JPH09288235A - Image forming lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming lens having a high image filed illuminance even in the case of using in an unmagnified state, and also capable of suppressing the fluctuation of various kinds of aberration. SOLUTION: The image forming lens 16 is constituted of three lenses; a 1st lens 41 constituted of a both-side convex lens, a 2nd lens 42 constituted of a both-side concave lens and a 3rd lens 43 constituted of a both-side convex lens positioned in this order form an object side, and the image side surface 43a of the 3rd lens 43 is made aspherical. Besides, the image forming lens is constituted so as to satisfy the following conditions; 0.25<f1 /f3 <0.75, 0.25<|f2 |/f<0.35 and 1.65<Nd2 <1.72, provided that (f1 ) denote the focal distance of the 1st lens 41, (f2 ) distance of the 3rd lens 43, (f) denotes the composite focal distance of all the lenses and Nd2 denotes the refractive index of the 2nd lens 42.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an image forming lens suitable as a lens for forming an image on an image pickup device such as.

[0002]

2. Description of the Related Art An image obtained by illuminating a developed photographic film which is being conveyed with illumination light from a light source and forming an image light transmitted through the photographic film on a solid-state image pickup device such as a CCD through an optical system. There is known a film image input device that outputs the image to a video device such as a CRT or a printer. In this film image input device, the projection light is imaged on the solid-state image pickup device at an equal magnification, and an optical system for this image formation is disclosed in, for example, Japanese Patent Laid-Open No. 3-8.
A triplet type lens described in Japanese Patent No. 1714 is used.

[0003]

The triplet lens has the advantage that it can maintain a good balance of various aberrations with a simple structure. However, if the triplet lens is used at equal magnification, the angle of view is widened. Along with this, there is a drawback that the image plane illuminance becomes dark. Therefore, a triplet type lens configured so that the illuminance on the image surface is bright is proposed by Japanese Patent Laid-Open No. 5-188284. However, when this lens is used at equal magnification, the field curvature becomes too large to be suppressed, and distortion aberration increases. As described above, it is extremely difficult to achieve both wide angle and high illuminance of the imaging lens, and in the conventional triplet type imaging lens, the F number is limited to about 5.

The present invention has been made in consideration of the above circumstances, and an object thereof is to provide an image forming lens which has a high image plane illuminance even when used at an equal magnification and which suppresses variations in various aberrations. .

[0005]

In order to achieve the above object, the imaging lens of the present invention comprises, in order from the object side, a first lens composed of a biconvex lens, a second lens composed of a biconcave lens, and a biconvex lens. The third lens is composed of three lenses, the image side surface of the third lens is formed as an aspherical surface, the focal length of the first lens is f ₁ , the focal length of the second lens is f ₂ , and the third lens is When the focal length of the lens is f ₃ , the combined focal length of all the lenses is f, and the refractive index of the second lens is Nd ₂ , 0.25 <f ₁ / f ₃ <0.75 0.25 <| f ₂ | / f <0.35 1.65 <Nd ₂ <1.72.

[0006]

In the present invention, the negative refractive power and refractive index of the second lens are set to appropriate values to correct spherical aberration that occurs when used at equal magnification and to suppress changes in field curvature. You can Further, by making the image-side surface of the third lens aspherical, it becomes possible to correct the field curvature that cannot be suppressed by the second lens and suppress distortion.

Further, by satisfying the conditional expression 0.25 <f ₁ / f ₃ <0.75, it is possible to suppress fluctuations in spherical aberration generated in the first lens and coma in the third lens, thereby maintaining a balance of various aberrations. It is possible to obtain good optical performance. When the value goes below the lower limit of this conditional expression, the refracting power of the first lens becomes too large, the spherical aberration generated in the first lens increases, and the second lens cannot be completely corrected. If the upper limit is exceeded, the third
The refracting power of the lens becomes too large, the coma aberration of the third lens increases, and the field curvature cannot be suppressed.

Further, by correcting the spherical aberration, the second lens is constructed so as to satisfy the conditional expression 0.25 <| f ₂ | / f <0.35 1.65 <N _d2 <1.72. In addition, it is possible to suppress changes in field curvature and maintain a good balance of various aberrations. If at least one of the above conditional expressions exceeds the lower limit, the negative refractive power of the second lens becomes excessive and spherical aberration increases, which cannot be suppressed. If any of the values exceeds the upper limit, the curvature of field will be deteriorated and the third lens will not be able to correct it, and distortion will increase.

The aspherical surface of the third lens has the following conditional expression: X = ch ² / [1 + √ {1- (1 + K) c ² h ² }] +
It is formed so as to satisfy Ah ⁴ + Bh ⁶ + Ch ⁸ + Dh ¹⁰ . In the formula, K, A, B, C, D
Represents the aspherical coefficient, c represents the reciprocal of the radius of curvature, and h represents the height of the ray from the optical axis. Here, by configuring the aspherical surface so as to satisfy the conditional expression −0.0007 <A <−0.00015, it is possible to correct on-axis and off-axis aberrations in a well-balanced manner. If the upper and lower limits of this conditional expression are exceeded, field curvature and spherical aberration will become excessive and correction will not be possible.

[0010]

FIG. 5 shows the structure of a film image input device. The film image input device 10 is
It is composed of a fluorescent lamp 11 for a light source, an image reading section 12 and a monitor 13. The photo film 14 is a fluorescent lamp 11.
And the image reading unit 12 are conveyed at a constant speed in the direction of the arrow in the figure. The image reading unit 12 includes a mirror 15 for bending the optical path, an imaging lens 16, a CCD line sensor 17,
And the image processing unit 18, and the CCD line sensor 17
The image processing unit 18 and the image processing unit 18 are formed on a single printed circuit board 19.

Illumination light emitted from the fluorescent lamp 11 passes through the photographic film 14 and projects a film image. The projection light transmitted through the photographic film 14 is reflected by the mirror 15 in a direction parallel to the film transport direction, and the imaging lens 1
An image is formed on the CCD line sensor 17 via 6. CC
The image signal of the image formed on the D line sensor 17 is sent to the image processing unit 18, where it is subjected to negative / positive conversion processing, gradation correction, color correction, etc., and then displayed on the monitor 13.

The image reading section 12 is integrally incorporated in the holding unit 20 shown in FIGS. 6 and 7. As shown in FIG. 7, the holding unit 20 includes a main body 21 that holds the mirror 15 and a lens frame 2 that holds the imaging lens 16.
2, and CCD line sensor 17 and printed circuit board 19
It is composed of an image pickup device frame 23 that integrally holds and a pressing plate 24. The main body portion 21, the lens frame 22, and the image pickup element frame 23 are each formed of a resin material having a light shielding property. The main body 21 is integrally provided with a dark box 25, and the mirror 15 is housed inside the dark box 25. The dark box 25 has a rear surface 25a opened, and an upper surface 25b is formed with an entrance 26 for allowing the projection light transmitted through the photographic film 14 to enter the dark box 25. Again dark box 25
On both side surfaces 25c and 25d of each of the
a, 27b are formed.

A lens barrel 28 is integrally provided on the front surface of the image pickup device frame 23. The lens barrel 28 is provided so as to be coaxial with the optical axis of the CCD line sensor 17 incorporated in the image pickup device frame 23, and the ribs 29a, 2 are provided above the lens barrel 28.
9b and an opening 30 are provided. The lens frame 22 holding the imaging lens 16 is fitted into the lens barrel 28,
Thereby, the imaging lens 16 and the CCD line sensor 17
And are coaxially positioned. The image pickup device frame 23 is positioned so that the front end portion of the lens frame 22 attached to the front surface enters into the dark box 25 from the open back surface 25a of the dark box 25, and then is screwed onto the main body portion 21.

The pressing plate 24 is made of a thin metal plate, is placed on the upper surface of the dark box 25, and is screwed onto the main body 21. The pressing plate 24 has an opening 24a formed at a position facing the entrance 26 formed in the dark box 25, and a pair of mirror holding pieces 31, 32 and a pressing piece 33.
a, 33b, and a hook-shaped locking piece 34 are integrally provided. The pair of mirror holding pieces 31 and 32 are provided in the dark box 2.
5 are provided so as to sandwich both side surfaces 25c and 25d, and mirror holding portions 31a and 32a formed by being bent inward are formed at the tips thereof. The mirror holders 31a and 32a project into the dark box 25 from the openings 27a and 27b formed on both side surfaces 25c and 25d of the dark box 25, and the mirror 15 is placed thereon.

Pressing pieces 33a, 33b and locking piece 34
Are provided so as to project above the lens barrel 28.
As shown in FIG. 6, the pressing pieces 33a and 33b are provided on the lens barrel 28.
The lens barrel 28 is pressed downward by coming into contact with ribs 29a and 29b provided on the upper part of the frame, and the movement of the image pickup device frame 23 in the vertical direction is restricted. Further, the locking piece 34 enters into the opening 30 formed in the lens barrel 28 and regulates the movement of the image pickup device frame 23 in the front-rear direction. With these, the lens frame 22 and the image pickup device frame 2 at the time when the pressing plate 24 is put on the dark box 25.
3 is temporarily fixed to the main body 21 together, and the imaging lens 1
6 and the CCD line sensor 17 are positioned. Then, if the image pickup device frame 23 and the pressing plate 24 are screwed, the positional relationship among the mirror 15, the imaging lens 16 and the CCD line sensor 17 is maintained in an appropriate state and fixed in the holding unit 20. The screwing positions 35a and 35b (see FIG. 7) of the image pickup device frame 23 on the main body 21 are
If the imaging lens frame 23 is formed long in a direction parallel to the optical axis 16a of the imaging lens 16, the image pickup device frame 23 can be moved in the front-rear direction, so that the focus position of the imaging lens 16 can be accurately adjusted.

FIG. 1 shows the lens configuration of the imaging lens of the present invention. The imaging lens 16 includes a first lens 41, a second lens 42, and a third lens 43 in order from the object side.
And is arranged on the image side of the diaphragm D. The first lens 41 and the third lens 43 are biconvex lenses, and the second lens 42 is a biconcave lens. The image-side surface 43a of the third lens 43 is formed in an aspherical shape. The diaphragm D is an object-side opening of the lens frame 22 that holds the imaging lens 16.

[0017]

【Example】

[First Example] The specifications of the imaging lens 16 are as follows. _{f = 13.44 f 1 = 5.09 f} 2 = -3.70 f 3 = 10.16 F no = 3.5 m = 0.92

In the above data, f is the composite focal length of the entire imaging lens 16, f ₁ is the focal length of the first lens 41, and f ₂
Is the focal length of the second lens 42, f ₃ is the focal length of the third lens 43, F _no is the F number, and m is the magnification.

The lens data of the imaging lens 16 is shown in Table 1 below.
Shown in The surface number i is a number given to the surface of each lens in order from the object side, and the surface distance d represents the lens thickness or air distance between the next surface and the next surface (unit is mm).

[0020]

[Table 1]

The aspherical surface is expressed by the conditional expression X = ch ² / [1 + √ {1- (1 + K) c ² h ² }] +
It is formed to satisfy Ah ⁴ + Bh ⁶ + Ch ⁸ + Dh ¹⁰ . In the equation, c represents the reciprocal of the radius of curvature (= 1 / R), and h represents the height of the light beam from the optical axis. Further, the aspherical surface coefficients are as follows. K = 0.000000 A = -0.452720E-03 B = -0.415715E-04 C = 0.308772E-05 D = -0.552697E-06

[F ₁ / f] which is the characteristic value of the imaging lens 16
₃ ”,“ | f ₂ | / f ”, and“ Nd ₂ ”are f ₁ / f ₃ = 0.5009942 | f ₂ | /f=0.2752976 Nd ₂ = 1.689312, The conditional expression 0.25 <f ₁ / f ₃ <0.75 0.25 <| f ₂ | / f <0.35 1.65 <Nd ₂ <1.72 is satisfied.

An aberration diagram of the imaging lens 16 is shown in FIG. In the figure, (A) shows spherical aberration, (B) shows astigmatism,
(C) represents distortion. In addition, the symbols c, d, and F in the spherical aberration diagram of FIG.
56.3 nm), d line (587.6 nm), F line (48
Aberrations for 6.1 nm) are shown. Further, FIG. 2 (B)
Symbols S and T in the astigmatism diagram of 1 represent aberrations with respect to a spherical section and a meridional section, respectively.

Next, the respective specifications and lens data of another embodiment of the structure of the imaging lens will be shown. Further, FIGS. 3 and 4 show aberration diagrams of spherical aberration, astigmatism, and distortion according to each example.

The "second embodiment" _{f = 13.15 f 1 = 4.94 f} 2 = -3.63 f 3 = 10.10 F no = 3.5 m = 0.92

The lens data of the image forming lens of the second embodiment are shown in Table 2 below.

[0027]

[Table 2]

The aspherical surface coefficients are as follows. K = 0.000000 A = -0.281501E-03 B = -0.470091E-04 C = 0.322713E-05 D = -0.365699E-06

The characteristic value of the image forming lens of the second embodiment is f ₁ / f ₃ = 0.4891089 | f ₂ | /f=0.2760456 Nd ₂ = 1.699305, and the conditional expression 0.25 < _{f 1 / f 3 <0.75 0.25} <| meets _{/ f <0.35 1.65 <Nd 2} <1.72 | f 2.

The "third embodiment" _{f = 13.50 f 1 = 5.32 f} 2 = -3.83 f 3 = 10.24 F no = 3.5 m = 0.92

The lens data of the image forming lens of the third embodiment are shown in Table 3 below.

[0032]

[Table 3]

The aspherical coefficients are as follows. K = 0.000000 A = -0.2555770E-03 B = -0.152171E-04 C = -0.461150E-06 D = -0.654243E-07

The characteristic value of the imaging lens of the third embodiment is f ₁ / f ₃ = 0.5195312 | f ₂ | /f=0.2837037 Nd ₂ = 1.689312, and the conditional expression 0.25 < _{f 1 / f 3 <0.75 0.25} <| meets _{/ f <0.35 1.65 <Nd 2} <1.72 | f 2.

[0035]

As described above, according to the present invention, the imaging lens is composed of three lenses of a biconvex lens, a biconcave lens, and a biconvex lens, and the refracting power of the biconcave lens is adjusted to use at an equal magnification. It is possible to correct the spherical aberration at the same time and suppress the change in the field curvature. Further, by making the image-side surface of the lens located closest to the image side aspherical, it becomes possible to correct the field curvature that cannot be suppressed by the biconcave lens and suppress distortion. As a result, even in an imaging lens configured to have a bright image plane illuminance, it is possible to maintain a good balance of various aberrations.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram of an imaging lens of the present invention.

FIG. 2 is an aberration diagram of the imaging lens shown in FIG.
(A) shows spherical aberration, (B) shows astigmatism, and (C) shows distortion.

FIG. 3 is an aberration diagram according to another lens configuration of the imaging lens of the present invention.

FIG. 4 is an aberration diagram of still another lens configuration of the imaging lens of the present invention.

FIG. 5 is a schematic diagram showing a configuration of an image input device.

FIG. 6 is an external view of a holding unit that houses the image reading unit illustrated in FIG.

FIG. 7 is an exploded view of the holding unit shown in FIG.

[Explanation of symbols]

 10 image input device 12 image reading unit 14 photo film 16 imaging lens 17 CCD line sensor 41 first lens 42 second lens 43 third lens D diaphragm

Claims (1)

[Claims] 1. A lens comprising, in order from the object side, a first lens composed of a biconvex lens, a second lens composed of a biconcave lens, and a third lens composed of a biconvex lens. The surface is formed into an aspherical surface, the focal length of the first lens is f ₁ , the focal length of the second lens is f ₂ , the focal length of the third lens is f ₃ , the combined focal length of all lenses is f, When the refractive index of the two lenses is Nd ₂ , 0.25 <f ₁ / f ₃ <0.75 0.25 <| f ₂ | / f <0.35 1.65 <Nd ₂ <1.72 An imaging lens characterized by satisfying the following respective conditions.