JPH06100721B2 - Wide angle photographic lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a wide-angle photographic lens having a five-element configuration suitable for a compact camera.

[Conventional technology]

As is known from Japanese Patent Application Laid-Open No. 58-62608, a photographic lens having five groups and five elements used in a conventional compact camera is provided with a power array in the order of positive, negative, negative, positive and negative from the object side. There is something I went to. In the lens described in this publication, the third lens (third lens from the object side) is a negative lens having a negative surface facing the object side, and the third lens and the fourth lens are separated from the second lens. This makes it easy to correct lateral chromatic aberration and coma.

In addition, Japanese Patent Publications No. 61-32654 and No. 60-260014 describe that the power arrangement from the object side is positive, negative, positive, positive, negative. A photograph of the former publication is shown. In the lens, the third lens is a positive meniscus lens having a negative surface facing the object side, and the second, third, and fourth lenses are arranged close to each other. The photographic lens described in the latter publication is designed compactly with a telephoto ratio of 0.91, and the third lens and the fourth lens are arranged close to each other.

Note that the photographic lenses known in the above-mentioned respective publications have in common that a biconcave lens is used as the second lens.

[Problems to be solved by the invention]

However, in the photographic lens disclosed in JP-A-58-62608, when the third lens and the fifth lens are plastic lenses in order to reduce the weight of the lens and reduce the cost, the amount of focus movement due to temperature change Has the drawback of becoming large.

In addition, in the photographic lens of Japanese Patent Publication No. 61-32654,
Since the fourth lens is in close proximity, in order to correct Petzval's sum and chromatic aberration of magnification while shortening the total lens length, at least one of the third and fourth lenses should have a low refractive index and high dispersion glass material (Ohara Corporation). It is necessary to use a F16) manufactured by MHI, which is disadvantageous in terms of cost and has a low degree of freedom in optical design.

Furthermore, the photographic lens described in JP-A-60-260014 has F
The dark lens is 4.0, and the third lens and the fourth lens are close to each other. Therefore, when trying to further shorten the total lens length, the second lens is set to the third lens.
You have to get close to the lens. However, for this purpose, the dispersion of the third lens or the fourth lens is increased,
It is difficult to correct the Petzval sum because the refractive index must be increased, or it is necessary to use a glass material for the second lens, such as SF03 manufactured by OHARA, which has a high refractive index but poor workability. Must result in increased costs.

[Object of the Invention]

The present invention has been made in view of the above-described prior art. In the case of reducing the cost by using two lenses out of the five-group, five-element configuration as a plastic lens, there is no focus movement due to temperature change and the total length is shortened. However, it is an object of the present invention to provide a wide-angle photographic lens capable of suppressing aberration fluctuation during focusing.

[Means for solving problems]

In order to achieve the above object, the present invention has a positive meniscus first lens, a negative meniscus second lens, and
Five-group five-element configuration in which a third lens having a positive meniscus, a biconvex fourth lens having a surface having a gentle curvature toward the object side, and a fifth lens having a negative meniscus having a negative surface having a strong curvature toward the object side are arranged. And the third lens and the fifth lens are made plastic lenses, and the surface on the object side of each of them is It is an aspherical surface.

Further, f ₁ and f ₄ are the focal lengths of the first lens and the fourth lens, K ₃ is the conical constant of the aspherical surface of the third lens, T _H is the telephoto ratio, and N ₁ and N ₇ are the first lens, respectively. , The refractive index of the fourth lens, R ₁ and R ₃ are the radii of curvature of the object-side surfaces of the first lens and the second lens, respectively, and d ₆ is the air gap between the third lens and the fourth lens. When R ₃ > 0.0, ・ ・ ・ (1) −1.6 <K ₃ <−0.9, ・ ・ ・ (2) The conditional expression of is satisfied.

The same plastic material is used for the third lens and the fifth lens, and these plastic lenses cancel the influence of temperature to eliminate the change in power. Further, the diaphragm is arranged between the third lens and the fourth lens, and each of the lens surfaces from the first lens to the third lens and the lens surface of the fifth lens has a spherical center on the diaphragm side. It is configured as a sphere.

[Action]

The expression (1) is a conditional expression when the principal point position of the entire system is moved to the object side by the second lens, and the expression (2) is for suppressing the spherical aberration to be small when the total lens length is shortened. It is a conditional expression. By the conditions of the expressions (1) and (2), the lens system can be compactly assembled without causing the focus movement.

Further, the expression (3) is a conditional expression for preventing coma flare when the lens system is made compact so as not to pose a problem in practical use, and the expression (4) uses the fourth lens as the third lens and the fifth lens as the fifth lens.
It is positioned approximately at the center of the lens and represents the condition for balancing the aberrations of the lens system and satisfactorily correcting the chromatic aberration of magnification. When the fourth lens approaches the third lens side, it becomes difficult to correct the chromatic aberration of magnification. On the contrary, when the fourth lens approaches the fifth lens side, the peripheral light amount becomes insufficient, and the size of the lens is inevitably increased. Therefore, it is not preferable.

The third lens and the fifth lens, which are plastic lenses, are mainly used for canceling the focus movement due to the temperature change, and are determined to reduce the power load. In particular, when the power of the fifth lens is increased, the asymmetry of the lens system becomes strong, and the aberration variation during focusing becomes large, which is not preferable.

An example in which the present invention is applied to a wide-angle photographic lens having an angle of view of 65 ° and an F number of about 2.8 will be described below with reference to optical system data and aberration diagrams.

First Embodiment FIG. 1 shows the first positive meniscus in order from the object side (left side).
Lens L1, second lens L2 of negative meniscus, third lens L3 of positive meniscus, biconvex fourth lens L4 with a surface having a gentle curvature toward the object side, negative meniscus lens with a strong negative surface toward the object side The 1st example of the present invention in which the 5th lens L5 is arranged is shown. The third lens L3 and the fifth lens L5 are each composed of a plastic lens, and are given surface numbers i in order from the object side, the radius of curvature at each surface is Ri, the surface spacing on the axis is di, and the refraction of the d line is The lens data of the first embodiment are as follows, where the index and the Abbe number are Ni and νi, respectively.

In the above data, R ₅ and R ₉ are aspherical surfaces of aspherical surfaces ASPH 1 and ASPH 2, where paraxial radius of curvature is r, conic coefficient is K, and aspherical coefficient is A, B, C. , These aspheric ASPH 1, ASPH 2
Is The above values of r, K, A, B and C are `` ASPH 1 '' r 10.10790 K −1.3036 A −0.245359 × 10 <sup>-4</sup> B −0.292012 × 10 <sup>-5</sup> C 0.2 × 10 <sup>-7</sup> `` ASPH 2 '' r −6.61556 K −0.33784 A 0.848371 × 10 ⁻⁶ B 0.0 C 0.0, focal length FL, back focus length BF, F _NO
Respectively FL = 34.0 BF = 10.10 F _NO = 2.88.

The values of the telephoto ratio T _H , the focal lengths f ₁ and f _{4 of} the first lens L1 and the fourth lens L4, which are used in connection with the conditional expressions (1) to (4) described above, are as follows. ing.

T _H = 0.956 f ₁ = 23.423 f ₄ = 38.074 In the first embodiment, the aberration characteristics shown in FIGS. 2A and 2B are obtained at the infinity focus position and the 1.0 m focus position. In the spherical aberration diagram, the solid line shows d
The line and the one-point left-handed rotation show the characteristic for the g-line, the solid line in the astigmatism diagram shows the characteristic for the sagittal surface, and the broken line shows the characteristic for the meridional surface.

Hereinafter, the lens data etc. of the second to sixth embodiments of the present invention shown in FIGS. 3, 5, 7, 9 and 11 will be shown.
These aberration diagrams are shown in FIG. 4, FIG. 6, FIG. 8, FIG.
Shown in Figure 12. The symbols such as the surface number i are used in common with those in FIG. Also in these examples, the third lens and the fifth lens are composed of plastic lenses,
The surface on the object side is an aspherical surface expressed by the above formula. Further, FIGS. 4 (A) and 4 (B) show the characteristics at infinity and a focus position of 1.0 m as in FIG. 2, and FIGS. 12 (A) and 12 (B) show infinity and 0.8. The characteristic at the focus position of m is shown. It should be noted that FIGS. 6, 8, and 10 show the characteristics at the infinite position.

[Second Embodiment] `` ASPH 1 '' r 10.42567 K -1.04377 A -0.430454 x 10 <sup>-4</sup> B -0.149333 x 10 <sup>-5</sup> C 0.0 `` ASPH 2 '' r -6.11880 K -0.31028 A 0.26950 x 10 ^-4 B 0.0 C 0.0 FL = 34.5 BF = 10.612 F _NO = 2.90 T _H = 0.945 f ₁ = 25.168 f ₄ = 41.396 [Third Embodiment] ASPH 1 r 13.25597 K −1.42878 A 0.0 B 0.0 C 0.0 ASPH 2 r −5.55423 K −0.30806 A 0.0 B 0.0 C 0.0 FL ＝ 34.5 BF ＝ 11.532 F _NO ＝ 2.88 T _H ＝ 0.946 f ₁ ＝ 24.416 f ₄ = 33.995 [Fourth Embodiment] ASPH 1 r 12.01219 K −1.07370 A 0.0 B 0.0 C 0.0 ASPH 2 r −5.44490 K −0.30601 A 0.0 B 0.0 C 0.0 FL ＝ 34.5 BF ＝ 11.833 F _NO ＝ 2.89 T _H ＝ 0.938 f ₁ ＝ 22.525 f ₄ = 31.33 [Fifth Embodiment] "ASPH 1" r 13.65251 K −1.11000 A 0.0 B 0.0 C 0.0 “ASPH 2” r −5.64314 K −0.34192 A 0.0 B 0.0 C 0.0 FL = 34.5 BF = 11.222 F _NO = 2.89 T _H = 0.942 f ₁ = 27.206 f ₄ = 37.04 [Sixth Embodiment] `` ASPH 1 '' r 8.65196 K -1.21721 A -0.403126 x 10 <sup>-5</sup> B -0.171 130 x 10 <sup>-5</sup> C -0.690591 x 10 <sup>-7</sup> `` ASPH 2 '' r -7.53166 K -0.48997 A -0.276328 x 10 ^-4 B -0.485445 × 10 ^-6 C 0.0 FL ＝ 34.5 BF ＝ 10.595 F _NO ＝ 2.90 T _H ＝ 0.944 f ₁ ＝ 22.634 f ₄ ＝ 42.656 In order to obtain a wide-angle photographic lens having an angle of view of 65 ° and an F number of about 2.8 as in the above examples, the first lens L1 is a positive meniscus lens having a strong convex surface facing the object side, and the second lens L2 and the third lens By providing an air lens with a thin air gap between the lens L3 and the object side surface of the third lens L3 as an aspherical surface, it becomes possible to satisfactorily correct spherical aberration and coma. In the prior art, the object-side surface of the second lens had to be concave in order to correct the aberration, whereas the configuration described above reduces the load of the second lens on the aberration correction. Since the lens can be configured by a concave lens having a convex surface facing the object side, it is advantageous in moving the principal point position to the object side and shortening the total length of the lens system.

Further, the fourth lens L4 acts as an image forming lens for the lens group from the first lens L1 to the third lens L3,
Astigmatism is corrected by the air gap between the third lens L3 and the fourth lens L4, and chromatic aberration of magnification and Petzval sum are corrected by selecting the glass material of the fourth lens L4. Further, the balance distribution between the powers of the first lens L1 to the third lens L3 and the powers of the fourth lens L4 and the fifth lens L5 makes it possible to control the aberration variation due to focusing. The fifth lens L5 is a negative lens for extending the focal length of the entire system, but the aspherical surface on the object side causes the fifth lens L5 to the fourth lens L4.
It is possible to satisfactorily correct aberrations that cannot be completely corrected by the above lens groups, especially distortion.

〔The invention's effect〕

As described above, according to the present invention, in constructing a wide-angle photographic lens, the third lens and the fifth lens in the five-group, five-lens configuration are made of plastic lenses, and the object side surface thereof is By making each aspherical surface, it is possible to suppress the focus movement with respect to the temperature change while reducing the cost. In addition, the overall lens system can be shortened while suppressing fluctuations in aberrations during focusing, which gives greater freedom in design compared to conventional lenses, and a wide-angle photographic lens with superior performance. Will be obtained.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment of the present invention. FIGS. 2A and 2B are aberration characteristic diagrams of the first embodiment when the in-focus position is at infinity and 1.0 m. FIG. 3 is a schematic configuration diagram showing a second embodiment of the present invention. FIGS. 4A and 4B are aberration characteristic diagrams of the second embodiment when the focus position is at infinity and 1.0 m. FIG. 5 is a schematic configuration diagram showing a third embodiment of the present invention. FIG. 6 is an aberration characteristic diagram of the third embodiment when the in-focus position is at infinity. FIG. 7 is a schematic configuration diagram showing a fourth embodiment of the present invention. FIG. 8 is an aberration characteristic diagram of the fourth embodiment when the focus position is at infinity. FIG. 9 is a schematic configuration diagram showing a fifth embodiment of the present invention. FIG. 10 is an aberration characteristic diagram of the fifth embodiment when the in-focus position is at infinity. FIG. 11 is a schematic configuration diagram showing a sixth embodiment of the present invention. 12 (A) and 12 (B) are aberration characteristic diagrams of the sixth embodiment when the in-focus position is at infinity and 0.8 m. L1 …… first lens L2 …… second lens L3 …… third lens L4 …… fourth lens L5 …… fifth lens

Claims (1)

[Claims] 1. A wide-angle photographic lens having a five-element structure, wherein a positive meniscus first lens, a negative meniscus second lens, a positive meniscus third lens, and an object side having a gentle curvature are arranged in order from the object side. It is composed of a convex fourth lens and a negative meniscus fifth lens having a strong negative curvature on the object side. Each of the third lens and the fifth lens has at least a surface on the object side substantially. In addition to the aspherical plastic lens represented by, f ₁ and f ₄ are the focal lengths of the first lens and the fourth lens, K ₃ is the conical constant of the aspherical surface of the third lens, and T _H is the telephoto ratio. ,
N ₁ and N ₇ are the refractive indices of the first lens and the fourth lens, R ₁ and
When R ₃ is the radius of curvature of the object-side surface of each of the first lens and the second lens, and d ₆ is the air space between the third lens and the fourth lens, R ₃ > 0.0, −1.6 <K ₃ <−0.9, A wide-angle photographic lens characterized in that the conditional expression is satisfied.