JP5358425B2 - High-performance large-aperture ratio imaging lens consisting of two lenses with an F-number of 3 or less - Google Patents

Abstract

Provided with a first lens, a diaphragm, and a second lens in this order from the object side, wherein the first surface of the first lens is a convex surface, the second surface thereof is a concave surface, the first surface of the second lens is a concave surface, and the second surface thereof is a convex surface. Each lens surface is formed with an aspheric surface, and the first surface of the second lens is formed with a diffractive optical surface that exhibits chromatic dispersion. Optical components satisfy the following conditional equations: (1) -0.11 < (L/R)/(V1/V2) < -0.07 and (2) 0.09 < De/f < 0.18, provided that R is the average radius of curvature of the first surface of the second lens, L is the distance from the diaphragm to the first surface of the second lens, V1 is the Abbe number of the e-line of the first lens material, V2 is the Abbe number of the e-line of the second lens material, De is the distance in the optical axis direction on the respective effective diameters of the first surface of the first lens and the second surface of the first lens, f is the focal distance of the entire system, and the sign of the radius of curvature is negative when the center of curvature is left of the refracting interface and is positive when right of the same.

Description

  The present invention relates to a small imaging lens that is intended to be mounted on, for example, a mobile phone and the like, and is composed of a two-lens configuration for photographing a wide-angle light such as white light and an object with a wide angle of view. A high-performance large-aperture ratio imaging lens having a value of 3 or less is provided.

  In recent years, with the shift from photographic film to image sensors such as CCD and CMOS, imaging lenses have rapidly become smaller and can be mounted on mobile phones and the like. In addition, technical problems such as restrictions on the light incident angle for electronic light receiving elements must also be satisfied.

  Furthermore, in recent years, as the number of pixels of the image pickup device has increased, as shown in Patent Documents 1 to 3, glass or plastic lenses having 2 to 4 sheets have become mainstream, while the popularization of mobile phones and the like has rapidly progressed. There is a strong market demand for high image quality even at a low price, and a higher-performance imaging lens with fewer components is desired.

JP 2007-298719 A JP 2005-326682 A JP 2005-284153 A

  Conventionally, as a small-sized and low-priced imaging lens, a lens composed of two aspherical plastic lenses is well known, and its production efficiency is high and cheaper than that of a three-lens configuration. There is a big difference compared to the three-sheet configuration. However, the three-sheet configuration is superior in design performance, but is more sensitive to tolerances than the two-sheet configuration, so the yield is worse and the parts cost is higher than the two-sheet configuration, and the assembly cost is significantly higher. There are disadvantages.

  In order to solve this problem, from the viewpoint of cost, the imaging lens inevitably has a two-lens configuration. However, since the number of components is small, it is difficult to correct chromatic aberration for broadband light, and a high spatial frequency. In contrast, sufficient resolution performance cannot be obtained.

  An object of the present invention is to solve these problems by effectively correcting chromatic aberration, and to provide a high-performance large-aperture-ratio imaging lens having an F value of 3 or less and having a high-performance two-lens configuration with a minimum configuration. To do.

  The imaging lens according to the present invention provides an imaging lens for imaging an object with a wide-angle light such as white light and a wide angle of view with a lens having two lenses, and has the following characteristics. ing.

In order from the object side, the lens is composed of two lens systems including a first lens made of a plastic material , an aperture, and a second lens made of a plastic material . The first surface of the first lens is convex and the second surface is concave. Diffraction in which the first surface of the second lens is a concave surface, the second surface is a convex surface, each surface of the lens is an aspheric surface, and exhibits a color dispersion function on the first surface of the second lens. An optical surface is arranged.

The imaging lens as described above is preferably configured such that each optical element satisfies the following conditional expression in a coordinate system in which the object direction is negative and the image plane direction is positive from the reference point.
(1) −0.11 <(L / R) / (V1 / V2) <− 0.07
(2) 0.09 <De / f ≦ 0.1 55
However,
R: Average radius of curvature of the first surface of the second lens The average radius of curvature represents the radius of curvature of the spherical surface obtained by approximating the displacement of the aspherical surface in the optical axis direction by the least square method.
L: Distance from the diaphragm to the first surface of the second lens V1: Abbe number of e-line of the first lens material V2: Abbe number of e-line of the second lens material De: First lens first surface and first lens first The distance f in the optical axis direction at each effective diameter of the two surfaces: focal length of the entire system

  A diffractive optical surface having a chromatic dispersion function is characterized in that a diffractive relief is formed on a lens surface, and the surface exhibits a chromatic dispersion that is larger than the chromatic dispersion of a lens material. The optical surface.

  The present invention realizes an imaging lens for satisfactorily imaging an object with wide-band light and a wide angle of view with a lens system having two lenses. The two-lens configuration secures a constant Petzval sum and achieves optimal aberration correction in order to achieve field curvature correction to accommodate a wide angle of view, and light at an optimal incident angle with respect to an electronic light receiving element. Is the number of components necessary to secure the exit angle of the light beam from the lens so that is incident.

  However, the two-lens configuration can correct the light aberration satisfactorily, but the chromatic aberration correction is insufficient, resulting in blurring depending on the chromatic aberration in the formed image with respect to the broadband light. Satisfactory resolution cannot be obtained for the electronic light receiving element.

  In recent years, the size of the light receiving element has been rapidly reduced, so that the entire lens system has been reduced in size, and the accuracy of individual components has become proportionally stricter. As a result, in order to keep the mass production quality constant, it is required to increase the accuracy of each part and ensure the assembly accuracy. As the number of components increases, manufacturing becomes difficult.

  The present invention facilitates manufacturing by configuring the lens system with a two-lens configuration, solves the above-mentioned problem by optimally arranging the diffractive optical surface that exhibits the chromatic dispersion function, and has an F-number consisting of two lenses. The present invention provides a high-performance large-aperture ratio imaging lens of 3 or less.

2 is a configuration diagram of an imaging lens according to Example 1. FIG. FIG. 4 is an aberration diagram of the imaging lens according to Example 1. FIG. 6 is a diagram illustrating diffraction efficiency when a diffractive optical surface is arranged on the first surface of the second lens relating to Example 1. 6 is a diagram illustrating diffraction efficiency when a diffractive optical surface is disposed on the second surface of the first lens relating to Example 1. FIG. 6 is an aberration diagram of the imaging lens according to Example 2. FIG. 10 is an aberration diagram of the imaging lens according to Example 3. FIG. FIG. 6 is an aberration diagram of an imaging lens according to a comparative example.

  Embodiments of the present invention will be described below with reference to the drawings. First, Example 1 will be described in detail as a representative example of the present invention. In the following embodiments, duplicate descriptions are omitted.

  FIG. 1 shows a configuration diagram of an imaging lens according to Embodiment 1 of the present invention.

  As shown in FIG. 1, the imaging lens according to the first exemplary embodiment includes, in order from the object side, a lens system including a first lens, a diaphragm R4, and a second lens. The first surface of the first lens is a convex surface, and the second surface is a second surface. The surface is configured as a concave surface, the first surface of the second lens is configured as a concave surface, the second surface is configured as a convex surface, each surface of the lens is configured as an aspheric surface, and a color dispersion function is provided on the first surface of the second lens. A diffractive optical surface is disposed. The lens uses a cycloolefin-based or polycarbonate-based plastic as a material in order to easily form an aspherical surface and obtain an optimum dispersion.

  In the present embodiment, virtual surfaces R3 and R5 corresponding to a lens frame for controlling the amount of light are arranged between the first lens and the second lens, and a diaphragm R4 is arranged between them. A cover glass composed of parallel planes R8 and R9 is disposed between the second lens second surface R7 and the imaging surface S. Further, in FIG. 1, d1, d2,..., D9 represent surface intervals, and X represents an optical axis.

  The lens surfaces R1, R2, R6, and R7 are formed of a basic aspherical surface expressed by Equation 1, and the lens surface R6 has an optical path difference function expressed by Equation 2 on the basic aspherical surface. Diffractive optical surfaces are formed.

Further, the imaging lens of Example 1 is configured to satisfy the following conditional expression.
(1) −0.11 <(L / R) / (V1 / V2) <− 0.07
(2) 0.09 <De / f ≦ 0.1 55
However,
R: Average radius of curvature of the first surface of the second lens The average radius of curvature represents the radius of curvature of the spherical surface obtained by approximating the displacement of the aspherical surface in the optical axis direction by the least square method.
L: Distance from the diaphragm to the first surface of the second lens V1: Abbe number of e-line of the first lens material V2: Abbe number of e-line of the second lens material De: First lens first surface and first lens first The distance f in the optical axis direction at the effective diameters of the two surfaces is the focal length of the entire system. The details of the present invention will be described below.

  First, the basic configuration of the imaging lens is, in order from the object side, a first lens whose first surface is a convex surface and a second surface is a concave surface, an aperture, a second lens whose first surface is a concave surface, and whose second surface is a convex surface. Each surface of the lens is aspherical, and a diffractive optical surface having a color dispersion function is disposed on the first surface of the second lens.

  In order to correct aberrations well over a wide field angle, it is necessary to correct field curvature well, and a surface with negative power is placed close to the stop for the purpose of field curvature correction. By reducing the Petzval sum, the surfaces having negative power are arranged on both sides of the diaphragm, and the first surface of the first lens and the second surface of the second lens are convex surfaces. It becomes possible to keep the distortion favorable by sandwiching the structure close to the concentric.

  However, in the two-lens configuration, there is no correction means for axial chromatic aberration, and color blurring occurs, resulting in degradation of resolution performance in a region with a high spatial frequency. The present invention solves this problem by providing a diffractive optical surface having a chromatic dispersion function.

  The diffractive optical surface is constituted by a relief that generates an optical path difference defined by an optical path difference function. Usually, the dispersion of glass is 25 to 80 in terms of the Abbe number of the e-line, and about −3. 5 and the reverse sign have the property of showing a dispersion that is approximately one digit larger. In addition, when correcting chromatic aberration with a general optical material such as glass, it is necessary to configure a lens by combining at least two types of materials having different dispersions. It is possible to effectively correct chromatic aberration by exhibiting a chromatic dispersion function. Table 8 described below compares the axial chromatic aberration of a lens having a structure similar to the present invention having no diffractive optical surface as Example 1 of the present invention and a comparative example, but from the g line to the C line of the comparative example. The spread of axial chromatic aberration at the wavelength of the spectral line is 0.025 mm, whereas the spread of the first embodiment is 0.006 mm, which is about 1/4, and the chromatic aberration is corrected extremely well. I understand.

  On the other hand, diffractive optical surfaces have extremely high diffraction efficiency with respect to the design reference wavelength, but there are drawbacks in that the diffraction efficiency decreases when the design reference wavelength is deviated or the light incident angle is increased. A design that is as small as possible is required.

  In a lens system composed of general glass or the like that does not use a diffractive optical surface, an optical element such as a chromatic aberration correction surface used for correcting chromatic aberration is generally arranged at a position relatively close to the stop. Similarly, by arranging the diffractive optical surface at a position close to the stop, it is possible to correct chromatic aberration both on-axis and off-axis. However, as in the first embodiment, when the angle of view increases with a relatively large aperture ratio such as F / 2.8, the incident angle of the light increases as the field angle increases, resulting in a sharp decrease in diffraction efficiency and a significant decrease in the contrast at the periphery. I will let you.

  The present invention solves this problem by arranging a diffractive optical surface on a concave surface appropriately separated from the stop in order to cope with a large angle of view. In order to solve the above-described problem that the diffraction efficiency is lowered due to the change in the incident angle of the light beam, it is desirable to arrange the diffractive optical surface on the surface close to the concentric with the aperture interposed therebetween. A diffractive optical surface is disposed on the first surface of the second lens.

  The imaging lens of the present invention mainly assumes an imaging lens unit for a camera mounted on a mobile phone, and the aperture position is set as much as possible in order to simultaneously satisfy the restrictions on the light incident angle required by the imaging device and the compactness. It is necessary to move to the object side. However, if the stop is placed in front of the first lens, the upper ray is greatly bounced up by the first lens second surface, making it difficult to correct aberrations. As a result, it is disposed near the second lens second surface. Become. For the same reason, the second lens second surface controls the light emission angle in accordance with the characteristics of the image sensor, so that the thickness of the second lens is increased. On the other hand, since the diffractive optical element has a very large dispersion, if it is arranged far from the stop, the chromatic aberration of magnification becomes too large and the correction becomes excessive.

  For the reasons described above, the present inventors have concluded that the position of the diffractive optical surface in the present invention is optimal for the first surface of the second lens immediately after the stop. However, since the chromatic aberration of magnification is affected by the balance between the diffractive optical surface being slightly away from the stop and the correction of ray aberrations other than chromatic aberration, the dispersion of the first lens and the second lens is appropriately taken. It is possible to optimally correct axial chromatic aberration and lateral chromatic aberration.

  Conditional expression (1) represents the positional relationship between the stop and the diffractive optical surface, and has a certain relationship in order to satisfactorily correct chromatic aberration and other light aberrations and maintain good diffraction efficiency. The distance between the stop and the diffractive optical surface differs depending on the dispersion of the material of the second lens. From the viewpoint of correcting the chromatic aberration of magnification, the ratio V1 / V2 of the Abbe numbers of the two lens materials and the average curvature of the diffractive optical surface The optimum value is determined by the balance of the three elements of the radius and the distance from the stop to the diffractive optical surface, and the value of (L / R) / (V1 / V2) is preferably approximately constant. Conditional expression (1) relates to this balance. When the value of (L / R) / (V1 / V2) increases, the lens surface relatively approaches the stop or the average radius of curvature of the surface increases, and the magnification is increased. Since chromatic aberration is undercorrected, it is necessary to optimally correct the chromatic aberration of magnification by reducing the value of (V1 / V2). Further, when the value of (L / R) / (V1 / V2) is reduced, the reverse phenomenon occurs, and the chromatic aberration of magnification must be optimally corrected by increasing the value of (V1 / V2). . According to each example described later, even though (L / R) and (V1 / V2) have different values, the ratio (L / R) / (V1 / V2) is about -0.09. The value is almost constant. Outside the range of the conditional expression, it becomes difficult to perform axial chromatic aberration correction and lateral chromatic aberration correction at the same time, adversely affecting other aberration corrections, and the desired performance cannot be realized.

  The average radius of curvature represents the radius of curvature of the spherical surface derived from the aspherical shape given by the aspherical shape formula by the least square method. Using an average radius of curvature means that it is difficult to accurately represent the shape of the periphery of the refractive surface with a paraxial radius of curvature on an aspherical surface, and the influence of a diffractive optical surface away from the aperture that affects chromatic aberration at large angles of view. This is useful for more accurately expressing the correlation between and the conditional expression.

  Conditional expression (2) is a condition relating to the first lens edge thickness, and is a restriction peculiar to a particularly small imaging lens such as an imaging lens for a mobile phone, and represents a relationship between the restriction on the mechanism and the performance. The main object of the present invention is a very small imaging lens having an aperture ratio of about F2.8 and a maximum half angle of view of about ω = 33 °. The lens configuration as in the present invention is generally advantageous for correcting astigmatism, coma aberration, distortion, and the like because the refractive surface is arranged in a shape close to concentric across the diaphragm, but is required by the image sensor. Since the exit ray angle is adjusted to a constant condition in accordance with the ray incident angle, the asymmetry of the power arrangement with respect to the stop is increased, and the aberration correction balance is lost. In addition, since the optical diffractive surface is arranged asymmetrically with respect to the stop, the stop position is arranged at a position farther from the image plane. As a result, it is preferable to reduce the thickness of the first lens in order to satisfactorily correct off-axis aberrations. However, in the case of the above-mentioned purpose of use, the edge thickness is relatively thin due to a large aperture ratio and a wide angle of view. Therefore, the lens thickness is limited due to the mechanism.

  Further, increasing the edge thickness of the first lens is not only a mechanism limitation due to the influence of a large aperture ratio and a wide angle of view, but also a method within the effective diameter of the first lens first surface and the second lens second surface. This increases the line angle, and in particular causes a problem in lens molding. Therefore, in the conditional expression (2), if De is below the lower limit, the lens cannot be held by the mechanism, and if De exceeds the upper limit, the performance is lowered and the normal angle of the lens surface is increased, causing a problem in lens molding processing. Result.

Although two lenses of a lens having a diffractive optical surface is already known from JP-Open 2004-191844 Patent Publication and Laid-Open 2008-89949 Patent Publication, both the diffractive optical surface to the basic structure lens having no diffractive optical surface In this example, the arrangement position and shape of the diffractive optical surface are not clearly defined. Due to the nature of diffractive optical surfaces having large dispersion, it is difficult to achieve effective effects unless the lens configuration, arrangement position, and shape thereof are optimized. Although the degree of freedom regarding the arrangement of the diffractive optical surface is large under specific conditions such as monochromatic light and a small angle of view, in order to make the diffractive optical surface act effectively in an imaging lens with white light and a wide angle of view as in this embodiment For this, the form of the present invention is optimal. Further, lenses related to an imaging lens having white light and a wide angle of view, which are the object of the present invention, are known from Japanese Patent Laid-Open No. 10-161020, etc., but all have a small aperture ratio of about F number: 8 as in the present invention. Since the performance and mechanical conditions required for a lens with a large aperture ratio are greatly different, there is no restriction as in conditional expression (2).

  The position of the diffractive optical surface of the present invention is arranged at the position of the surface interval L immediately after the stop. This is to make the change in the incident angle of the off-axis light beam as small as possible. In terms of light aberration, design is possible even if a diffractive optical surface is arranged on the second surface of the first lens, but the change in the incident angle of the principal ray on the diffractive optical surface becomes large and the diffraction efficiency decreases. There is. FIG. 3 and FIG. 4 show the diffraction efficiency of the first lens second surface and the second lens first surface compared with the principal ray incident angle in the first embodiment. It can be seen that the diffraction efficiency decreases when the angle of view becomes larger than this, but it is understood that the decrease in diffraction efficiency is smaller when the diffractive optical surface is arranged on the first surface of the second lens.

  In Example 1, R = −0.354, L / R = −0.168, V1 / V2 = 1.873, (L / R) / (V1 / V2) = − 0.090, De = 0. 107, which is in the range defined by conditional expression (1) and conditional expression (2).

  Table 1 shows the radius of curvature R (mm) of each lens of Example 1, the surface spacing d (mm) on the optical axis of each surface, the refractive index n and the Abbe number v at the e-line of the lens material. Further, the lower part of the table shows the focal length f, F number, half angle of view ω, and values corresponding to conditional expression (1) and conditional expression (2) of the entire system of the first embodiment. In Table 1 and the following tables, numbers corresponding to the respective symbols are sequentially increased from the object side.

  Table 2 shows the values of the constants of the aspheric coefficient in Example 1 and the optical path difference function on the diffractive optical surface.

  The imaging lens according to the second embodiment has substantially the same lens configuration as that of the first embodiment, but the Abbe numbers of the first lens and the second lens are equal, and the absolute value of the average radius of curvature R increases, so As the distance L to the first surface of the two lenses becomes smaller, the value of L / R becomes relatively smaller. As a result, in Example 2, R = −0.546, L / R = −0.092, V1 / V2 = 1.0, (L / R) / (V1 / V2) = − 0.092, De. = 0.139, which is in the range defined by conditional expression (1) and conditional expression (2).

  Table 3 shows the curvature radius R (mm) of each lens of Example 2, the surface distance d (mm) on the optical axis of each surface, the refractive index n and the Abbe number v at the e-line of the lens material. Further, the lower part of the table shows the focal length f, F number, half angle of view ω, and values corresponding to conditional expression (1) and conditional expression (2) of the entire system of the first embodiment. In Table 1 and the following tables, the numbers corresponding to the respective symbols are sequentially increased from the object side.

  Table 4 shows the values of the constants of the aspheric coefficient and the optical path difference function on the diffractive optical surface in Example 2.

  The imaging lens according to the third embodiment has substantially the same lens configuration as that of the first embodiment, but the distance L from the stop to the optical diffraction surface is larger than that of the first embodiment, and the edge thickness De of the first lens is larger. This makes the mechanical structure and lens processing easy. As a result, in Example 3, R = −0.424, L / R = −0.165, V1 / V2 = 1.877, (L / R) / (V1 / V2) = − 0.088, De. = 0.155, which is within the range defined by conditional expression (1) and conditional expression (2).

  Table 5 shows the curvature radius R (mm) of each lens of Example 3, the surface distance d (mm) on the optical axis of each surface, the refractive index n and the Abbe number v at the e-line of the lens material. Further, the lower part of the table shows the focal length f, F number, half angle of view ω, and values corresponding to conditional expression (1) and conditional expression (2) of the entire system of the first embodiment. In Table 1 and the following tables, numbers corresponding to the respective symbols are sequentially increased from the object side.

Table 6 shows the values of the constants of the aspheric coefficient and the optical path difference function on the diffractive optical surface in Example 3.

  The effect of the optical diffractive surface will be described based on a comparative example. A lens material that reduces chromatic aberration as much as possible without a diffractive optical surface is used, and the lens configuration is similar to that of the example. Although it does not exist, it becomes a form similar to Example 2.

  Table 7 shows the radius of curvature R (mm) of each lens of the comparative example, the surface distance d (mm) on the optical axis of each surface, the refractive index n and the Abbe number v at the e-line of the lens material. The lower part of the table shows values corresponding to the focal length f, F number, and half angle of view ω of the entire system of the comparative example.

  Table 8 shows the values of the constants of the aspheric coefficient in the comparative example and the optical path difference function in the diffractive optical surface.

  Table 9 shows the amount of axial chromatic aberration in Examples 1, 2, and 3 and the comparative example. Note that the amount of chromatic aberration represents the width of chromatic aberration in the range of wavelengths from 435.8 nm to 656.3 nm as “spread” with reference to the wavelength of e-line of 546.1 nm. The numerical value is in mm.

  As described above, according to the imaging lens of the present invention, it is possible to correct chromatic aberration very well with two components by arranging the diffractive optical surface at an optimum position, and the other Aberrations can be corrected to a practical level, and high-efficiency production can be provided, and a low-cost lens that has a large aperture and sufficient image quality and is compatible with high image quality can be provided. This is particularly effective in the field of imaging lenses for mobile phones, where there is a strong demand for downsizing and cost reduction.

R1 Aspherical surface R1 of the first surface of the first lens R2 Aspherical surfaces R3 and R5 of the first surface of the second lens R5 A virtual surface corresponding to a lens frame for controlling the light beam R4 Aperture R6 The diffractive optical surface of the first surface of the second lens is Aspherical surface R7 formed Aspherical surfaces R8 and R9 of the second surface of the second lens Curvature radii d1 to d8 of the cover glass Axial surface distance X Optical axis S Imaging surface

Claims (2)

In order from the object side, an imaging lens including a lens system including a first lens made of a plastic material , an aperture, and a second lens made of a plastic material ,
The first surface of the first lens is a convex surface, the second surface is a concave surface, the first surface of the second lens is a concave surface, the second surface is a convex surface, and each surface of the lens is an aspheric surface, and diffractive optical surface der the first face of the second lens to exhibit color dispersion function Ri, F value consisting of two configurations, wherein a satisfies child the following condition (2) is 3 or less high performance Large aperture ratio imaging lens.
However,
(2) 0.09 <De / f ≦ 0.155
De: distance in the optical axis direction at each effective diameter of the first surface of the first lens and the second surface of the first lens
f: Focal length of the entire system The object side surface of the first lens and the second lens is the first surface, and the image side surface is the second surface. 2. The two-element configuration according to claim 1, wherein each optical element is configured to satisfy the following conditional expression in a coordinate system in which the object direction is negative from the reference point and the image plane direction is positive. A high-performance large-aperture ratio imaging lens having an F value of 3 or less.
(1) −0.11 <(L / R) / (V1 / V2) <− 0.07
However,
R: Average radius of curvature of the first surface of the second lens Note that the average radius of curvature represents the radius of curvature of the spherical surface obtained by approximating the displacement of the aspherical surface in the optical axis direction by the least square method. L: First surface of the second lens from the stop Distance V1: Abbe number of e-line of first lens material V2: Abbe number of e-line of second lens material

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