JPH0250445B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a wide-angle lens with an angle of view of about 57 degrees and a brightness of about F2.8, and particularly to a wide-angle lens used in optical systems with relatively small screen sizes such as disk cameras and facsimiles. Conventionally, this type of lens system had a small focal length, so no consideration was given to making the lens compact, and it had a large telephoto ratio. Recently, lenses for disk cameras have appeared that have a reduced telephoto ratio by using an aspherical surface. The present invention uses a configuration of four lenses in four groups to provide a wide-angle lens that uses only spherical surfaces without using any aspherical surfaces, has a small telephoto ratio, has good aberration correction, and is suitable for the above-mentioned applications. In order to achieve this objective, the lens system is constructed in order from the object side: a first lens group consisting of a biconvex lens with a strongly convex surface facing the object side, and a second lens group consisting of a biconcave lens with a strongly concave surface facing the image side. It consists of two lens groups, a third lens group that is a biconvex lens, and a fourth lens group that is a negative meniscus with a convex surface facing the image side, f: focal length of the entire system f _1.2 : the first lens group and the second lens group Combined focal length with the lens group f _1.2.3 : Combined focal length from the first lens group to the third lens group D ₆ : Distance between the third lens group and the fourth lens group R ₇ : Object in the fourth lens group Radius of curvature Ni: When the refractive index of the glass material of the i-th lens group is -0.05/f<1/f1.2<0.15/f...(1) 1.27/f<1/f1, 2, 3<1.55 /f...(2) 0.22f< _D6 <0.36f...(3) _N1 , _N2 , _N4 >1.75, _N3 >1.68...It is a wide-angle lens that satisfies each condition of (4). , the telephoto ratio can be pushed to about 1.2 or less. In the lens of the present invention, an aperture is placed between the second lens group and the third lens group in order to improve the conditions for off-axis light flux. Therefore, the negative distortion generated in the third lens group is corrected by using a biconvex lens in the first lens group. Conditions (1) and (2) are necessary to maintain appropriate spherical aberration, coma aberration, and distortion, and to reduce the telephoto ratio. If the lower limit of condition (1) is exceeded, the focal length of the first lens group becomes longer or the focal length of the second lens group becomes shorter, resulting in excessive coma flare and large under distortion. Furthermore, since the principal point position moves toward the image side, the telephoto ratio increases. On the other hand, if the upper limit is exceeded, the focal length of the first lens group becomes shorter or the focal length of the second lens group becomes longer, resulting in large under-spherical aberration. If the lower limit of condition (2) is exceeded, excessive coma flare will occur and the telephoto ratio will further increase. On the other hand, if the value is larger than the upper limit, under-coma flare, large under-distortion aberration, and under-spherical spherical aberration will occur. Condition (3) is necessary to keep the telephoto ratio small while keeping various aberrations within appropriate ranges. If condition (3) exceeds the lower limit, in order to keep the telephoto ratio small, 1/f _1.2 should be greater than the upper limit of condition (1), or 1/f _1.2.3 should be greater than the upper limit of condition (2). Must be. Conversely, when condition (3) exceeds the upper limit, the telephoto ratio decreases, but the fourth lens group is far away from the third lens group, so correction of spherical aberration and coma aberration becomes difficult.
It becomes difficult. Condition (4) is a necessary condition for keeping the Petzpar sum of the entire system within an appropriate range, spherical aberration and coma aberration within an appropriate range, and a necessary condition for keeping the telephoto ratio small. Refractive index of convex lens group
Since the radius of curvature of the convex lens group in which N ₁ and N ₃ are smaller than the lower limit becomes smaller, the Petz Pearl sum becomes larger and the curvature of field becomes larger. Furthermore, spherical aberration becomes undercorrected. Conversely, when the refractive indices N ₂ and N ₄ of the concave lens group become smaller than the lower limit of condition (4), the radius of curvature of the concave lens group becomes small, the Petzpar sum becomes small, and the image plane curves toward the image side. As the point distance difference increases, excessive coma flare occurs. Even if the refractive index of each lens group is smaller than the lower limit of condition (4), each aberration can be well corrected, but since the present invention is aimed at a lens system with a small focal length, the processing of each lens group is Ease of use must also be considered. The refractive index of the convex lens group is the condition (4)
If it becomes smaller than the lower limit of , the aberration described above will worsen.
This is because when the refractive index of the concave lens group becomes small, the radius of curvature of the convex lens group becomes small, so lens processing becomes difficult unless the axial thickness of the convex lens is increased. However, if the axial thickness of the convex lens group is increased, it becomes impossible to keep the telephoto ratio of the lens system small. Furthermore, in order to keep spherical aberration and coma within appropriate ranges, it is desirable that _R7 satisfy the following conditions. −0.6f<<R ₇ <−0.32f (5) Examples of the lens system of the present invention will be shown below. Next, examples of the present invention will be shown. In the examples, R is the radius of curvature, D is the interplanar distance, N is the refractive index, V is the Abbe number, and T is the telephoto ratio. Example 1 f=10 F _Np =2.87 2W=57°30′

[Table] T=1.197 1/f _1.2 = 0.0538/f 1/f _1.2.3 =
1.46811/f Spherical aberration, astigmatism, and distortion aberration curves are shown in Figure 2. Example 2 f=10 F _Fp =2.87 2w=57°24′

[Table] T=1.198 1/f _1.2 = 0.0415/f 1/f _1.2.3 =
1.452/f Spherical aberration, astigmatism, and distortion aberration curves are shown in Figure 3. Example 3 f=10 F _Np =2.87 2w=57°31′

[Table] T=1.199 1/f _1.2 = 0.0982/f 1/f _1.2.3 =
1.4688.f Figure 4 shows the spherical aberration, astigmatism, and distortion aberration curves. Example 4 f=10 F _Np =2.87 2w=50°26′

【table】

[Table] T=1.195 1/f _1.2 = 0.0181/f 1/f _1.2.3 =
1.4300/f Figure 5 shows the spherical aberration, astigmatism, and distortion aberration curves. Example 5 f=10 F _Np =2.87 2w=57°32′

[Table] T=1.199 1/f _1.2 = 0.0312/f 1/f _1.2.3 =
1.4500/f Figure 6 shows the spherical aberration, astigmatism, and distortion aberration curves. Example 6 f=10 F _Np =2.87 2w=56°26′

[Table] T=1.188 1/f _1.2 = 0.0681/f 1/f _1.2.3 =
1.4143/f Spherical aberration, astigmatism, and distortion aberration curves are shown in FIG. Example 7 f=10 F _Np =2.87 2w=56°12′

[Table] T=1.156 1/f _1.2 = 0.1241/f 1/f _1.2.3 =
1.4734/f Spherical aberration, astigmatism, and distortion aberration curves are shown in Figure 8. Example 8 f=10, F _Np =2.87 2w=56°12′

【table】

[Table] T=1.196 1/f _1.2 = 0.0599/f 1/f _1.2.3 =
1.3523/f Spherical aberration, astigmatism, and distortion aberration curves are shown in Figure 9.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a lens according to an embodiment of the present invention, FIGS. 2, 3, 4, 5, 6, and 7.
, FIG. 8, and FIG. 9 are respectively Example 1, Example 2, Example 3, Example 4, Example 5, Example 6,
FIG. 7 is an aberration curve diagram of Example 7 and Example 8.

Claims (1)

[Claims] 1. In order from the object side, a first lens group consisting of a biconvex lens with a strongly convex surface facing the object side, a second lens group consisting of a biconcave lens with a strongly concave surface facing the image side, and a biconvex lens. It is composed of a third lens group and a fourth lens group which is a negative meniscus with a convex surface facing the image side, f: Focal length of the entire system f _1.2 : Combined focal length of the first lens group and the second lens group f _1.2.3 : Composite focal length D from the first lens group to the third lens group ₆ : Distance between the third and fourth lens groups R ₇ : Radius of curvature on the object side of the fourth lens group Ni: i-th When the refractive index of the glass material of the lens group is -0.05/f<1/f1.2<0.15/f1.27/f<1/f1,2,3<1.55/f0.22f< _D6 <0.36f _N1 , Wide-angle lens that satisfies the following conditions: N ₂ , N ₄ ＞1.75N ₃ ＞1.68