JP2002365530A - Photographic lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a compact photographic lens having high resolution and being constituted of a small number of lenses. SOLUTION: This photographic lens is constituted by arranging an aperture diaphragm nearest to an object side, and then arranging a 1st lens having positive refractive power (hereinafter referred to as positive lens), a 2nd lens having negative refractive power (hereinafter referred to as negative lens), a 3rd lens which is a positive lens and a 4th lens which is a negative lens, in this order starting from the object side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to a CCD (charged device) such as a digital still camera, a surveillance camera, and a PC camera (an imaging device attached to a personal computer).
The present invention relates to a high-performance and compact photographing lens used for a small-sized imaging device using an imaging device such as a coupled device.

[0002]

2. Description of the Related Art The rapid spread of digital still cameras (hereinafter, DSCs) for general use in recent years has been remarkable. A DSC is an imaging device that is structurally configured to electrically capture a still image formed by a photographing lens using a CCD or other imaging device (hereinafter, a CCD) and record the image in a built-in memory or a memory card. At first, the LCD monitor could be used as a viewfinder for shooting and as a monitor for playing back captured images, so it was popularized by appealing its immediateness and convenience compared to silver halide cameras, but on the other hand, silver It has been pointed out that the resolution of the photographed image is lower than that of the salt camera, which is a disadvantage. However, in this regard, DSCs are approaching the resolution of silver halide cameras in the range of restrictions such as the print size of the popular format in terms of resolution because of the rapid spread and supply of CCDs with a large number of pixels at low cost. It has been improved and commercialized.

[0003] Turning now to a conventional DSC photographing lens, it can be seen that the structure is similar to a VTR photographing lens because a CCD is used to capture an image despite high pixels. However, since higher performance is required in terms of required resolution and image quality, the configuration is often complicated, and the size of the optical system and the CCD screen size are the same. The taking lens for DSC becomes larger than the taking lens for VTR. The features of the conventional DSC photographing lens will be outlined below.

[0004] 1. High image quality Recently, the number of CCD pixels is 3 million pixels to 400 pixels.
10,000 pixels have also been announced in DSCs for the general public. VT
Since the screen size is different from the 350,000 pixel class image sensor used for R, direct comparison does not make much sense, but there is a difference of about 10 times if the screen size is ignored. It will be. That is, required for the taking lens,
It is considered that the accuracy (degree of difficulty) of aberration correction also has a difference of this difference. In order to increase the number of pixels of a CCD, a method of increasing the number of pixels by reducing the pixel pitch without increasing the screen size as much as possible has been adopted at present. The pixel pitch of the announced CCD of 1.3 million pixels is about 4.2 μm. Therefore, even if it is assumed that the minimum circle of confusion is twice the pixel pitch, 8.4 μm
And the minimum circle of confusion of a 35 mm silver halide camera is about 33
It can be said that the resolution required for the photographing lens of the digital still camera is about four times that of the silver halide camera.

[0005] 2. The image-side telecentricity is good. The image-side telecentricity means that the principal ray of the light beam for each image point becomes almost parallel to the optical axis after exiting the final surface of the optical system. Means to intersect almost vertically. In other words, the exit pupil position of the optical system is sufficiently away from the image plane. This is because the color filter on the CCD is located slightly away from the imaging surface,
This is because when the light is obliquely incident, the aperture efficiency is substantially reduced (referred to as shading). In particular, most recent high-sensitivity CCDs have a microlens array disposed immediately in front of the imaging surface. In this case, similarly, if the exit pupils are not sufficiently separated, the aperture efficiency will be reduced in the periphery.

[0006] 3. A large back focus is required. A space for inserting some optical elements is generally required between the CCD and the optical system of the photographing lens, as well as a protective glass plate due to the structure of the CCD and the space behind it. It is said. An optical low-pass filter (hereinafter referred to as OLPF) inserted for the purpose of preventing a moiré phenomenon or the like generated due to the periodic structure of the CCD, and the relative visibility of the human eye by reducing the sensitivity in the infrared wavelength region of the CCD. The infrared absorption filter is also inserted between the optical system and the CCD for the purpose of approximating the above.

[0007]

As described above, the conventional DS
The photographing lens C generally has three features (conditions). For items (2) and (3), the color filter of the CCD and the arrangement of the micro lens array were reviewed, and For items (1) and (2), reviewing the materials of the OLPF and other materials and reviewing the CCD structure from the ground up have shown signs of improvement. Awareness of gender is creating an environment for developing photographic lenses that take advantage of their characteristics.

The present invention has been made in view of the above circumstances, and provides a compact photographic lens having a high resolution and a small number of components.

[0009]

According to the present invention, there is provided a photographing lens comprising:
An aperture stop is arranged closest to the object side, and thereafter, in order from the object side, a first lens having a positive refractive power (hereinafter a positive lens), a second lens having a negative refractive power (hereinafter a negative lens), and a positive lens. A third lens and a fourth lens which is a negative lens, wherein at least one of the refracting surfaces forming the entire system has an aspherical refracting surface; Satisfies the following conditional expression (1), the power of the second lens satisfies the following conditional expression (2), and the Abbe number of the first lens and the second lens is the following conditional expression: (3) is satisfied. (Claim 1) (1) 1.5 <f / f ₁ <2.5 (2) 1.5 <f / | f ₂ | <2.5 (However, since the absolute value is f ₂ <0) ( 3) 15 <ν ₁ −ν ₂ where f: combined focal length of the entire lens system f ₁ : focal length of the first lens f ₂ : focal length of the second lens ν ₁ : Abbe number of the first lens ν ₂ : Abbe number of the second lens

The basic features of the lens configuration of the photographic lens of the present invention include the first lens having a large positive power, the second lens having a large negative power, and a relatively small positive power following the first lens. And a fourth lens having the smallest negative power on the image plane side, and having a so-called telephoto type power arrangement in which positive, negative, positive, negative and positive powers precede. That is. Further, in order to correct chromatic aberration, main achromatization is performed by the first lens and the second lens having large power. Accordingly, the first lens and the second lens mainly correct spherical aberration, coma, and chromatic aberration near the axis, and the third lens and the fourth lens mainly correct distortion that is off-axis aberration. It has the effect of correcting aberrations and maintaining good telecentricity.

Under such an overall configuration, conditional expression (1)
Is a conditional expression regarding an appropriate power distribution of the first lens. If the upper limit of conditional expression (1) is exceeded, the power of the first lens will be excessive, and spherical aberration and chromatic aberration will be large. Conversely, if the lower limit is exceeded, it is advantageous for correcting monochromatic aberration, but the overall length of the lens system is increased, and compactness is impaired.

Conditional expression (2) is a conditional expression relating to the power of the negative lens. It is the second lens and the fourth lens that have negative power in the entire lens system. However, since much of the negative power is left to the second lens,
The condition relating to the power of the second lens can be said to be a conditional expression relating to the power distribution of the negative lens in the entire lens system. This negative power is a conditional expression for correcting chromatic aberration and spherical aberration generated in the first lens and the third lens, which are positive lenses. Therefore, when the value exceeds the upper limit of the conditional expression, the color correction becomes excessive, and it is difficult to reduce the size. Conversely, if the lower limit is exceeded, color correction will be insufficient, and it will be difficult to correct spherical aberration and coma.

Conditional expression (3) is an achromatic condition of the first lens and the second lens having the main power,
Below the lower limit, the positive and negative lens powers become excessively large, causing monochromatic aberration and processing problems. Above the upper limit, the occurrence of monochromatic aberration due to the decrease in the refractive index of the positive lens also becomes excessive.

The shape of the image side surface of the first lens satisfies the following conditional expression (4), and the shape of the image side surface of the third lens satisfies the following conditional expression (5). It is preferable that the shape of the object side surface of the fourth lens satisfies the following conditional expression (6). (Claim _{2) (4) 0.8 <|} ( for although the absolute value _{r 2 <0) <2.5 (} 5) 0.25 <| | r 2 / r 1 r 6 | / f <0 .45 (However, the absolute value is r ₆ <0) (6) 1.2 <r ₇ /f<2.0, where r ₁ : radius of curvature of the object side surface of the first lens r ₂ : radius of the first lens Radius of curvature of the image side r ₆ : radius of curvature of the image side of the third lens r ₇ : radius of curvature of the object side of the fourth lens

Conditional expression (4) is a conditional expression relating mainly to the shape of the first lens for correcting spherical aberration. Therefore, if the upper limit of the conditional expression is exceeded, a large negative spherical aberration will occur, making it difficult to correct the lens located behind the first lens, and excessively generating coma. Conversely, if the lower limit is exceeded, it is advantageous for off-axis aberration correction, but it is not possible to have a means for correcting spherical aberration generated excessively on the image side surface of the first lens.

Conditional expression (5) relates to the shape of the third lens, and it is necessary that the third lens has a loose positive meniscus shape with the convex surface facing the image side. In addition, by giving a loose positive power, the first
The objective of the present invention is to correct off-axis aberrations while reducing the power of the lens and the second lens. If the upper limit is exceeded, the off-axis principal ray angle becomes too low, and the fourth lens cannot be corrected, so that the telecentricity deteriorates. If the lower limit is exceeded, although it is advantageous in terms of telecentricity, off-axis coma flare increases and performance deteriorates.

Conditional expression (6) shows conditions relating to the shape of the fourth lens, and mainly relates to telecentricity and distortion on the image side. Since these aberrations are balanced in the range of the conditional expression, the image-side telecentricity and the distortion are degraded both above the upper limit and below the lower limit.

Further, it is preferable that the second lens to the fourth lens are made of a resin material, except that the first lens is made of a glass material. The use of an aspherical surface, which is the feature of (1), and the cost reduction can be easily achieved.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to specific numerical examples. In the following first to third embodiments, the aperture stop S (the surface is S
1, S2), and thereafter the first lens L in order from the object side.
1, a second lens L2, a third lens L3, and a fourth lens L4 are arranged. A parallel plane glass LP is disposed between the fourth lens L4 and the image plane with an air gap. Although the parallel plane glass LP is actually a quartz optical filter, there is no problem in the optical description of the present invention, so it is expressed as a single parallel plane glass having the same thickness.

As is well known, the aspherical surface used in each embodiment has an aspherical surface formula: Z = (Y ² / Y) when the Z axis is taken in the optical axis direction and the Y axis is taken in the direction orthogonal to the optical axis. r) [1 + √ ｛1- (1 + K) (Y /
r) ² }] + A · Y ⁴ + B · Y ⁶ + C · Y ⁸ + D ·
A curved surface obtained by rotating the curve given by Y ¹⁰ + ‥‥ around the optical axis. The paraxial radius of curvature: r, the conic constant: K, and the higher order aspherical coefficients: A, B, C, D To define the shape. In the notation of the conical constant and the higher order aspheric coefficient in the table, "E and the number following it" represent "10th power". For example, “E−4” means 10 ⁻⁴ , and this numerical value is multiplied by the immediately preceding numerical value.

[Embodiment 1] The first embodiment of the photographing lens of the present invention.
Table 1 shows a numerical example of the embodiment. FIG. 1 is a diagram showing the lens configuration, and FIG. 2 is a diagram showing various aberrations. In the tables and drawings, f is the focal length of the entire lens system, F _no is the F number,
2ω is full angle of the lens, b _f is the back focus. The back focus _bf is an air-equivalent distance from the image side surface of the fourth lens to the image surface. In addition, R is the curvature radius, D is the lens thickness or distance between lens, N _d is the refractive index of the d line, the [nu _d designates the Abbe number of d line. D, g, and C in the spherical aberration diagram are aberration curves at respective wavelengths. C. Is a sine condition. In the astigmatism diagram, S indicates sagittal, and M indicates meridional.

[0022]

[Table 1]

Embodiment 2 Table 2 shows a numerical example of the second embodiment. FIG. 3 is a diagram showing the lens configuration, and FIG. 4 is a diagram showing various aberrations.

[Table 2]

[Embodiment 3] Table 3 shows a numerical example of the third embodiment. FIG. 5 is a diagram illustrating the lens configuration, and FIG. 6 is a diagram illustrating various aberrations.

[Table 3]

Next, the values corresponding to the conditional expressions (1) to (6) for Examples 1 to 3 are summarized in Table 4.

[Table 4]

As evident from Table 4, the numerical values of the first to third embodiments satisfy the conditional expressions (1) to (6), and are evident from the aberration diagrams in the respective embodiments. Thus, each aberration is well corrected.

[0027]

According to the present invention, it is possible to provide a compact photographic lens having a high resolution and a small number of components. In addition, since the aperture stop is located closest to the object side, taking advantage of the feature that the taking lens is inconspicuous when viewed from the object side, it is especially used for surveillance cameras and PC cameras (imaging devices attached to personal computers). It can be applied to products that have high performance, are compact, and make use of the shape characteristics.

[Brief description of the drawings]

FIG. 1 is a lens configuration diagram of a first embodiment of a taking lens according to the present invention.

FIG. 2 is a diagram showing various aberrations of the taking lens of the first embodiment.

FIG. 3 is a lens configuration diagram of a second embodiment of the taking lens according to the present invention;

FIG. 4 is a diagram showing various aberrations of the taking lens according to the second embodiment;

FIG. 5 is a lens configuration diagram of a third embodiment of the taking lens according to the present invention;

FIG. 6 is a diagram illustrating various aberrations of the photographing lens according to the third example;

Claims (3)

[Claims] An aperture stop is disposed closest to the object side, and thereafter, has a positive refractive power (hereinafter referred to as a positive lens) in order from the object side.
Refraction constituting the entire system in a photographic lens including a lens, a second lens having a negative refractive power (hereinafter referred to as a negative lens), a third lens as a positive lens, and a fourth lens as a negative lens. At least one of the surfaces is an aspherical refracting surface, and satisfies the following conditional expression (1) with respect to the power of the first lens, and the following conditional expression (2) with respect to the power of the second lens: Satisfies the following conditional expression (3) with respect to the Abbe numbers of the first lens and the second lens. (1) 1.5 <f / f ₁ <2.5 (2) 1.5 <f / | f ₂ | <2.5 (However, the absolute value is f ₂ <0) (3) 15 <ν ₁₋ ν ₂ where f: combined focal length of the entire lens system f ₁ : focal length of the first lens f ₂ : focal length of the second lens ν ₁ : Abbe number of the first lens ν ₂ : Abbe of the second lens number 2. The imaging lens according to claim 1, further comprising the following conditional expression (4) with respect to the shape of the image side surface of said first lens, and the following condition with respect to the shape of the image side surface of said third lens: 2. The photographing lens according to claim 1, wherein Expression (5) is satisfied, and the following conditional expression (6) is satisfied with respect to the shape of the object side surface of the fourth lens. ₍₄₎ 0.8 _<| (for although the absolute value of _{r 2 <0) <2.5 (} 5) 0.25 <| | r 2 / r 1 r 6 | / f <0.45 ( provided that absolute values for _{r 6 <0) (6)} 1.2 <r 7 /f<2.0 However, _{r 1:} radius of curvature of the object side surface of the first lens radius r _2: curvature radius of the image side surface of the first lens r ₆ : radius of curvature of the image side surface of the third lens r ₇ : radius of curvature of the object side surface of the fourth lens 3. The photographic lens according to claim 1, wherein the first lens is made of a glass material, and the other second to fourth lenses are made of a resin material. The photographing lens according to claim 1 or 2, wherein: