JPS633286B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a so-called circumferential fisheye lens that is bright with an aperture ratio of 1:2.8, has a long backfocus that is more than 4.5 times the focal length, and has an angle of view of 180° in the entire circumferential direction. This relates to an equidistant projection type fisheye lens in which distortion is controlled so that the angle of view is directly proportional to the angle of view. A fisheye lens is a lens designed to project a spherical surface onto a flat surface by intentionally creating distortion, and is also used in general photography by taking advantage of the special distortion effect. In order to intentionally create distortion and ensure a long back focus as an interchangeable lens for single-lens reflex cameras,
A negative lens group with strong refractive power is usually placed in the front group. Therefore, in general, it becomes difficult to correct lateral chromatic aberration, and field curvature and astigmatism increase, making it difficult to maintain high optical performance. In particular, with so-called equidistant projection type fisheye lenses where the image height is proportional to the angle of incidence, it is necessary to uniquely determine the amount of distortion for the angle of incidence and control this.
The degree of difficulty increases further. The present invention overcomes the above-mentioned difficulties and has an aperture ratio of 1:2.8, with a back focus that is close to the focal length.
It provides an equidistant projection type fisheye lens with a magnification of more than 4.5 times. First, to explain the lens configuration of the present invention, it consists of, in order from the object side, a first lens and a second lens that are negative meniscus lenses with their convex surfaces facing the object side, a third lens that is a negative lens, and a fourth lens that is a positive lens. A third lens group consisting of a laminated lens of a positive fifth lens and a negative sixth lens; a laminated lens of a positive seventh lens and a negative eighth lens; 9th lens
This fisheye lens is composed of a lens group consisting of a bonded lens and a tenth lens, which is a positive lens, and is characterized by satisfying the following conditions. (1) 3＜｜f〓｜/f＜6, f〓＜0 (2) 0.2＜d ₆ /f＜0.7 (3) 0.75＜r ₆ /r ₇ ＜1.15 (4) −1.9＜r ₁₃ / f<−1.5 (5) 2<r ₁₆ /f<3.5 (6) 〓 _N −〓 _P ＞0.18 (7) 〓 _P −〓 _N ＞20 where f: Focal length of the entire system f: The focal length of the first lens group Focal length d ₆ : Air distance between the third and fourth lenses r ₆ : Radius of curvature of the image side surface of the third lens r ₇ : Radius of curvature of the object side surface of the fourth lens r ₁₃ : Between the seventh lens and Radius of curvature of the bonding surface with the 8th lens r ₁₆ : Radius of curvature of the bonding surface with the 9th lens and the 10th lens 〓 _N : Average value of the refractive index of the negative lens in the 1st lens group 〓 _P : The 1st lens Average value of the refractive index of the positive lenses in the group〓 _P : Average value of the Atsube number of the positive lenses in the 1st lens group〓 _N : Average value of the Atsube numbers of the negative lenses in the 1st lens group Next, each of the above conditions will be explained. do. Condition (1) relates to the refractive power of the first lens group. conditions
If the upper limit of (1) is exceeded, the refractive power of the first lens group will become too small, making it difficult to secure the necessary back focus, or the overall length of the lens will become longer in order to secure the back focus. arise. Conversely, if the lower limit of condition (1) is exceeded, the refractive power of the first lens group becomes too large, positive spherical aberration and chromatic aberration increase, and the Petzval sum becomes too small, resulting in an increase in field curvature. It becomes difficult to maintain good performance. Condition (2) relates to the air gap between the third and fourth lenses. All lenses from the first lens to the third lens are negative lenses and generate negative distortion, but in the case of an equidistant projection type fisheye lens like the present invention, it simply reflects light at an incident angle of 90°. On the other hand, it is necessary not only to generate 100% negative distortion, but also to take an arrangement that imparts a certain fixed distortion to light at any incident angle. In the present invention, by arranging the fourth lens, which is a positive lens, at an appropriate distance from the third lens, the height at which light beams incident from various angles pass through the fourth lens is adjusted, and the height of the fourth lens is adjusted. By controlling the amount of positive distortion that occurs, the first
The final amount of distortion is controlled by skillfully adjusting the balance with the negative distortion generated in the negative lens groups from the lens to the third lens. If condition 2 is not met, it becomes difficult to realize the above-mentioned point. Furthermore, if the upper limit of condition (2) is exceeded, there will be a drawback that the occurrence of positive spherical aberration will increase too much. Condition (3) is related to condition (2) and relates to correction of spherical aberration and chromatic aberration occurring within the first lens group. conditions
(3) If the lower limit of , chromatic aberration remains, and even if the upper limit of condition (3) is exceeded, the radius of curvature of the object side surface of the fourth lens becomes too small, and high-order positive spherical aberration and chromatic aberration still remain. However, good performance cannot be maintained. Condition (4) relates to the radius of curvature of the bonding surface between the seventh lens and the eighth lens. Condition (4) sets the bonding surface to act more strongly on the axial light beam than on the off-axis light beam, and in conjunction with conditions (6) and (7) described later, mainly axial chromatic aberration It is also effective in correcting spherical aberration. If the lower limit of condition (4) is exceeded, the effect of correcting longitudinal chromatic aberration and spherical aberration will be weakened, and the chromatic aberration of magnification of the off-axis light beam will increase, which is not preferable. If the upper limit is exceeded, the effect of correcting longitudinal chromatic aberration and spherical aberration will be excessive, which is undesirable. Condition (5) relates to the radius of curvature of the bonding surface between the ninth lens and the tenth lens. Condition (5) is also the condition described below.
In relation to (6) and (7), it contributes to correction of longitudinal chromatic aberration, as well as correction of lateral chromatic aberration of off-axis light beams and correction of coma aberration. If the lower limit of condition (5) is exceeded, it is effective in correcting lateral chromatic aberration and coma aberration, but large positive axial chromatic aberration occurs, which is undesirable. When the upper limit of condition (5) is exceeded, the effect of correcting lateral chromatic aberration and coma aberration is weakened, and the bonding surface loses its meaning. Condition (6) relates to the difference in refractive index between the glass material used for the positive lens and the glass material used for the negative lens in the first lens group. As described above, since the lens of the present invention uses a strong negative lens group as the first lens group, the Petzval sum tends to decrease, and field curvature and astigmatism worsen. Condition (6) is to correct this decrease in the Petzval sum. If this condition (6) is not met, the effect of correcting the decrease in Petzval's sum will be weakened, and field curvature and astigmatism will worsen and cannot be corrected, which is not preferable. At the same time, the effect of correcting various aberrations on each bonding surface in the aforementioned first lens group is undesirably weakened. Condition (7) relates to the difference in Abbe number between the glass material used for the positive lens and the glass material used for the negative lens in the first lens group. As described above, the bonding surfaces of the seventh lens and the eighth lens, and the bonding surfaces of the ninth lens and the tenth lens in the lens group contribute to the correction of axial chromatic aberration and lateral chromatic aberration, respectively. If condition (7) is not met, the effect of correcting each chromatic aberration on each bonding surface will be lost,
Unable to maintain good performance. Note that, if necessary, it is possible to insert a parallel plane plate as a filter between the sixth lens and the seventh lens, and this is within the scope of the present invention. Examples of the present invention will be shown below. Here, f is the focal length of the entire system, f _B is the back focus, r is the radius of curvature of each lens surface, d is the lens thickness or lens spacing,
n is the d-line refractive index of each lens, and ν is the Abbe number of each lens. Example 1

【table】 Example 2

【table】

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a lens configuration diagram of Example 1 of the present invention, FIG. 2 is a diagram of various aberrations of Example 1, FIG. 3 is a lens configuration diagram of Example 2 of the present invention, and FIG. 4 is a lens configuration diagram of Example 2. It is a diagram of various aberrations.

Claims (1)

[Claims] 1. In order from the object side, a first lens and a second lens of a negative meniscus lens both having convex surfaces facing the object side;
a third lens group consisting of a negative third lens and a positive fourth lens; a third lens group consisting of a bonded lens of a positive fifth lens and a negative sixth lens; and a seventh positive lens. It is composed of a laminated lens consisting of a lens and an eighth lens which is a negative lens, and a laminated lens consisting of a ninth lens which is a negative lens and a tenth lens which is a positive lens, and is required to satisfy the following conditions. Features a fisheye lens. (1) 3＜｜f〓｜/f＜6, f〓＜0 (2) 0.2＜d ₆ /f＜0.7 (3) 0.75＜r ₆ /r ₇ ＜1.15 (4) −1.9＜r ₁₃ / f<−1.5 (5) 2<r ₁₆ /f<3.5 (6) 〓 _N −〓 _P ＞0.18 (7) 〓 _P −〓 _N ＞20 where f: Focal length of the entire system f _I : Focal length of the 1st lens group Focal length d ₆ : Air distance between the third and fourth lenses r ₆ : Radius of curvature of the image side surface of the third lens r ₇ : Radius of curvature of the object side surface of the fourth lens r ₁₃ : Between the seventh lens and Radius of curvature of the bonding surface with the 8th lens r ₁₆ : Radius of curvature of the bonding surface with the 9th lens and the 10th lens 〓 _N : Average value of the refractive index of the negative lens in the 1st lens group 〓 _P : The 1st lens Average value of the refractive index of the positive lens in the group〓 _P : Average value of the Atsube number of the positive lens in the 1st lens group〓 _N : Average value of the Atsube number of the negative lens in the 1st lens group