JP2008090150A - Imaging lens and imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging lens for a digital camera having a high imaging performance while obtaining a flat image surface from the center to the periphery and having a bright open F value and being compact, and an imaging device using the imaging lens. Is an issue.
In order from the object side, an aperture stop S, a first biconvex lens L1 having a positive refractive power, and a negative meniscus second lens having a negative refractive power and having a convex surface facing the object side. A fourth lens L2, a positive meniscus third lens L3 having a convex surface directed toward the image side, and a fourth lens configured so that the vicinity of the optical axis has a negative refractive power and the positive refractive power increases toward the periphery of the lens. An imaging lens 1 in which a lens L4 is disposed.
[Selection] Figure 1

Description

  The present invention relates to a novel imaging lens and imaging apparatus. Specifically, a high-performance and compact imaging lens for a small-sized imaging device mainly using a CCD (charged coupled device) such as a digital still camera or a PC camera or a CMOS (Complementary Metal-Oxide Semiconductor) imaging device, and The present invention relates to an imaging device using the imaging lens.

  In recent years, with the popularization of game devices for general homes, digital cameras as image input devices for capturing photographed person videos in a computer are becoming popular. Such a digital camera is required to have high resolution of captured images, miniaturization of a camera device, a large aperture ratio as a camera specification, and a low distortion and low cost.

  Conventionally, as a large-diameter and compact imaging lens, for example, those described in Patent Document 1 and Patent Document 2 can be cited.

JP 2002-365529 A Japanese Patent Laid-Open No. 2005-4028

  In the embodiment described in Patent Document 1, an imaging lens having a positive / negative / positive refractive power configuration is shown, but when the total length is shortened, the exit angle of the off-axis ray from the second lens group increases, In particular, there is a problem that the outward coma becomes excessive and correction is very difficult. In addition, there is a large difference between the field curvature from the center to the vicinity of 50% image height and the field curvature from the 70% image height to 100% image height, and it has been difficult to obtain a flat image surface from the center to the periphery.

  In the example described in Patent Document 2, the refractive power of the fourth lens is positive during the configuration, and it is difficult to shorten the back focus.

  In view of the above-described problems, the present invention provides an imaging lens for a digital camera having high imaging performance while obtaining a flat image surface from the center to the periphery and having a bright open F value and being compact, and the imaging lens. It is an object to provide a used imaging device.

  An imaging lens according to an embodiment of the present invention includes, in order from the object side, an aperture stop, a first biconvex lens having positive refractive power, and a negative lens having negative refractive power and having a convex surface facing the object side. A second meniscus lens, a positive third meniscus lens with a convex surface facing the image side, and a structure in which the vicinity of the optical axis has a negative refractive power and the positive refractive power increases toward the periphery of the lens. The fourth lens is arranged.

  In addition, an imaging apparatus according to an embodiment of the present invention includes an imaging lens and an imaging element that converts an optical image formed by the imaging lens into an electrical signal, and the imaging lens sequentially includes an aperture stop, The first lens having a positive refractive power and a biconvex shape, the second lens having a negative refractive power and having a convex surface facing the object side, and a positive meniscus shape having a convex surface facing the image side The third lens and the fourth lens configured so that the vicinity of the optical axis has a negative refractive power and the positive refractive power increases as it goes to the periphery of the lens are arranged.

  According to the present invention, while having a flat image surface from the center to the periphery, the open F value is bright and compact, it has high imaging performance.

  The best mode for carrying out the imaging lens and imaging apparatus of the present invention will be described below with reference to the drawings.

  The imaging lens according to the present invention includes, in order from the object side, an aperture stop, a first biconvex lens having a positive refractive power, and a negative meniscus second lens having a negative refractive power and having a convex surface facing the object side. A lens, a positive meniscus third lens having a convex surface directed toward the image side, and a fourth lens configured so that the vicinity of the optical axis has a negative refracting power and the positive refracting power increases toward the periphery of the lens. Arranged.

  Therefore, according to the present invention, it is possible to obtain an imaging lens having high imaging performance while obtaining a flat image surface from the center to the periphery and having a wide open F value and being compact.

  The fourth lens has a negative refractive power, but this can shorten the back focus and contribute to compactness.

  Giving the lens group closest to the image plane side to have a negative refractive power is a necessary condition for shortening the back focus, but at the same time the exit pupil position is close to the image plane, and the peripheral luminous flux to the image sensor is reduced. There was also a problem that the incident angle was increased. As shown in FIG. 5A, when the incident angle of the peripheral light beam Lp is small as shown in FIG. 5A, the total amount of light is received by the light receiving unit Rl. As shown in FIG. 4, in the case of a large size, the light flux Lp is generated by the light shield Sd provided between the light receiving unit Rl and the microlens Ml in order to prevent the incidence of noise light such as stray light or irregular reflection light to the light receiving unit Rl. Is kicked, which reduces the amount of ambient light.

  Therefore, in the imaging lens of the present invention, the fourth lens is configured such that the vicinity of the optical axis has a negative refractive power, and the positive refractive power increases as it goes to the periphery of the lens. As a result, both the shortening of the back focus and the securing of the amount of peripheral light can be achieved by preventing the incident angle of the peripheral luminous flux from entering the image sensor.

In the imaging lens according to the embodiment of the present invention, the image side surface of the fourth lens is constituted by an aspheric surface, and T is the portion of the aspherical surface that is the image surface side of the fourth lens that protrudes to the most image side. It is desirable to satisfy the following conditional expression (1), with the distance (sag amount) measured along the optical axis from the surface apex on the optical axis and f as the combined focal length of the entire lens system.
(1) 0.02 <T / f <0.09
FIG. 6 shows an upper half of an example of the fourth lens. The left side of FIG. 6 is the object side, and the right side is the image side. In FIG. 6, the optical axis ax of the portion Ap that protrudes to the most image side on the image side surface. The distance T measured along the optical axis ax from the upper surface vertex Pp is shown.

  Conditional expression (1) indirectly defines the refractive power of the periphery of the fourth lens.

  If the amount of protrusion to the image side of the portion of the image side surface that protrudes most to the image side exceeds the upper limit value of conditional expression (1), the entrance pupil position at the most peripheral image height becomes too shallow, so desirable shading characteristics Cannot be obtained. On the other hand, if the lower limit of conditional expression (1) is not reached, the local curvature at the lens periphery decreases, and the external coma generated in the second lens group cannot be canceled out, thus satisfying the imaging performance. It can not be made.

  As the imaging lens of the present invention, when the total length is shortened with a positive / negative / positive configuration, the exit angle of the peripheral light flux from the second lens becomes large, and outward coma is remarkably generated and correction thereof becomes difficult. By giving positive refracting power to the periphery of the portion of the fourth lens having negative refracting power, it becomes possible to produce inward coma in the fourth lens. A function of canceling out the outward coma aberration can be provided, which is a preferable configuration for correcting coma aberration.

In the imaging lens according to the embodiment of the present invention, the first lens is preferably a biconvex lens. It is possible to maintain good imaging performance while shortening the overall length. Furthermore, it is desirable to satisfy the following conditional expression (2), where f1 is the focal length of the first lens and f is the combined focal length of the entire system.
(2) 0.44 <f1 / f <0.87
Conditional expression (2) is mainly for correcting spherical aberration. If the lower limit of conditional expression (2) is not reached, the negative spherical aberration becomes excessive, and correction with a lens behind the first lens becomes difficult. Further, since coma aberration is excessively generated, it is not preferable. On the contrary, if the upper limit of conditional expression (2) is exceeded, it is advantageous for correcting off-axis coma, but spherical aberration occurring on the image side surface of the first lens can be corrected by the subsequent lens. Disappear.

  In the imaging lens according to the embodiment of the present invention, it is desirable that the second lens is a negative refractive meniscus lens having a convex surface on the object side.

  If the object side surface of the second lens becomes concave and becomes a biconcave lens, a light beam having a high image height will cause outward coma on both the object side surface and the image side surface. Attempting to correct them with subsequent lenses, especially for peripheral image heights of 70% to 100%, must increase the positive refractive power at the periphery of the fourth lens, The exit pupil position at the image height becomes too close, and the desired shading characteristics cannot be obtained.

  On the other hand, if the object-side surface is convex to the object side, the coma in the direction that cancels out the outward coma aberration can be produced on the surface, and therefore, in the second lens group. It is easy to suppress the occurrence of coma in

In the imaging lens according to an embodiment of the present invention, the third lens is a positive meniscus lens having a convex surface facing the image plane side, f3 is the focal length of the third lens, and f is the combined focal length of the entire system. It is desirable that the following conditional expression (3) is satisfied.
(3) 0.5 <f3 / f <3.9
Conditional expression (3) defines the focal length of the third lens. When the focal length of the third lens becomes longer than the upper limit of conditional expression (3), the power of the third lens becomes weaker, especially the coma. This is not desirable because it causes deterioration of aberration and astigmatism. If the focal length of the third lens becomes shorter than the lower limit value of the conditional expression (3), the back focus increases, making it impossible to form a compact lens and increasing astigmatism, which is not desirable.

  In the imaging lens according to the embodiment of the present invention, it is desirable that all the lenses from the first lens to the fourth lens are plastic lenses in order to realize cost reduction and thickness reduction at the same time. The plastic lens is low in the cost of the material itself, and there are fewer restrictions on lens processing than a glass lens in realizing a reduction in thickness. That is, a plastic lens capable of reducing the lens center thickness and the edge thickness is more advantageous for reducing the thickness than a glass lens.

  Hereinafter, specific embodiments of the imaging lens of the present invention and numerical examples in which specific numerical values are applied to these embodiments will be described.

  Note that an aspherical surface is employed in the imaging lens of the present invention. Therefore, in the orthogonal coordinate system in which the aspherical surface deformation amount ΔH (h) is an orthogonal coordinate system in which the vertex of the surface is the origin and the optical axis direction is the X axis, the direction perpendicular to the optical axis is h, the vertex curvature radius is r, and the conic constant is K. It is assumed that the fourth, sixth, eighth, tenth, and twelfth aspherical coefficients are defined by Equation 1 as A, B, C, D, and E.

  FIG. 1 shows the lens configuration of a first embodiment of the imaging lens of the present invention. The imaging lens 1 includes, in order from the object side, an aperture stop S, a biconvex first lens L1 having a double-sided aspherical surface, and a negative meniscus shape convexing on the object side and a double-sided second surface. Lens L2, a third lens L3 having a positive meniscus shape that is convex on the image surface side and having both surfaces aspherical surfaces, has a negative refractive power near the optical axis, and a positive refractive power increases toward the periphery of the lens. The fourth lens L4 configured as described above and having both aspherical surfaces is arranged. Here, the glass block G at the end of the optical system is arranged as a substitute for a low-pass filter, a lid glass of an image sensor, or the like. That is, when the imaging lens is incorporated into a small information device or the like and actually used, a low-pass filter, a lid glass of the imaging element, or the like is disposed between the fourth lens and the light receiving surface IMG of the imaging element. Therefore, in the first embodiment, at the time of designing the imaging lens, a filter made of a glass material equivalent to BK7 (trade name) manufactured by OHARA Co., Ltd. is placed close to the focus surface (IMG). Considered the effects of low-pass filter and image sensor lid glass.

Table 1 shows lens data of Numerical Example 1 in which specific numerical values are applied to the imaging lens 1 according to the first embodiment. In Table 1 and other lens data tables, Si is the i-th surface from the object side, R is the paraxial radius of curvature of the i-th surface, D is the i-th surface and i + 1-th surface from the object side. Nd is the refractive index with respect to the d-line (λ = 587.6 nm) of the medium having the i-th surface on the object side, and νd is the d-line of the medium having the i-th surface on the object side. , F is the total focal length, Fno is the F number, ω is the half angle of view, and ∞ for the radius of curvature R is that the surface is a plane. Further, regarding the aspherical surface, K is a conic constant, A is a 4th order, B is a 6th order, C is an 8th order, D is a 10th order, and E is a 12th order aspherical coefficient. In Table 1 and the following tables showing aspheric coefficients, “E-i” represents an exponential expression with a base of 10, that is, “10- ⁱ ”. For example, “0.12345E-05” represents “ 0.12345 × 10 ⁻⁵ ”.

  FIG. 2 shows longitudinal aberrations (spherical aberration, astigmatism, distortion) of Numerical Example 1. The diagram shows spherical aberration, where the solid line is for the d line and the broken line is the g line (λ = 435.8 nm). In the graph showing astigmatism, the vertical axis represents the image height, the horizontal axis represents the defocus, the solid line represents the sagittal image plane, and the broken line represents the meridional image plane. Further, the vertical axis of the graph showing distortion aberration represents the image height, and the horizontal axis represents%.

  FIG. 3 shows the lens configuration of a second embodiment of the imaging lens of the present invention. The imaging lens 2 includes, in order from the object side, an aperture stop S, a biconvex first lens L1 having a double-sided aspherical surface, and a negative meniscus shape convexing on the object side and a double-sided second surface. Lens L2, a third lens L3 having a positive meniscus shape that is convex on the image surface side and having both surfaces aspherical surfaces, has a negative refractive power near the optical axis, and a positive refractive power increases toward the periphery of the lens. The fourth lens L4 configured as described above and having both aspherical surfaces is arranged. Here, the glass block G at the end of the optical system is arranged as a substitute for a low-pass filter, a lid glass of an image sensor, or the like. That is, when the imaging lens is incorporated into a small information device or the like and actually used, a low-pass filter, a lid glass of the imaging element, or the like is disposed between the fourth lens and the light receiving surface IMG of the imaging element. Therefore, in the second embodiment, at the time of designing the imaging lens, a filter made of a glass material equivalent to BK7 (trade name) manufactured by OHARA Co., Ltd. is positioned in the immediate vicinity of the focus surface (IMG). Considered the effects of low-pass filter and image sensor lid glass.

  Table 2 shows lens data of Numerical Example 2 in which specific numerical values are applied to the imaging lens 2 according to the second embodiment.

  FIG. 4 shows longitudinal aberrations (spherical aberration, astigmatism, distortion) of Numerical Example 2. In the graph showing spherical aberration, the solid line is for d line, the broken line is for g line, In the diagram showing the point aberration, the vertical axis represents the image height, the horizontal axis represents the defocus, the solid line represents the sagittal image plane, and the broken line represents the meridional image plane. Further, the vertical axis of the graph showing distortion aberration represents the image height, and the horizontal axis represents%.

  Table 3 shows values corresponding to the conditional expressions (1) to (3) in Numerical Example 1 and Numerical Example 2.

  As shown in the respective tables and aberration diagrams, the imaging lenses according to Numerical Example 1 and Numerical Example 2 both satisfy conditional expressions (1) to (3), and the total length is as short as 6 mm or less. The open F-number is also bright at around 3.0, and further shows high imaging performance with each aberration corrected satisfactorily.

  Next, the imaging apparatus of the present invention will be described.

  The imaging device of the present invention is an imaging device including an imaging lens and an imaging element that converts an optical image formed by the imaging lens into an electric signal. A biconvex first lens having negative refractive power, a negative meniscus second lens having negative refractive power with the convex surface facing the object side, and a positive meniscus second lens with the convex surface facing the image side A third lens and a fourth lens configured so that the vicinity of the optical axis has negative refractive power and the positive refractive power increases as it goes to the periphery of the lens are arranged. That is, the image pickup lens according to the present invention and an image pickup device that converts an optical image formed by the image pickup lens into an electric signal are provided.

  As described above, since the imaging lens is small and has high performance, the imaging device can also be configured to be small and have high performance, and is portable, referred to as a small device such as a mobile phone, a notebook type, particularly a mobile computer. It is suitable to be mounted on a portable information device such as a personal computer suitable for the above, PDA (Personal Digital Assistant).

  FIG. 7 shows a block diagram of the simplest configuration example of the imaging apparatus of the present invention.

  The imaging device 10 includes an imaging lens 20 and an imaging element 30 that converts an optical image formed by the imaging lens 20 into an electrical signal. As the image pickup device 30, a device using a photoelectric conversion device such as a CCD or a CMOS can be used. The output of the image sensor 30 is subjected to various processes by the image processing circuit 40 and sent to the control unit 50. As the imaging lens 20, the imaging lens 1 according to the first embodiment and the imaging lens 2 according to the second embodiment can be used. Or you may use the imaging lens of this invention by forms other than above-described embodiment.

  And the process by the various operation signals input into the control part 50 from the outside is performed. For example, when a display command is input to the control unit 50, an image is displayed on a display unit, for example, the liquid crystal display 60, and when a recording operation command is input to the control unit 50, an image is displayed on a predetermined recording medium 70. Recorded as a signal. Note that the display unit 60 and the recording medium 70 described above may be provided in the imaging apparatus itself, such as a digital still camera, or may be connected via an external connection unit such as a USB (Universal Serial Bus). It may be a display means or a recording medium provided in an information device such as a personal computer or a PDA connected.

  FIG. 8 shows an example in which the imaging apparatus of the present invention is built in a personal computer.

  A personal computer 80 shown in FIG. 8 is a notebook personal computer, and a main body portion 81 and a display portion 82 are connected to each other by a hinge 83 so that the display portion 82 can be connected to the main body portion 81. The display unit 82 covers the upper surface of the main body 81 when not in use.

  On the upper surface of the main body 81, there are arranged a keyboard portion 81a on which various keys such as a character input key and a function operation key are arranged, a pointer operation portion 81b, and the like. A Versatile Disc) drive, an FD (Floppy Disk) drive, a recording medium insertion / removal port 81c such as a memory card slot, an external device connection part 81d such as a USB connector, and the like are provided.

  A liquid crystal display panel 82a is provided on the inner surface of the display portion 82, that is, the surface facing the upper surface of the main body 81 when the two portions are closed, and further, the liquid crystal display panel 82a is provided. A front lens 82c of a built-in camera (not shown) is desired to the outside through a facing hole 82b formed at a location close to the upper corner of the lens. The imaging apparatus of the present invention can be used for this built-in camera. In this case, the display means 60 described with reference to FIG. 7 is a liquid crystal display panel 82a, and the recording medium 70 is a DVD-R, DVD-RAM, DVD-RW, floppy disk (registered) that is inserted and removed from the insertion / removal port 81c. Trademark), a memory card, and the like.

  As described above, by incorporating the imaging device of the present invention in the personal computer 80, for example, it can be used as a camera unit of a videophone using the personal computer 80 and used for a video conference or the like. The game software installed in the computer 80 and the built-in camera can be used in cooperation with each other.

  It should be noted that the specific structures, shapes, and numerical values shown in the embodiments and numerical examples described above are only examples of the implementations that are carried out in carrying out the present invention, and thus the technology of the present invention. The scope should not be interpreted in a limited way.

It is a figure which shows the lens structure of 1st Embodiment of this invention imaging lens. It is a figure which shows the longitudinal aberration (spherical aberration, astigmatism, distortion aberration) of the numerical example 1 which applied the specific numerical value to 1st Embodiment. It is a figure which shows the lens structure of 2nd Embodiment of this invention imaging lens. It is a figure which shows the longitudinal aberration (spherical aberration, astigmatism, distortion aberration) of Numerical Example 2 which applied the specific numerical value to 2nd Embodiment. It is an expanded sectional view of the principal part explaining the relationship between the incident angle to an image sensor, and received light quantity. It is a figure for demonstrating the shape of a 4th lens. It is a block diagram showing an embodiment of an imaging device of the present invention. It is a schematic perspective view which shows the example which incorporated the imaging device of this invention in the personal computer.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Imaging lens, 2 ... Imaging lens, S ... Aperture stop, L1 ... 1st lens, L2 ... 2nd lens, L3 ... 3rd lens, L4 ... 4th lens, 10 ... Imaging device, 20 ... Imaging lens, 30 ... Image sensor

Claims (3)

In order from the object side, an aperture stop, a biconvex first lens having a positive refractive power, a negative meniscus second lens having a negative refractive power and having a convex surface facing the object side, and a convex surface on the image side A third lens having a positive meniscus shape facing the lens and a fourth lens configured so that the vicinity of the optical axis has a negative refractive power and the positive refractive power increases toward the periphery of the lens. A characteristic imaging lens. The imaging lens according to claim 1, wherein the following conditional expression (1) is satisfied.
(1) 0.02 <T / f <0.09
However,
T: Distance (sag amount) measured along the optical axis from the surface apex on the optical axis of the most aspherical portion of the aspherical surface on the image plane side of the fourth lens.
f: The combined focal length of the entire lens system. An imaging device including an imaging lens and an imaging element that converts an optical image formed by the imaging lens into an electrical signal,
The imaging lens includes, in order from the object side, an aperture stop, a first biconvex lens having a positive refractive power, and a negative meniscus second lens having a negative refractive power and having a convex surface facing the object side. A positive meniscus third lens with a convex surface facing the image side and a fourth lens configured so that the refractive power in the vicinity of the optical axis has a negative refractive power and the positive refractive power increases toward the periphery of the lens. An imaging apparatus characterized by comprising:

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