, and positive lenses
4. The teleconverter lens according to claim 3, wherein said rear group comprises one negative lens.