JP3295027B2 - Retrofocus type large aperture ratio wide-angle lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retrofocus type wide-angle lens having a large aperture ratio suitable for a digital camera using an image pickup device such as a CCD.

[0002]

2. Description of the Related Art Imaging lenses used in digital cameras are:
In order to provide a low-pass filter or an infrared light cut filter in the rear part, a long back focus is required, and in order to exert the performance of the imaging device, an exit pupil-image plane,
It is necessary to increase the distance between them, and there are drawbacks such as the complexity of the lens system and the increase of negative distortion, and it is difficult to design, and it is particularly difficult to increase the aperture ratio. .
8 was the limit.

[0003]

In recent years, with the increase in the number of pixels of the CCD, which is an image pickup device, the pixel pitch has become smaller.
High resolution has come to be required. In order to satisfy this requirement for high resolution, it has become an important issue to minimize not only the axial and off-axis chromatic aberrations but also the chromatic coma in addition to the various aberrations at the reference wavelength.

[0004]

According to the present invention, a first lens of a negative meniscus lens having a convex surface directed to the object side in order from the object side and a negative lens having a convex surface directed to the object side are formed to form a wide-angle lens having a large aperture ratio. A first meniscus lens comprising a second lens
A lens having a three-group configuration including a third group of a positive lens system including a group, an aperture, a second group that is a third lens of a biconvex positive lens, and a fifth lens cemented with a fourth lens of the biconvex positive lens. Wherein the concave surface on the image side of the first lens and the convex surface on the image side of the third lens are aspherical surfaces.
In the above-described lens, the concave surface on the image side of the negative meniscus lens having the convex surface facing the object side of the first lens and the convex surface on the image side of the third lens of the biconvex positive lens are formed of a transparent material outside the spherical glass lens. It can also be constituted by joining an aspherical layer made of aluminum.

[0005] In order to increase the distance between the back focus and the exit pupil-image plane, it is necessary to use a retrofocus type. It must be said that it is not suitable as a wide-angle lens. In order to correct this drawback, there are many types in which the negative lens of the first group is a composite negative lens of a positive lens and a negative meniscus, but at an intermediate angle of view, a barrel-shaped depiction due to negative distortion due to the negative lens is produced. In the peripheral portion having a high angle of view, correction of positive distortion by the positive lens acts strongly, so that a pincushion-type depiction due to the positive distortion results in a Jinkasa type distortion. In the composite negative lens of this type, the chromatic aberration of magnification is the intermediate angle of view because the positive lens has a reverse dispersion of a material having a high dispersion and a material having a low dispersion for the negative lens in order to correct chromatic aberration. In this case, overcorrection occurs, and undercorrection occurs in the peripheral area with a high angle of view.In recent years, wide-angle lenses for digital cameras that support a high number of pixels, which require high resolution, suffer from chromatic aberration of magnification and core aberration of color. And good results cannot be obtained.

In the present invention, in order to avoid the above-mentioned cause of chromatic aberration and to obtain a large aperture ratio, the first lens unit includes a negative meniscus having a convex surface directed toward the object side in order from the object side.
A composite negative lens system composed of a lens and a second negative meniscus lens having a convex surface facing the object side. The negative lens has a large correction effect on the first lens, and has a large correction effect on the image side. The aspherical surface, whose curvature becomes weaker, is adopted.
As for the aspherical surface, the concave surface on the image side has a large effect of correcting distortion, and the first lens is better. When an aspherical surface is used for the second lens, the negative distortion generated by the first lens must be corrected together, so that the aspherical coefficient must be increased, and the effect is not obtained much. As a result, despite the fact that only one aspherical surface is used in the first lens unit, a result with little distortion and no habit is obtained.
The chromatic aberration of magnification and the chromatic coma were also made extremely good over the entire angle of view. When a non-spherical layer made of a transparent material is bonded to the outside of a spherical glass lens on the image-side concave surface of a negative meniscus lens having a convex surface facing the object side as the first lens, the curvature decreases toward the periphery. Is adopted. In this case, it is desirable that the radius of curvature of the cemented spherical surface is equal to or slightly larger than the radius of curvature of the apex of the aspherical layer. As a result, the thickness of the aspherical layer does not differ greatly between the center and the peripheral portion, and the effect of correcting negative distortion can be obtained.

The second group is composed of a third biconvex positive lens. The present invention is based on the second aspect in terms of the large aperture ratio of F / 2.
It is effective to give a group a strong positive refractive power. Therefore, the radius of curvature of the spherical surface on the object side is also positive, but a stronger positive refractive power is given to the image side. In order to correct the resulting undercorrected spherical aberration, the image-side convex surface employs an aspheric surface whose curvature becomes weaker toward the periphery,
The spherical aberration could be corrected successfully. Third of the second group
In the case where the lens is a biconvex glass spherical lens and an aspherical layer made of a transparent material is bonded to the convex surface on the image side, the radius of curvature of the cemented spherical surface may be near the apex radius of curvature of the aspherical layer. In this case, the aspherical surface also has a curvature that becomes weaker toward the periphery. Since the aspheric coefficient is weaker than that of the first lens of the first group, the thickness of the aspheric layer and the difference between the center and the peripheral portion are small, and the effect of correcting spherical aberration is large.

If a strong positive refractive power is given to the cemented positive lens system of the fourth lens which is a positive biconvex lens of the third group and the fifth lens which is a negative lens, a strong positive refractive power is generated near the image plane. , Negative distortion increases. Therefore, it is better to avoid a very strong positive refractive power. Accordingly, the first lens, which is a negative meniscus with the convex surface facing the object side of the first group,
Excessive aspheric coefficient on the image side can be avoided. Third
The cemented positive lens system of the group functions to improve axial chromatic aberration, magnification chromatic aberration, and chromatic coma. Note that a parallel plane composed of a low-pass filter, an infrared light cut filter, a cover glass of a CCD, and the like is disposed behind the third group. The present invention satisfies the following conditions based on the features of the lens configuration described above. _{2.5 <│f L1 │ / f <} 5 ··· (1) 0.7 <f L1 / f L2 <2.0 ··· (2) 1.2 <f G2 /f<1.6 · (3) 6 <f _G3 / f _G2 <20 (4) r _G2F > 1.5│r _G2R │ (5) where f: focal length of the whole system f _L1 : on the object side Focal length of the first lens of the negative meniscus with the convex surface facing When the aspherical layer made of a transparent material is joined to the image side of the spherical glass lens, the aspherical layer is included. F _L2 : convex surface on the object side The focal length of the negative second meniscus lens f _G2 : the focal length of the second group, which is the third lens of the biconvex positive lens. The aspherical layer made of a transparent material is joined to the image side of the spherical glass lens. F _G3 : focus of the positive lens system of the third group in which the fourth lens and the negative fifth lens of the biconvex positive lens are joined. Point distance r _G2F : radius of curvature of the object-side spherical surface of the second group which is the third lens of the biconvex positive lens r _G2R : vertex of the image-side aspheric surface of the second group which is the third lens of the biconvex positive lens Radius of curvature In the case of joining an aspherical layer made of a transparent material to the image side of a spherical glass lens, the apex radius of curvature of the aspherical layer

Condition (1) defines the range of the refractive power of the first negative meniscus lens. If the lower limit is exceeded, the back focus becomes longer, but the negative distortion increases and cannot be compensated for by increasing the aspherical coefficient of the concave surface on the image side. When it exceeds the upper limit, the back focus becomes shorter,
The object of the present invention cannot be achieved.

Conditional expression (2) defines the refractive power distribution of the first negative meniscus lens and the second negative meniscus lens of the first group of composite negative lenses. When the value exceeds the lower limit, the refractive power of the first negative meniscus lens is strong, so that the negative distortion increases. When the upper limit is exceeded, the refractive power distribution of the first negative meniscus lens is weak, which is advantageous for reducing negative distortion, but shortens the back focus.

Conditional expression (3) defines the range of the positive refractive power of the second group, which is the third lens. It is advantageous to increase the positive refractive power of the second lens unit in order to obtain the large aperture ratio, which is the object of the present invention. Cannot be reached. When the value exceeds the upper limit, it is advantageous to increase the back focus. However, if the positive refractive power distribution of the cemented positive lens system of the third group is increased, the focal length of the entire system can be shortened and the large aperture ratio can be reduced. Cannot be obtained, resulting in an increase in negative distortion.

Conditional expression (4) defines the range of the positive refractive power of the second and third lens units. The feature of the present invention is the second
A large positive aperture is assigned to the group to obtain a large aperture ratio. By reducing the positive index assignment of the cemented lens system of the third group close to the image plane, the convex surface is directed to the object side. The negative meniscus does not increase the aspherical coefficient of the concave surface on the image side of the first lens, thereby preventing an increase in negative distortion. The positive refracting power distribution of the lens system is increased, which causes an increase in negative distortion. When the value exceeds the upper limit, the positive refractive power allotment of the cemented lens system of the third group becomes weak, and the back focus becomes short, so that the object of the present invention cannot be achieved.

Conditional expression (5) defines the ratio of the radius of curvature of the object-side spherical surface of the biconvex third lens which is the second group to the vertex radius of curvature of the image-side aspheric surface. Here, in the case where the biconvex third lens of the second group joins an aspherical layer made of a transparent material on the image side of the spherical glass lens, the radius of apex of the aspherical layer is the apex. If the range of conditional expression (5) is exceeded, the second
The positive refractive power of the surface on the object side of the group is strong, the back focus is short, and the object of the present invention cannot be achieved.

[0014]

EXAMPLES Examples 1 to 5 of the retrofocus type large aperture ratio wide angle lens of the present invention are shown in Tables 1 to 5. The meanings of the symbols used in this description are as follows. f: focal length of the whole system r _i : radius of curvature of a spherical or parallel plane or apex radius of curvature of an aspheric surface r _ip : aspherical layer r _i made of a transparent material is joined to the image side of a spherical glass lens The radius of curvature of the cemented spherical surface _di : sequentially the axial thickness of the lens or the parallel plane or the air gap _dip : the axis of the aspherical layer made of a transparent material made of a transparent material on the image side of the spherical glass lens r _i upper thickness n _i: sequentially lens or refractive index n _ip for the material of the d-line of the parallel plane: aspheric layer when bonding the aspherical layer made of a transparent material on the image side of the spherical glass lens (refractive index n _i) Ν _i : Abbe number of lens or parallel plane material in sequence ν _ip : When an aspherical layer made of a transparent material is joined to the image side of a spherical glass lens (Abbe number ν _i ) Abbe number of aspherical layer material X: distance from a tangent plane at the vertex of the lens surface to a point on the aspheric surface h: height from the optical axis C: curvature of the aspherical vertex (C = 1 / r) K: conical constant A _2i : aspherical surface When the coefficient

(Equation 1) It is represented by 1 and 4 show a first embodiment and a third embodiment of a retrofocus type large aperture ratio wide-angle lens according to the present invention.
FIG. 2 is a configuration sectional view and an optical path diagram of FIG. (Hereinafter referred to as margins.)

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

FIG. 2, FIG. 3, FIG. 5, FIG. 6 and FIG. 7 show a first example of a retrofocus type large aperture ratio wide angle lens according to the present invention.
It is an aberration curve figure of thru / or a 5th example. Negative distortion is small and straightforward. Spherical aberration, astigmatism, and coma as well as lateral and off-axis chromatic aberration of magnification and chromatic coma, which are off-axis, are corrected very well. , F / 2, it can sufficiently cope with a digital camera having a large number of pixels and a small pixel pitch.

[0016]

The wide-angle lens which has a long back focus, a large distance between the exit pupil and the image plane, and is compatible with high resolution, has an aperture ratio of F / 2.8 which is limited to F / 2. In addition, the number of lenses was reduced to five, and the configuration was realized. One aspheric surface was used in each of the front group and the rear group, where the most effect was obtained.

[Brief description of the drawings]

FIG. 1 is a configuration sectional view and an optical path diagram of a first embodiment of a retrofocus type large aperture ratio wide angle lens according to the present invention.

FIG. 2 is an aberration curve diagram of the first embodiment.

FIG. 3 is an aberration curve diagram of the second embodiment.

FIG. 4 is a configuration sectional view and an optical path diagram of Embodiment 3 of a retrofocus type large aperture ratio wide angle lens according to the present invention.

FIG. 5 is an aberration curve diagram of the third embodiment.

FIG. 6 is an aberration curve diagram of the fourth embodiment.

FIG. 7 is an aberration curve diagram of the fifth embodiment.

Continuation of front page (56) References JP-A-10-111454 (JP, A) JP-A-61-162021 (JP, A) JP-A-1-191820 (JP, A) JP-A-2-284108 (JP) JP-A-4-261510 (JP, A) JP-A-8-160296 (JP, A) JP-A-5-323187 (JP, A) JP-A-3-288113 (JP, A) 4-107407 (JP, A) JP-A-9-15492 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21 / 04 G02B 25/00-25/04

Claims (2)

(57) [Claims] 1. A first group of a negative meniscus lens having a convex surface facing the object side in order from the object side and a second lens of a negative meniscus lens having a convex surface facing the object side, a diaphragm, and a biconvex positive lens. In a three-group lens composed of a third group of a positive lens system including a second lens group as a third lens and a fourth lens of a biconvex positive lens and a fifth lens of a negative lens cemented to the first lens, concave on the image side of the convex surface of the third lens on the image side retrofocus large aperture ratio wide are aspherical
A retrofocus large aperture ratio wide angle lens that satisfies the following conditions for square lenses
lens. Serial _{2.5 <| f L1 | / f} <5 ··· (1) 0.7 <f L1 / f L2 <2.0 ··· (2) 1.2 <f G2 /f<1.6 _{··· (3) 6 <f G3} / f G2 <20 ··· (4) r G2F> 1.5 | r G2R | ··· (5) provided that f: the focal point of the entire system distance f _L1: the object side Meniscus first lens with convex surface facing
Aspherical layer made of transparent material on the image side of the focal length spherical glass lens
In the one that, f _L2 which is intended to include the aspheric layer: second lens of a negative meniscus having a convex surface directed toward the object side
Focal length f _G2 : focal length of the second group which is the third lens of the biconvex positive lens
Joining aspherical layer made of a transparent material on the image side of the release spherical glass lens
In the one that, itself comprising an aspherical layer that f _G3: contact of the fourth lens and a negative fifth lens of a double convex positive lens
Focal length r _{G2F of the combined} positive lens system of the third group : Object of the second group which is the third lens of the biconvex positive lens
Radius of curvature r _G2R of the spherical surface on the image side: image side of the second group which is the third lens of the biconvex positive lens
Aspherical apex radius of curvature transparent to image side of spherical glass lens
When joining an aspherical layer made of a material,
Vertex radius of curvature of face layer 2. The lens according to claim 1, wherein
The image-side concave surface of the negative meniscus lens having the convex surface facing the object side of the lens and the image-side convex surface of the third lens of the biconvex positive lens are formed by bonding an aspherical layer made of a transparent material to the outside of a spherical glass lens. A retrofocus type large aperture ratio wide-angle lens, which can be configured.