JPH09159912A - Wide-angle lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a small-sized and simple wide-angle lens by constituting the lens of a 1st lens being a biconcave lens and a 2nd lens whose lens surface on an image side has intense curvature and which has positive power from an object side and making at least one surface of both lenses aspherical. SOLUTION: Wide angle is attained by setting the lens as a retrofocus type that the 1st lens is a negative lens and a 2nd lens is a positive lens. Then, the biconcave lens is set as the 1st lens and the rear-side principal point of the 1st lens is set closer to the object side than the lens surface on the image side, while the curvature of the lens surface on the image side of the 2nd lens is made intenser than that of the lens surface on the object side, so that the front-side principal point of the 2nd lens is closer to the image side. Therefore, even when a distance between the 1st lens and the 2nd lens is made short, a distance between the principal points of both lenses becomes long. Therefore, the increase of the power of both lenses is restrained, and aberration is easily corrected. Namely, by making the distance between the principal points long with reference to the distance between both lenses, the distance between both lenses is shortened and the lens is miniaturized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact and simple wide-angle lens suitable for surveillance cameras, videophones and the like.

[0002]

2. Description of the Related Art Optical systems used in surveillance cameras, videophones and the like are desired to be small and have a wide angle. In response to this request, JP-A-4-107407 and JP-A-4
No. 3,349,418 or Japanese Patent Laid-Open No. 6-67091 discloses a two-group, two-element configuration. Each of these is a retrofocus type, and the distance between the first lens and the second lens is as large as 1.5 times or more the focal length of the entire lens system. Therefore, the effective diameter of the first lens becomes large, and miniaturization is insufficient.

Further, Japanese Patent Laid-Open No. 6-67089 discloses that
An optical system having a two-group, two-element configuration, in which the distance between the first lens and the second lens is small and which is more compact than the optical system is disclosed, but this optical system has a narrow field angle of 40 ° or less. ,
Insufficient as a wide-angle lens.

[0004]

As described above, the conventional lens is insufficient in downsizing, or the angle of view is narrow even with a compact lens, which is suitable for a surveillance camera or a videophone. The lens was not always satisfactory.

The present invention has been made to solve the above problems. That is, it is an object of the present invention to provide a small and simple wide-angle lens suitable for a surveillance camera, a videophone and the like.

[0006]

The object of the present invention is achieved by adopting the following constitution.

The first lens is a biconcave lens in order from the object side, and the second lens having a positive power on the image side lens surface has a stronger curvature than the object side. Alternatively, the wide-angle lens is characterized in that the second lens has at least one aspherical surface.

Further, it is desirable that the aspherical surface is disposed on the image side lens surface of the second lens, and the curvature thereof gradually decreases as the distance from the optical axis increases.

Further, it is desirable that the first lens has at least one aspherical surface whose curvature gradually decreases with distance from the optical axis, and satisfies the following conditional expression.

| R ₂ / r ₁ | ≦ 0.2 (1) where r _{1 is} the radius of curvature of the object-side lens surface of the first lens r _{2 is the} radius of curvature of the image-side lens surface of the first lens The wide-angle lens preferably satisfies the following conditional expression.

0.3 ≦ d ₂ /f≦1.2 (2) where d _{2 is} the axial upper surface distance between the first lens and the second lens f is the focal length of the entire lens system

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION

(A) In the optical system of the present invention, the first lens is a negative lens and the second lens is a second lens.
A wide angle has been achieved by using a retro focus type lens called a positive lens. In addition, the first lens is biconcave, and the rear principal point of the first lens is brought closer to the object side than the image side lens surface, while the curvature of the image side lens surface of the second lens is set to be smaller than that of the object side lens surface. By making it stronger, the front principal point of the second lens is brought closer to the image side. Therefore, even if the lens distance between the first lens and the second lens is reduced, the principal point distance between the first lens and the second lens is relatively large. Therefore, it is possible to suppress the increase in the power of the first lens and the second lens, and it is easy to correct the aberration.

That is, by increasing the distance between the principal points with respect to the lens distance between the first lens and the second lens, the distance between the first lens and the second lens can be shortened, thereby achieving miniaturization. ing.

Further, when the lens distance between the first lens and the second lens is made small, the curvature of the image side lens surface of the first lens becomes very strong. Therefore, if the effective diameter of the first lens is increased by widening the angle of the optical system, it becomes difficult to process the lens.

Therefore, if the first lens is a biconcave lens,
Since the curvature of the lens surface on the image side can be weakened as compared with the case of using a meniscus lens, processing of the lens becomes relatively easy.

(B) By using at least one aspherical surface, it is possible to excellently correct aberrations. In particular, the image-side lens surface of the second lens is an aspherical surface, and the curvature becomes gradually weaker as the distance from the optical axis increases, so that the inward coma aberration can be favorably corrected.

At this time, it is desirable that the following conditional expression be satisfied.

0.005f ≦ | Δ _max | ≦ 0.035f (3) where f is the focal length of the entire system, and Δ _max is the light from the reference spherical surface due to the aspheric surface at the maximum effective diameter of the image side surface of the second lens. It is the amount of deviation in the axial direction.

When the value goes below the lower limit of the expression (3), the coma aberration is insufficiently corrected, and the inward coma aberration remains. On the contrary, when the value exceeds the upper limit of the expression (3), the coma aberration is overcorrected, and the outward coma aberration occurs.

In order to correct coma aberration better,
It is desirable that the expression (3) satisfy the following condition.

0.010f ≦ | Δ _max | ≦ 0.028f (4) (c) Further, by using an aspherical surface whose curvature gradually becomes weaker as it moves away from the optical axis for the first lens, the distortion aberration is corrected. be able to. However, if the upper limit of conditional expression (1) is exceeded, it becomes difficult to correct distortion.

In order to satisfactorily correct the distortion aberration,
Conditional expression (1) preferably satisfies the following condition.

| R ₂ / r ₁ | ≦ 0.1 (d) When d ₂ exceeds the upper limit of conditional expression (2), the total length of the optical system increases and the size of the first lens increases. Not desirable. If d ₂ becomes smaller than the lower limit of the conditional expression (2), it becomes difficult to correct coma and astigmatism, and it becomes impossible to secure a sufficient back focus, which is not desirable.

In order to make the optical system sufficiently small, it is desirable that the upper limit of conditional expression (2) be 1.0. Further, in order to correct coma and astigmatism better, it is desirable that the lower limit of conditional expression (2) is 0.4.

[0025]

Next, examples of the present invention will be described. Here, r is the radius of curvature, d is the surface spacing, nd is the refractive index for the d-line, and νd is the Abbe number for the d-line.

The aspherical surface is represented by the following equation.

[0027]

[Equation 1]

Here, X: length of a perpendicular line drawn from a point on the aspherical surface away from the optical axis to the tangent plane of the aspherical surface vertex h: distance from the optical axis K, A _2i (i = 2, 3) ... 5): aspherical surface coefficient r: radius of curvature of the reference spherical surface.

Further, f is the focal length, fb is the back focus, F _NO. Is the F number, and ω is the half angle of view.

The back focus is represented by an air conversion length.

Next, Table 1 shows Example 1. FIG.
Is a sectional view of the wide-angle lens of Embodiment 1 according to the present invention, and FIG. 2 is an aberration diagram of the wide-angle lens of Embodiment 1.

[0032]

[Table 1]

Next, Table 2 shows Example 2 of the present invention. FIG.
Is a sectional view of the wide-angle lens of Embodiment 2 according to the present invention, and FIG. 4 is an aberration diagram of the wide-angle lens of Embodiment 2.

[0034]

[Table 2]

Next, Table 3 shows Example 3 of the present invention. FIG.
Is a sectional view of the wide-angle lens of Embodiment 3 according to the present invention, and FIG. 6 is an aberration diagram of the wide-angle lens of Embodiment 3.

[0036]

[Table 3]

Next, Table 4 shows Example 4. Note that FIG.
Is a sectional view of the wide-angle lens of Embodiment 4 according to the present invention, and FIG. 8 is an aberration diagram of the wide-angle lens of Embodiment 4.

[0038]

[Table 4]

Next, Table 5 shows Example 5 of the present invention. Incidentally, FIG.
10 is a sectional view of a wide-angle lens of Embodiment 5 according to the present invention, FIG.
6A is an aberration diagram of a wide-angle lens of Example 5. FIG.

[0040]

[Table 5]

[0041]

The structure of the invention according to claim 1 has an effect of facilitating correction of aberration of the lens, facilitating downsizing of the lens and workability of the lens.

The configuration of the invention according to claim 2 has an effect of making it possible to favorably correct coma aberration.

The structure of the invention according to claim 3 has an effect of making it possible to favorably correct distortion.

The structure of the invention according to claim 4 has an effect that coma aberration and astigmatism can be favorably corrected, and the lens can be downsized.

As described above, according to the present invention, it is possible to provide a small-sized and simple high-performance wide-angle lens suitable for a surveillance camera or a videophone.

[Brief description of the drawings]

FIG. 1 is a sectional view of a wide-angle lens according to a first exemplary embodiment.

FIG. 2 is an aberration diagram of a wide-angle lens in Example 1.

FIG. 3 is a sectional view of a wide-angle lens according to a second embodiment.

FIG. 4 is an aberration diagram of a wide-angle lens in Example 2.

FIG. 5 is a sectional view of a wide-angle lens according to a third exemplary embodiment.

FIG. 6 is an aberration diagram of a wide-angle lens in Example 3.

FIG. 7 is a sectional view of a wide-angle lens in Example 4.

FIG. 8 is an aberration diagram of a wide-angle lens in Example 4.

FIG. 9 is a sectional view of a wide-angle lens in Example 5.

FIG. 10 is an aberration diagram of a wide-angle lens in Example 5.

Claims (4)

[Claims] 1. A first lens, which is a biconcave lens in order from the object side, and a second lens having a positive power, the lens surface on the image side having a stronger curvature than that on the object side. A wide-angle lens, wherein one lens or the second lens has at least one aspherical surface. 2. The wide-angle lens according to claim 1, wherein the aspherical surface is arranged on a lens surface of the second lens on the image side, and the curvature thereof gradually decreases as the distance from the optical axis increases. 3. The at least one aspherical surface of the first lens is such that the curvature thereof gradually becomes weaker as the distance from the optical axis increases.
The wide-angle lens according to claim 1 or 2, which has a surface and satisfies the following conditional expression. | R ₂ / r ₁ | ≦ 0.2 where r _{1 is} the radius of curvature of the object side lens surface of the first lens r _{2 is the} radius of curvature of the image side lens surface of the first lens 4. The wide-angle lens according to claim 1, wherein the wide-angle lens satisfies the following conditional expression. 0.3 ≦ d ₂ /f≦1.2 where d _{2 is} the axial upper surface distance between the first lens and the second lens f is the focal length of the entire lens system