JPH1195096A - Image pickup lens system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the increase of an aperture ratio, obtain a sufficient image angle, correct various aberration and attain miniaturization by composing an image pickup lens system of first to third lenses respectively with positive, negative and positive refracting power, and a fourth lens with positive refracting power with an image side face formed in a nonspherical face. SOLUTION: An image pickup lens system 1 is composed of four lenses that are a first lens L1, a meniscus lens, convex on the object side and having negative refracting power, a second lens L2 with a convex facing the image face side, having positive refracting power, a third lens L3 concave on the image face side, having negative refracting power, a fourth lens L4 with the image face side face formed in a nonspherical face, having positive refracting power, in this order from the object side to the image face side, and a diaphragm IR arranged between the second and third lenses L2, L3, and a low-pass filter FL arranged between the fourth lens L4 and an image face 2. In this image pickup lens system 1, an image face curve is corrected to the image side by the action of the fourth lens L4 with the image face side formed in a nonspherical face, having positive refracting power, particular by the action of a face S8 formed in a nonspherical face.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide-angle imaging lens system most suitable for a small-sized imaging device.

[0002]

2. Description of the Related Art In recent years, a small-sized image pickup apparatus, for example, a video camera using a solid-state image pickup device, is small-sized and
There is a demand for an imaging lens system having a large aperture ratio (bright) and a wide angle of view.

Further, in the above-mentioned small-sized image pickup apparatus, a so-called low-pass filter needs to be arranged between the image pickup lens system and the image plane in order to cut high-frequency components of the image, so that a long back focus is required. ing.

[0004]

However, in general, in order to increase the aperture ratio of the imaging lens system and increase the angle of view, the number of lenses constituting the imaging lens system increases, There is a problem that the lens system becomes expensive.

For example, in an imaging lens system described in Japanese Patent Application Laid-Open No. 9-166748, an F value is 2.0 and a half angle of view is 30.
Although a high-degree and high-performance imaging lens system with good performance is obtained, it is expensive because the number of lens components is as large as five.
It is insufficient in obtaining an inexpensive imaging lens system.

Therefore, the imaging lens system of the present invention is compact,
Another object is to obtain an imaging lens system having a large aperture ratio and a wide angle of view at low cost.

[0007]

According to the present invention, there is provided an imaging lens system comprising:
In order to solve the above problems, a first lens having a negative refractive power, a second lens having a positive refractive power, and a third lens having a negative refractive power are arranged in order from the object side to the image plane side. ,
The fourth lens has a positive refractive power and a fourth lens which is joined to the third lens and has an aspheric surface on the image side.

Therefore, in the imaging lens system of the present invention,
An imaging lens system that is small, has a large aperture ratio, and has a sufficient angle of view can be obtained at low cost.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the imaging lens system of the present invention will be described with reference to the accompanying drawings. still,
1 to 4 show a first embodiment, and FIGS.
Shows a second embodiment.

First, common items in each embodiment will be described. In the following description,
"S _i" is the i-th surface counted from the object side, "r _i" is the curvature of the surface S _i radius, "d _i" is the i-th surface from the object side and the i
The surface interval on the optical axis (xx) between the (+1) th surface and “ _ni ” is the d-line (wavelength 58) of the i-th surface from the object side.
7.6 nm), and “ν _i ” is _i from the object side.
Shall respectively the Abbe number at the d-line in th surface S _i, also shows "f" is the focal length in the e-line on the lens system (wavelength 546.1 nm), "FNO" is an open F value, respectively Shall be.

The lenses used in the embodiments include those having a lens surface formed by an aspherical surface. Therefore, in each of the following tables, “(ASP)” is added to the right side of the numerical value of r _{i on} the lens surface constituted by the aspherical surface.

The aspherical shape is defined by the following equation.

[0013] ^{Z = C · H 2/1} + √ [1- (1 + K) ·
C ² · H ² ] + 〓 (A _2i · H ²ⁱ ) where “Z” is the height “H” (= √ (X) of the tangent plane and the spherical surface at the vertex of the aspheric surface from the optical axis xx. ² + Y ² ))
, "C" indicates the curvature (1 / r) of the aspherical vertex, "K" indicates the conic constant, and " _A2i " indicates the 2i-th order aspherical coefficient.

The imaging lens systems 1 and 1A in each embodiment include, in order from the object side to the image plane side, a first lens L1, which is a meniscus lens having a negative refractive power and a convex surface on the object side, and an image plane. Second with positive refractive power with the convex surface facing the side
A lens L2, a third lens L3 having a concave refractive surface on the image surface side and having a negative refractive power, a fourth lens L4 having a positive refractive power having an aspherical surface on the image surface side, and a second lens L4; It comprises a stop IR arranged between the lens L2 and the third lens L3, and a low-pass filter FL arranged between the fourth lens L4 and the image plane 2.

[0015] Then, in the imaging lens system 1 and 1A, the action of the fourth lens L4 image side having a positive refractive power is aspheric, in particular, by the action of the surface S ₈ which is aspheric, the image plane The curvature is largely corrected toward the image side.

The first lens L1 and the second lens L2 are arranged such that "f
_{1 and 2,} "the first lens L1 when the focal length by combination of the second lens _{L2, -0.15 <f / f 1,2} <0.08 ( hereinafter, referred to as" Condition (1).) Is satisfied.

Furthermore, the length from the imaging lens system 1 and the first surface of the lens S ₁ of 1A until the eleventh surface S ₁₁ (image plane) "L",
Assuming that the back focus of the entire lens system is “f _b ”, the following condition is satisfied: 2.5 <L / f _b <3.8 (hereinafter referred to as “conditional expression (2)”).

Hereinafter, the conditional expressions (1) and (2) will be described.

In the conditional expression (1) which defines the relationship between the focal length f at e-line in the entire lens system and the focal length f _{1,2 obtained} by combining the first lens L1 and the second lens L2, f
The reason for defining / f _1,2 to be within such a range is as follows.

That is, the value of f / f _1,2 is the lower limit value.
Becomes 0.15 or less, collapse distribution of power at the whole lens system, along with distortion and astigmatism becomes worse, the burden on the surface S ₈ is an aspherical surface of the fourth lens L4 increases, its shape Is complicated. On the other hand, if the value of f / f _1,2 exceeds the upper limit of 0.08, sufficient back focus cannot be obtained, and a low-pass filter is provided between the fourth lens L4 and a solid-state imaging device (not shown). This is because a sufficient space for arranging the FL cannot be obtained.

The imaging lens systems 1 and 1A are provided with an aperture I
The front group GR1 (first lens L1 and second lens L2) and the rear group GR2 (third lens L3 and fourth lens L)
4), the tolerance of eccentricity of the front group GR1 and the rear group GR2 is relaxed by setting the value of f / f _1,2 within the range defined by the conditional expression (1). And the effect that the assemblability of the imaging lens systems 1 and 1A is improved.

Further, a first lens L1 and a second lens L2
When plastic is used as the material of the first lens, the first lens L1 and the second lens L2 cancel each other to reduce the influence of the shift of the focal position due to the temperature change, which is a drawback of the plastic lens. Will be able to do it.

In conditional expression (2) which defines L / f _b which is the relationship between the length L from the first lens surface S ₁ to the image surface S ₁₁ and the back focus f _b of the entire lens system, the first lens
Is due to following reasons length L to the image plane S ₁₁ is defined to be within a range of 3.8 times to 2.5 times the back focus f _b of the entire lens system from the surface S ₁ .

That is, the value of L / f _b is the lower limit of 2.5
In the following cases, the refractive powers of the front group GR1 and the rear group GR2 increase, and it becomes difficult to correct the negative distortion. In addition, a space for arranging the aperture IR is secured. In addition to this, high precision is required for the distance before and after the aperture IR,
This is because the productivity of the imaging lens systems 1 and 1A deteriorates. On the other hand, if the value of L / f _b exceeds the upper limit of 3.8 or more, the total length of the imaging lens systems 1 and 1A becomes longer, and as a result, the outside of the lens is required to secure the peripheral light amount. It is necessary to increase the diameter, which is contrary to the purpose of miniaturization.

Next, details of each of the imaging lens systems 1 and 1A will be described.

FIGS. 1 to 4 show a first embodiment of the imaging lens system according to the present invention.
1 shows an embodiment of the present invention.

Table 1 shows each value of the imaging lens system 1.

[0028]

[Table 1]

As shown in Table 1, the first surface S ₁ , second surface S ₂ , third surface S ₃ and eighth surface S ₈ of the lens are aspherical. Therefore, Table 4 shows the fourth, sixth, and
Eighth and tenth order aspherical coefficients A ₄ , A ₆ , A ₈ and A
_{Shows 10} .

[0030]

[Table 2]

Note that "E" in Table 2 means an exponential expression with a base of 10 (the same applies to Table 4 described later).

FIG. 2 shows a spherical aberration diagram of the imaging lens system 1, and FIG.
FIG. 4 shows an astigmatism diagram of the imaging lens system 1, and FIG.
Are shown respectively. In the spherical aberration diagram, the solid line is the e-line, and the dashed line is the g-line (wavelength 435.8n).
m), the dashed line indicates the value on the C line (wavelength 656.3 nm), and in the astigmatism diagram, the solid line indicates the value on the sagittal image plane, and the dashed line indicates the value on the meridional image plane (second embodiment described later). The same applies to FIG. 6 showing a spherical aberration diagram and FIG. 7 showing an astigmatism diagram in the embodiment.)

FIGS. 5 to 8 show a second embodiment of the imaging lens system according to the present invention.
1 shows an embodiment of the present invention.

Table 3 shows each value of the imaging lens system 1A.

[0035]

[Table 3]

As shown in Table 3, the first surface S ₁ , second surface S ₂ , third surface S ₃ and eighth surface S ₈ of the lens are aspherical. Therefore, Table 4 shows the fourth, sixth, and
Eighth and tenth order aspherical coefficients A ₄ , A ₆ , A ₈ and A
_{Shows 10} .

[0037]

[Table 4]

FIG. 6 shows a spherical aberration diagram of the imaging lens system 1A, FIG. 7 shows an astigmatism diagram of the imaging lens system 1A, and FIG. 8 shows a distortion aberration diagram of the imaging lens system 1A.

Finally, in each of the above embodiments, f,
Table 5 shows values of FNO and the half angle of view ω, and Table 6 shows values of f / f _1,2 and L / f _b in conditional expressions (1) and (2).

[0040]

[Table 5]

[0041]

[Table 6]

As described above, in the imaging lens system of the present invention, 2.0 lenses can be realized by using only four lenses.
A video having a sufficiently bright FNO of 6 and 1.80, a sufficiently long back focus f _{b of} approximately L / 3, and a half angle of view ω of 31 degrees or more, and in which various aberrations are well corrected. A small-sized imaging lens system suitable for a small-sized imaging device using a solid-state imaging device such as a camera can be obtained at low cost.

[0043]

As apparent from the above description, the imaging lens system of the present invention is a retrofocus type imaging lens system in which the back focus is longer than the focal length of the entire lens system. In order to the side, a first lens having a negative refractive power and a second lens having a positive refractive power
Since it is composed of a lens, a third lens having a negative refractive power, and a fourth lens having a positive refractive power, which is joined to the third lens and whose image-side surface is formed by an aspheric surface,
A small-sized imaging lens system having a sufficient angle of view and sufficiently correcting various aberrations can be obtained at low cost without making the aperture ratio unnecessarily large.

According to the second aspect of the present invention, the first
Since at least one of the surfaces of the lens and the second lens is formed by an aspherical surface, distortion of the entire imaging lens system can be satisfactorily corrected. Further, even when the first lens and the second lens are formed by plastic as in the invention described in claim 3, the first lens is not moved by the first lens with respect to the movement of the focal position due to a temperature change which is a defect of the plastic lens. Since the lens and the second lens act so as to cancel each other, the influence can be reduced.

According to the fourth and fifth aspects of the present invention, since the condition of -0.15 <f / f _1,2 <0.08 is satisfied, the refractive power in the entire lens system is satisfied. In addition, it is possible to prevent the distortion and the astigmatism caused by the collapse of the distribution of the aberrations from being deteriorated, to reduce the load on the fourth lens, and to obtain a sufficient back focus value.

Further, in the inventions described in claims 6 and 7, since the condition of 2.5 <L / f _b <3.8 is satisfied, the productivity and the negative of the entire lens system are satisfied. A small-sized imaging lens system having a sufficient angle of view and sufficiently correcting various aberrations can be obtained at low cost without deteriorating distortion.

Furthermore, in the invention described in claims 8 to 14, since the concave meniscus lens having the convex surface facing the object side is used for the third lens, the third lens has a lens front composed of the first lens and the second lens. It is possible to reduce the influence of the eccentricity of the group and the rear group consisting of the third lens and the fourth lens.

It should be noted that the specific shapes and structures of the respective parts shown in each of the above-described embodiments are merely examples of the embodiment of the present invention.
These should not be construed as limiting the technical scope of the present invention.

[Brief description of the drawings]

FIG. 1 together with FIGS. 2 to 4 show a first embodiment of an imaging lens system according to the present invention.
FIG. 1 is a schematic diagram showing a lens configuration.

FIG. 2 is a diagram illustrating spherical aberration.

FIG. 3 is a diagram illustrating astigmatism;

FIG. 4 is a diagram illustrating distortion.

FIG. 5 shows a second embodiment of the imaging lens system according to the present invention together with FIGS.
FIG. 1 is a schematic diagram showing a lens configuration.

FIG. 6 is a diagram illustrating spherical aberration.

FIG. 7 is a diagram illustrating astigmatism;

FIG. 8 is a diagram illustrating distortion.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Imaging lens system, 1A ... Imaging lens system, 2 ... Image plane, L
1: first lens, L2: second lens, L3: third lens, L4: fourth lens

Claims (14)

[Claims] 1. A retrofocus type imaging lens system having a back focus longer than the focal length of the entire lens system, comprising: a first lens having a negative refractive power in order from the object side to the image plane side; A second lens having a negative refracting power, a third lens having a negative refracting power, a fourth lens having a positive refracting power, which is joined to the third lens and whose image-side surface is formed by an aspheric surface. An imaging lens system comprising: 2. The imaging lens system according to claim 1, wherein at least one of the first lens and the second lens has an aspheric surface. 3. The imaging lens system according to claim 2, wherein the first lens and the second lens are formed of plastic. 4. The imaging lens system according to claim 2, wherein the following condition is satisfied. −0.15 <f / f _1,2 <0.08 where f: focal length of the entire lens system, f _1,2 : focal length by combining the first lens and the second lens. 5. The imaging lens system according to claim 3, wherein the following condition is satisfied. −0.15 <f / f _1,2 <0.08 where f: focal length of the entire lens system, f _1,2 : focal length by combining the first lens and the second lens. 6. The imaging lens system according to claim 2, wherein the following condition is satisfied. 2.5 <L / f _b <3.8, where L: distance from the lens surface closest to the object side to the image plane, f _b : the back focus value of the entire lens system. 7. The imaging lens system according to claim 3, wherein the following condition is satisfied. 2.5 <L / f _b <3.8, where L: distance from the lens surface closest to the object side to the image plane, f _b : the back focus value of the entire lens system. 8. The imaging lens according to claim 1, wherein the third lens is a concave meniscus lens having a convex surface facing the object side. 9. The imaging lens according to claim 2, wherein the third lens is a concave meniscus lens having a convex surface facing the object side. 10. The imaging lens according to claim 3, wherein the third lens is a concave meniscus lens having a convex surface facing the object side. 11. The imaging lens according to claim 4, wherein the third lens is a concave meniscus lens having a convex surface facing the object side. 12. The imaging lens according to claim 5, wherein the third lens is a concave meniscus lens having a convex surface facing the object side. 13. The imaging lens according to claim 6, wherein the third lens is a concave meniscus lens having a convex surface facing the object side. 14. The imaging lens system according to claim 7, wherein the third lens is a concave meniscus lens having a convex surface facing the object side.