JP2000171700A - Wide-angle lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a wide-angle lens where some aberration is excellently compensated in spite of simple constitution by constituting the lens of 1st and 2nd lenses having negative and positive refractive power in order from an object side, and making 1st and 2nd lenses have at least one aspherical surface and satisfy a specified condition. SOLUTION: This lens is constituted of the 1st lens having the negative refractive power and the 2nd lens being a biconvex lens having the positive refractive power in order from the object side. The 1st and the 2nd lenses have at least one aspherical surface. They satisfy conditional expressions ω>45 deg. and 0.5<d2/f<1.5. Provided that ω is the half viewing angle of the wide-angle lens, (f) is the focal distance of an entire system, and d2 is axial distance between the 1st and the 2nd lenses. Furthermore, they satisfy conditional expression 1.0<|f1|/f<1.5 and 0.7<|r4|/r3<1.3. Provided that f1 is the focal distance of the 1st lens, r3 is the radius of curvature of the object side surface of the 2nd lens and r4 is the radius of curvature of the image side surface of the 2nd lens.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a camera or a PC board camera using an image sensor such as a CCD or an artificial retinal LSI. And a wide-angle lens having a simple configuration of two lenses.

[0002]

2. Description of the Related Art In recent years, applications of an optical system using an image pickup device such as a CCD or an artificial retinal LSI have been widespread.
Among them, for surveillance cameras, there is an increasing need for a wide-angle lens that is compact, has a small number of lenses, and has a half angle of view of 45 ° or more.

As a two-lens configuration satisfying such a wide angle requirement, for example, the invention described in Japanese Patent Application Laid-Open No. 6-67091 is known.

[0004]

However, in these inventions, the entire length of the lens with respect to the focal length is slightly large, and thus the invention does not sufficiently meet the demand for compactness.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a wide-angle lens which is compact and has good correction of various aberrations despite its simple structure of two lenses.

[0006]

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

That is, in order from the object side, a first lens having a negative refractive power and a bi-convex second lens having a positive refractive power are provided, and the first lens and the second lens have at least one non-convex surface. A wide-angle lens having a spherical surface and satisfying the following conditional expressions.

Ω> 45 ° (1) 0.5 <d2 / f <1.5 (2) where ω: half angle of view of the wide-angle lens f: focal length of the whole system d2: A wide-angle lens, characterized by satisfying the following conditional expression.

1.0 <| f1 | / f <1.5 (3) 0.7 <| r4 | / r3 <1.3 (4) where f1: focal point of the first lens Distance r3: radius of curvature of the object side surface of the second lens r4: radius of curvature of the image side surface of the second lens Further, the first lens has a negative meniscus shape with the convex surface facing the object side, and A wide-angle lens having a stop between two lenses.

Now, the operation of the wide-angle lens according to the present invention will be described.

In the present invention, a first lens having a negative refractive power and a biconvex second lens having a positive refractive power are arranged in order from the object side.
By using a retrofocus type constituted by a lens, a wide-angle lens satisfying the expression (1) having a half angle of view of 45 ° or more is achieved. By making at least one surface of the first lens and the second lens aspherical, a wide-angle lens in which various aberrations are satisfactorily corrected can be obtained.

Hereinafter, the conditional expressions will be described.

Conditional expression (2) defines the relationship between the axial distance between the first lens and the second lens and the focal length of the entire system. If the upper limit of this condition is exceeded, the lens spacing becomes too wide, and both the overall length and the first lens diameter become large, resulting in an increase in the size of the entire system. If the lower limit of the condition is exceeded, it becomes difficult to secure the necessary back focus.

In addition, conditional expression (2) takes into account the aberration correction balance at a large angle of view that satisfies expression (1).
More preferably, the following conditional expressions should be satisfied.

0.7 <d2 / f <1.2 Conditional expression (3) defines the relationship between the refractive power of the first lens and the refractive power of the entire system under the conditional expression (2). . If the upper limit of this condition is exceeded and the refractive power of the negative first lens becomes too weak, the positive Petzval sum increases and the field curvature becomes undesirably large. If the lower limit of this condition is exceeded and the refractive power of the negative first lens becomes too strong, astigmatism and distortion occurring here will increase. In addition, deterioration of optical performance when the lens is decentered also becomes conspicuous, and the required accuracy in manufacturing becomes severe.

Conditional expression (4) limits the range of the radius of curvature of the second lens in order to favorably correct coma and the like under conditional expressions (2) and (3). Exceeding the range of the conditional expression is not preferable because inward or outward coma becomes conspicuous.

In a wide-angle lens satisfying the conditional expression (1) of the present invention, in order to obtain better aberration correction and compactness, a first lens having a negative refractive power is provided by a negative lens having a convex surface facing the object side. It is preferable to have a meniscus shape, and to arrange the diaphragm between the first lens and the second lens. By making the first lens a negative meniscus lens, off-axis aberrations generated in the first lens can be suppressed to a small value.

With respect to the stop position, since the height of the off-axis light beam passing through the first lens is lower by disposing it between the two lenses than in the case where it is disposed behind the second lens, the necessary peripheral light amount can be reduced. The first lens diameter can be kept small while securing. Further, since the image side surface of the first lens is not so deep, it is possible to avoid occurrence of higher-order aberrations and difficulty in surface processing.

[0019]

Embodiments of the present invention will be described below.

In the embodiment of the present invention, both the first lens and the second lens are formed of a plastic material. The merits of the plastic material include the fact that the optical system is made lighter and that an aspherical surface can be easily added because it is manufactured by injection molding. In each embodiment, better aberration correction is realized by using an aspherical surface for a total of four surfaces.

In the following symbols, f is the focal length of the entire system (mm), ω is the half angle of view (°), r is the radius of curvature (mm),
d is the axial distance (mm), nd is the refractive index for d-line,
νd is the Abbe number.

The symbol * in the surface No represents an aspheric surface. In an orthogonal coordinate system with the vertex of the surface as the origin and the optical axis direction as the X axis, the curvature of the vertex is C, the conic constant is K, the aspheric coefficient is A4,
A6 is represented by the following equation.

[0023]

(Equation 1)

Example 1 f = 2.3 F-number = 2.4 ω = 47.0 d2 / f = 1.04 | f1 | /f=1.30 | r4 | /r3=1.00 1 shows lens data of Example 1, Table 2 shows aspheric coefficient data, FIG. 1A shows a lens cross-sectional view, and FIG. 1B shows aberration charts.

[0025]

[Table 1]

[0026]

[Table 2]

Example 2 f = 2.08 F-number = 2.5 ω = 64.6 d2 / f = 0.82 | f1 | /f=1.25 | r4 | /r3=0.89 3 shows lens data of Example 2, Table 4 shows aspheric coefficient data, FIG. 2A shows a lens cross-sectional view, and FIG. 2B shows aberration charts.

[0028]

[Table 3]

[0029]

[Table 4]

Example 3 f = 2.20 F-number = 2.5 ω = 64.9 d2 / f = 0.91 | f1 | /f=1.37 | r4 | /r3=1.11 5 shows lens data of Example 3, Table 6 shows aspheric coefficient data, FIG. 3A shows a lens cross-sectional view, and FIG. 3B shows aberration charts.

[0031]

[Table 5]

[0032]

[Table 6]

[0033]

According to the present invention, it is possible to provide a wide-angle lens in which various aberrations are satisfactorily corrected irrespective of a simple configuration having a compact number of lenses of two.

[Brief description of the drawings]

FIG. 1 is a sectional view and an aberration diagram of a lens according to a first embodiment.

FIG. 2 is a sectional view and an aberration diagram of a lens according to a second embodiment.

FIG. 3 is a sectional view and an aberration diagram of a lens according to a third embodiment.

Claims (3)

[Claims] 1. In order from an object side, a first lens having a negative refractive power and a biconvex second lens having a positive refractive power are provided. The first lens and the second lens have at least one non-convex lens. A wide-angle lens having a spherical surface and satisfying the following conditional expressions. ω> 45 ° 0.5 <d2 / f <1.5, where ω: half angle of view of the wide-angle lens f: focal length of the entire system d2: axial distance between the first lens and the second lens 2. The wide-angle lens according to claim 1, wherein the following conditional expression is satisfied. 1.0 <| f1 | / f <1.5 0.7 <| r4 | / r3 <1.3, where f1: focal length of the first lens r3: radius of curvature of the object side surface of the second lens r4: number Radius of curvature of image side of two lenses 3. The first lens has a negative meniscus shape with the convex surface facing the object side, and the first lens and the second lens have a negative meniscus shape.
3. The wide-angle lens according to claim 1, further comprising a stop between the lenses.