JP2002162561A - Shooting lens - Google Patents

Abstract

(57) [Problem] To provide a compact photographing lens having high resolution, a small number of components, a long back focus, and good image side telecentricity. SOLUTION: An aperture stop is arranged closest to an object side, and thereafter, is constituted by four lenses of a first lens, a second lens, a third lens and a fourth lens in order from the object side, wherein the first lens is an image. A meniscus lens having a convex shape on the side;
The lens is a lens having a positive refractive power (hereinafter referred to as a positive lens), the third lens is a lens having a negative refractive power, and the fourth lens is a positive lens which is bonded to the third lens. Lens.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art In recent years, in addition to conventional cameras using silver halide films, for example, 35 mm format cameras, digital still cameras have rapidly become widespread as image pickup apparatuses of a new genre. The digital still camera can easily take a picture by using an attached liquid crystal monitor as a viewfinder, and can also enjoy playing the picture taken on the spot. In addition, as a tool for inputting still images to personal computers and the like that have spread to ordinary households,
Also, with the increase in resolution of color printers and the like, they have been used for printing purposes as in conventional cameras. A digital still camera is an imaging device that is structurally configured to electrically capture a still image formed by a photographing lens with a CCD or other imaging device (hereinafter referred to as a CCD) and record the captured still image in a built-in memory or a memory card. At first, the LCD monitor can be used as a viewfinder for shooting and as a monitor for playing back captured images. 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, recently, with the rapid spread of digital still cameras, CCD cameras with a large number of pixels have been supplied at a low price. It is being improved and commercialized with the force approaching.

In order to increase the number of pixels of the CCD, a method of increasing the screen size while maintaining the pixel pitch and a method of reducing the pixel pitch while maintaining the screen size can be considered. In general, a method of increasing the number of pixels by reducing the pixel pitch without changing the screen size is given priority because the number of pieces to be taken per unit wafer is reduced and the cost is increased. For example, the pixel pitch of a recently announced CCD for a digital still camera having a number of effective pixels of 3 million pixels is about 3.5 μm. Therefore, even if it is assumed that the minimum circle of confusion is twice the pixel pitch, it is 7.0 μm, which is 35 μm.
Since the minimum circle of confusion of the mm-size silver halide camera is considered to be about 33 μm, it can be said that the resolving power required for the photographing lens of the digital still camera is about five times that of the silver halide camera.
This also means that the area of each pixel for taking in light is reduced, and as a result, the output sensitivity of the sensor is reduced. As a countermeasure, attempts have been made to improve by placing a microlens array immediately before each pixel. However, when the pixel pitch is on the order of 3 μm, the film sensitivity becomes lower than the ISO100 of the film sensitivity when viewed effectively, and the open F value of the photographing lens is reduced. Unless it is small and a bright lens, it becomes difficult to use.

On the other hand, as an optical system using a CCD, VT
There is a photographic lens of the R camera, and comparing the characteristics of the photographic lens of the digital still camera and the VTR camera,
It can be considered that the size of the image circle is almost the same, and more specifically, the telecentricity of the image side is required as described later. The taking lens is more similar to the taking lens of a digital still camera. Therefore, the use of a photographic lens for a VTR camera in a digital still camera has been performed at the beginning of widespread use. V
TR cameras are also being developed, and those that have been digitally processed and feature high image quality have recently been commercialized. However, the required resolution due to the nature of viewing reproduced images on a television or monitor is a digital still camera. A 350,000 pixel class that is one digit smaller than the CCD used in the above is sufficient. The pixel pitch of this class of CCD is about 5.6 μm. Therefore, in order to use such a photographic lens for a VTR camera in a digital still camera using a CCD having more than 1 million pixels or a CCD of a 3 million pixel class, the lack of resolution becomes crucial, and the use is unbearable. . Also, the required level of the amount of distortion with respect to the photographing lens is different depending on the difference between a moving image and a still image, and in a digital still camera, stricter aberration correction including distortion is required.

As described above, in an optical system using an image sensor such as a CCD, the telecentricity on the image side must be designed well. The telecentricity on the image side means that the principal ray of the ray bundle for each image point becomes almost parallel to the optical axis after exiting the final surface of the optical system, that is, crosses the image plane almost perpendicularly. . In other words, it is required that the exit pupil position of the optical system be sufficiently away from the image plane. This is because the color filter on the CCD is located at a position slightly distant from the imaging surface, so that when the light beam enters obliquely, the substantial aperture efficiency is reduced (referred to as shading). In many CCDs of the sensitivity type, a microlens array is arranged immediately before the imaging surface. In this case as well, if the exit pupils are not sufficiently separated, the aperture efficiency decreases in the periphery. Also,
A quartz optical filter (optical low-pass filter) or CCD inserted between the optical system and the CCD in order to prevent the moire phenomenon caused by the periodic structure of the CCD
In order to reduce the sensitivity in the infrared wavelength range of the infrared ray and bring it closer to the human eye's relative luminous efficiency, the effective thickness of the infrared absorption filter inserted between the optical system and the CCD is also reduced on the optical axis and around the optical axis. In this respect, it is necessary to design the image-side telecentricity of a photographing lens for a digital still camera well.

[0006]

As described above, a photographing lens for a digital still camera is required to have a resolution approximately five times as high as that of a silver halide camera, and at the same time, to improve telecentricity on the image side and to improve optical performance. A quartz optical filter, infrared absorption filter, etc. must be inserted between the system and the image plane,
It is required to get enough back focus. In addition, while the F-number is small and functions such as a zoom lens are generally required due to a situation such as a decrease in the sensitivity of the CCD, a further compactness is also required. In order to do so, advanced optical design techniques such as the effective introduction of aspherical lenses are required.

SUMMARY OF THE INVENTION In view of the circumstances described above, an object of the present invention is to provide a compact photographing lens having a high resolution, a small number of components, a long back focus, and good image side telecentricity.

[0008]

According to the present invention, there is provided a photographing lens comprising:
An aperture stop is arranged closest to the object side, and thereafter the first lens, the second lens, the third lens, and the fourth lens
The first lens is a meniscus lens having a convex shape on the image side, the second lens is a lens having a positive refractive power (hereinafter referred to as a positive lens), and the third lens is a third lens.
The lens is a lens having a negative refractive power, and the fourth lens is a positive lens that is configured to adhere to the third lens. In the aperture stop and the photographic lens configured by arranging the first lens to the fourth lens, the power of the second lens satisfies the following conditional expression (1) and is configured by bonding. The combined power of the third lens and the fourth lens satisfies the following conditional expression (2). (Claim 1) (1) 0.8 <f ₂ /f<2.0 (2) 2.0 <| f _3.4 | / f where f: synthetic focal length of the entire lens system f ₂ : second the focus of the second lens distance f 3 _{· 4:} composite focal length of the third lens and the fourth lens

Conditional expression (1) relates to the power of the second lens. In the lens configuration of the photographic lens of the present invention, the second lens basically has a large positive power, and the other lenses have chromatic aberration generated by the second lens.
And has the function of correcting monochromatic aberration in a concentric shape. Therefore, when the power of the second lens is reduced below the upper limit, the power of the first lens or the cemented lens (hereinafter, the cemented lens) composed of the third lens and the fourth lens becomes excessively large, and the concentric structure is formed. Collapse and various aberrations worsen. Also,
If the power of the second lens becomes larger than the lower limit, it becomes difficult to correct chromatic aberration, spherical aberration, and the like generated in the second lens by another lens.

Conditional expression (2) relates to the combined power of the third lens and the fourth lens constituting the cemented lens. The third lens has a strong negative refractive power, but has an important meaning with respect to Petzval sum, that is, field curvature and chromatic aberration. If the lower limit is exceeded, that is, if the power of the third lens is large, favorable conditions are obtained for the field curvature and chromatic aberration, but disadvantageous conditions for spherical aberration and coma. Conversely, when the value exceeds the upper limit, that is, when the power of the third lens is small, spherical aberration and coma are advantageous, but field curvature and chromatic aberration are disadvantageous.

The shape of the object-side and image-side surfaces of the first lens satisfies the following conditional expression (3), and the image-side shape of the fourth lens and the object-side shape of the third lens. Satisfies the following conditional expression (4), and satisfies the following conditional expression (5) with respect to the Abbe numbers of the third lens and the fourth lens constituting the cemented lens. It is preferable that the following conditional expression (6) is satisfied in relation to the refractive indices of the third lens and the fourth lens. (Claim 2)

(3) 0.8 <r ₁ / r ₂ <1.2 (4) 1.0 <r ₇ / r ₅ <1.5 (5) 15 <ν ₄ −ν ₃ (6) 72 _{<n 3} and 1.72 _{<n 4,} however, _{r 1:} curvature of the object side of the first lens radius r _2: curvature of the image side of the first lens radius r _5: curvature of the object side of the third lens radius r ₇ : radius of curvature of the fourth lens on the image side ν ₃ : Abbe number of the third lens ν ₄ : Abbe number of the fourth lens n ₃ : refractive index to d-line of the third lens n ₄ : d-line of the fourth lens Refractive index for

Conditional expression (3) is a condition relating to the relationship between the shape of the object side and the image side surface of the first lens. The feature of the shape of the first lens is that it is generally concentric with the aperture stop. By adopting such a shape, spherical aberration and coma can be satisfactorily corrected and distortion can be maintained satisfactorily. Correcting aberrations and coma in a well-balanced manner adversely affects distortion.

The conditional expressions (4) to (6) are conditional expressions relating to the third lens and the fourth lens constituting the cemented lens. Comparing the refractive index and Abbe number of the glass material before and after the cemented surface (sixth surface) between the third lens and the fourth lens, the difference in refractive index is substantially equal, and the difference in Abbe number is substantially equal. There are differences in the values. In other words, the primary purpose of this cemented surface is to correct chromatic aberration, and there is almost no correction capability for correction of other aberrations, and there is almost no difference in refractive index. It may be considered as one lens.

Thus, when looking at conditional expression (4),
It can be said that this conditional expression is a conditional expression regarding the shapes on the object side and the image side when the cemented lens is considered as one lens. That is, by keeping the value of r ₇ / r _{5 in} the range of the conditional expression (4), the surface becomes concentric with respect to the light beam incident on the third lens, the occurrence of various aberrations is suppressed to a small value, and the chromatic aberration is favorably reduced. Can be corrected. Therefore, if the value exceeds the upper and lower limits in the conditional expression (4), the concentric shape and the effect are lost, and astigmatism, distortion, and telecentricity are rapidly deteriorated. Conditional expression (5) is
This is a condition relating to the distribution of Abbe numbers of the third lens and the fourth lens constituting the cemented lens. This condition is important for satisfactorily correcting the chromatic aberration of magnification. If the lower limit value is exceeded, the correction of the chromatic aberration becomes insufficient.

Conditional expression (6) is a condition relating to the distribution of the refractive indices of the third lens and the fourth lens which also constitute the cemented lens. In order to correct the chromatic aberration of magnification together with the conditional expression (5), the third lens and the fourth lens must each have a combination of strong positive and negative powers, and therefore have a surface with a strong curvature (that is, a surface that is difficult to process).
However, by adopting a high refractive index glass material according to the conditional expression (6), the surface shape becomes easy to process.
It is also important for the appropriateness of Petzval sum,
If the lower limit is exceeded, the radius of curvature will be small, making it difficult to process,
Also, the field curvature becomes an inappropriate amount.

Further, it is preferable that the first lens and the second lens are made of a resin material, and each of the first lens and the second lens has at least one or more aspherical refracting surfaces. This makes it possible to correct more advanced spherical aberration, coma, and astigmatism at a low cost for a bright light beam with a small F value and a light beam with a wide angle of view.
In addition, distortion and telecentricity can be controlled.

[0018]

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 the first lens L
1, the second lens L2, the third lens L3, and the fourth
The lens L4 is composed of four lenses, and a plane parallel glass LP is disposed between the fourth lens and the image plane with an air gap. The parallel plane glass LP is
Actually, CCD cover glass, quartz optical filter,
And an infrared absorption filter. Since there is no problem in the optical description of the present invention, it is represented by a single plane-parallel glass having the same total thickness.

As is well known, 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, the aspheric surface used in each embodiment is expressed by the following equation: Z = (Y ² / r) [1 + √ ｛1- (1 + K) (Y / r)
Y ² }] + A · Y ⁴ + B · Y ⁶ + C · Y ⁸ + D · Y ¹⁰
A curved surface obtained by rotating the curve given by + ‥‥ around the optical axis, by giving a paraxial radius of curvature: r, a conical constant: K, and higher order aspherical coefficients: A, B, C, D 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 may be 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, 2
full angle of ω lens, b _f is the back focus.
The back focus _bf is the image side surface of the fourth lens (the seventh
Plane) to the image plane. 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.

[0021]

[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 numerical examples 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, Table 4 collectively shows values corresponding to the conditional expressions (1) to (6) for the first to ninth embodiments.

[Table 4] As is evident from Table 4, the numerical values relating to each of the examples 1 to 3 satisfy the conditional expressions (1) to (6), and are evident from the aberration diagrams in each example. Each aberration is well corrected.

[0025]

According to the present invention, it is possible to provide a compact photographic lens having a high resolution, a small number of components, a long back focus, and good image side telecentricity. 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 is possible to expect the advantage of high performance and further utilizing 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] 1. An aperture stop is disposed closest to the object side, and is composed of four lenses of a first lens, a second lens, a third lens, and a fourth lens in order from the object side, wherein the first lens is A meniscus lens having a convex shape on the image side;
The lens is a lens having a positive refractive power (hereinafter referred to as a positive lens), the third lens is a lens having a negative refractive power, and the fourth lens is a positive lens which is bonded to the third lens. Lens. The aperture stop, and the first
In the taking lens configured by arranging the fourth lens from the lens, the third lens configured to be adhered satisfies the following conditional expression (1) regarding the power of the second lens.
A photographing lens, characterized by satisfying the following conditional expression (2) with respect to the combined power of the lens and the fourth lens. (1) 0.8 <f ₂ /f<2.0 (2) 2.0 <| f _3.4 | / f, where f: composite focal length of the entire lens system f ₂ : focal length of the second lens f _{3 · 4:} composite focal length of the third lens and the fourth lens 2. The imaging lens according to claim 1, further comprising the following conditional expression (3) with respect to the shape of the object-side and image-side surfaces of the first lens, and the image side of the fourth lens: And the shape of the third lens on the object side satisfies the following conditional expression (4), and relates to the following conditional expression (5) in relation to the Abbe numbers of the third lens and the fourth lens which are bonded and formed. The photographing lens according to claim 1, wherein the following conditional expression (6) is satisfied in relation to the refractive indices of the third lens and the fourth lens. _{(3) 0.8 <r 1 /} r 2 <1.2 (4) 1.0 <r 7 / r 5 <1.5 (5) 15 <ν 4 -ν 3 (6) 1.72 <n ₃ and 1.72 _{<n 4,} however, _{r 1:} curvature of the object side of the first lens radius r _2: curvature of the image side of the first lens radius r _5: curvature of the object side of the third lens radius r _7: No. Radius of curvature of the fourth lens on the image side ν ₃ : Abbe number of the third lens ν ₄ : Abbe number of the fourth lens n ₃ : refractive index of the third lens for d-line n ₄ : refractive index of the fourth lens for d-line 3. The photographing lens according to claim 1, wherein said first lens and said second lens are made of a resin material.
A photographic lens having a refracting surface having an aspherical shape equal to or more than a surface.