JPH0211883B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a photographic lens with a rear aperture for small lens shutter cameras, and particularly to a wide-angle lens with a simple and compact configuration for use in popular cameras. For popular compact lens shutter cameras, it is advantageous to use a wide-angle lens with a rear diaphragm in order to make it easier to attach to the camera and to make the entire camera including the lens and the lens itself more compact. be. As this type of lens, there is a triplet type lens which has a simple structure for use in popular machines, and some of these lenses are well known. (For example, Japanese Patent Publication No. 48-5494, Japanese Patent Publication No. 50-2807, etc.) However, these known triplet lenses have a half angle of view of about 27 degrees, and in order to make the camera even more compact, it is necessary to It is necessary to increase the angle to about 32°. However, when a triplet lens with a rear diaphragm is used to widen the angle of view, it is generally difficult to reduce vignetting near the maximum angle of view and prevent sharpening at the minimum aperture. One of the ways to reduce vignetting is to
Increasing the aperture of the lens not only goes against the objective of making the lens more compact, but also makes it difficult to increase the periphery of the enlarged first lens in an optical system with a simple configuration such as a triplet lens. It is impossible to eliminate the significant coma flare caused by the passing light rays. This invention improves the triplet type lens,
The objective is to obtain a lens with a brightness of an aperture ratio of about F4.0, a half angle of view of about 32 degrees, and small vignetting without the above-mentioned difficulties. In other words, a positive meniscus single lens, a negative double-concave single lens, and a positive double-convex single lens with their convex surfaces facing the object side are arranged in order from the object side, with a slight air gap between them, and both lenses are placed on the image side. In an optical system with an aperture placed close to a convex lens, f: Focal length of the entire system f _i : Focal length of the i-th lens from the object side f _1.2 : Combined focal length of the two lenses on the object side d _i : Distance between the i-th surface from the object side Σd: Distance between the first and sixth surfaces 0.005f<d ₄ <0.015f (1) 0.23f<Σd<0.26f (2) 0.88f<f ₁ ＜1.10f (3) −0.29f＜f ₂ ＜−0.25f (4) −0.61f＜f _1.2 ＜−0.44f (5) A triplet wide-angle lens with small vignetting This is what I was able to obtain. In order to reduce vignetting, the total thickness of the lens system Σd and _d4 must be small,
Conditions (1) and (2) relate to this. d ₄ is the condition (1)
If the value is larger than the upper limit, vignetting at the peripheral angle of view will increase, and the lens back will become longer, making it difficult to make the lens compact. Although it is desirable that d ₄ be as small as possible, it goes without saying that the lens area must be large enough for the light beam corresponding to the effective F-number to pass through. The lower limit value arises because of the luminous flux corresponding to a brightness of about _F4 . If Σd is larger than the upper limit of condition (2), vignetting at the peripheral surface angle becomes large, and the lens becomes less compact. On the other hand, if it is smaller than the lower limit, the vignetting will be smaller and the lens system will be more compact, but the Petzval sum and over-corrected spherical aberration of third and higher orders will increase, the curvature of the spherical aberration curve will become significantly large, and other It becomes impossible to correct it depending on the conditions. Since the lens of this invention is a wide-angle lens with a half angle of view of as much as 32 degrees, it is not possible to reduce the vignetting of the peripheral angle of view just by satisfying conditions (1) and (2). Therefore, by weakening the refractive power of the first lens, increasing the refractive power of the second lens, and increasing the negative refractive power of the composite system of the first lens and the second lens, the outer rays of the peripheral angle of view are is bent significantly with negative refractive power to reduce vignetting at peripheral angles of view. That is, f ₁ is smaller than the lower limit of condition (3), f ₂ is smaller than the lower limit of condition (4), or
When f _1.2 is smaller than the lower limit of condition (5), the negative refractive power becomes too small, making it impossible to reduce the vignetting of the peripheral angle of view. Furthermore, the Petzval sum of the entire system increases, and the curvature of field becomes significantly large. vice versa
When f ₁ , f ₂ , and f _1.2 are each larger than the upper limit values of conditions (3), (4), and (5), the negative refractive power becomes stronger and vignetting at the peripheral angle of view can be made very small. , the back focus becomes long and it becomes impossible to make the lens compact. Furthermore, the Petzval sum becomes too small, and the overcorrected spherical aberration increases significantly. In order to perform aberration correction that is well-balanced as a whole while satisfying the above conditions, it is desirable to further satisfy the following conditions. r _i : Radius of curvature of the i-th lens surface from the object side n _i : Refractive index of the i-th lens material from the object side 0.25f<r ₁ <0.31f (6) −0.70f<r ₃ ＜−0.50f (7) 0.25f＜r ₄ ＜0.34f (8) −0.58f＜r ₆ ＜−0.42f (9) 0.09f＜d ₁ ＜0.13f (10) 0.26f＜f ₃ ＜0.32f (11) 1.7＜n ₁ (12) 1.65＜n ₂ (13) 1.7＜n ₃ (14) Conditions (6)(7)(8)(9) are necessary to maintain good spherical aberration and Petzval sum. These are the conditions. r ₁ is greater than the upper limit of condition (6) or r ₆ is the condition
If it is smaller than the lower limit of (9), the amount of under-corrected high-order spherical aberration will be small, and the spherical aberration curve of the entire system will remain largely curved due to over-corrected high-order spherical aberration. Become. Conversely, if r ₁ becomes smaller than the lower limit of condition (6) or r ₆ becomes larger than the upper limit of condition (9), then
Spherical aberration is significantly undercorrected, making it difficult to correct spherical aberration as a whole system. When r ₃ becomes larger than the upper limit of condition (7), overcorrected higher-order spherical aberration that cannot be corrected by other means occurs. Conversely, when the value is smaller than the lower limit, negative astigmatism is reduced and the image plane becomes under-corrected. If r ₄ is larger than the upper limit of condition (8), spherical aberration will be undercorrected, and if r 4 is smaller than the lower limit, significant coma flare will occur at the peripheral angle of view. When d ₁ becomes larger than the upper limit of condition (10), vignetting at the peripheral angle of view becomes large, and even more significant coma flare occurs. Conversely, if it is smaller than the lower limit, the Petzval sum becomes too small and the curvature of the spherical aberration curve becomes large. If f ₃ is larger than the upper limit of condition (11), the curve of the spherical aberration curve will become large, and if f 3 is smaller than the lower limit, the aforementioned r ₆ will inevitably become larger, resulting in under-correction of spherical aberration, and even larger curves. Coma flare occurs. Conditions (12), (13), and (14) are necessary to maintain good Petzval sum and spherical aberration, and to suppress the occurrence of coma flare. When n ₁ and n ₃ become smaller than the lower limit values of conditions (12) and (14), the aforementioned r ₁ and r ₆ become strong, spherical aberration becomes insufficiently corrected, and the Petzval sum increases. If n ₂ is made smaller than the lower limit of condition (13) in order to reduce the Petzval sum, the aforementioned r ₄ will become small, and large coma flare will occur at the peripheral angle of view. Examples of this invention will be shown below. The symbols in the table are as follows. f _B : Back focus L: Distance from the final lens surface to the aperture W: Half angle of view example 1

[Table] A curve diagram of spherical aberration, astigmatic distortion aberration, and lateral aberration of the first example is shown in FIG. Example 2

[Table] Spherical aberration, astigmatism, distortion aberration of the second example,
A curve diagram of lateral aberration and lateral aberration is shown in FIG. Example 3

[Table] Spherical aberration, astigmatism, distortion aberration of the third example,
FIG. 4 shows a curve diagram of lateral aberration and lateral aberration. Example 4

【table】

[Table] Spherical aberration, astigmatism, distortion aberration of the fourth example,
FIG. 5 shows a curve diagram of lateral aberration and lateral aberration. Example 5

【table】

[Table] Spherical aberration, astigmatism, and distortion of the fifth example,
FIG. 6 shows a curve diagram of lateral aberration and lateral aberration. In FIGS. 2 to 6, the horizontal axis of the lateral aberration curve indicates the incident height of the light ray at the aperture position, and it is shown that vignetting is small up to the peripheral angle of view in all the examples.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a first embodiment of the lens of the present invention, and FIGS. 2 to 6 are aberration curve diagrams of the first to fifth embodiments, respectively.

Claims (1)

[Claims] 1. A positive meniscus single lens, a negative double-concave single lens, and a positive double-convex single lens, each having a convex surface facing the object side, are arranged in order from the object side with a slight air distance from each other, In an optical system with an aperture placed close to the lens on the image side, f: Focal length of the entire system f _i : Focal length of the i-th lens from the object side f _1.2 : Combined focus of the two lenses on the object side Distance d _i : Distance between the i-th surface from the object side Σd: Distance between the first and sixth surfaces 0.005f<d ₄ <0.015f 0.23f<Σd<0.26f 0.88f<f ₁ <1.10 A triplet lens with a rear diaphragm characterized by satisfying the following conditions: f −0.29f<f ₂ <−0.25f −0.61f<f _1.2 <−0.44f.