JP2005257806A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To excellently form an optical image on a compact many-pixel imaging device. <P>SOLUTION: The imaging lens is constituted of an optical diaphragm 11, a 1st lens L1 made of glass, whose both surfaces are made spherical and which has positive refractive power, a 2nd lens L2 made of glass, whose both surfaces are made aspherical and which has negative refractive power, and a 3rd lens L3 made of glass, whose both surfaces are made aspherical and which has positive refractive power in order from an object side. When it is assumed that the refractive index of the 1st lens L1 is Nd<SB>1</SB>and the Abbe number thereof is νd<SB>1</SB>, the refractive index of the 2nd lens L2 is Nd<SB>2</SB>and the Abbe number thereof is νd<SB>2</SB>, and the refractive index of the 3rd lens L3 is Nd<SB>3</SB>and the Abbe number thereof is νd<SB>3</SB>, the imaging lens satisfies (1) Nd<SB>1</SB>>1.45, νd<SB>1</SB>>70, (2) 1.61>Nd<SB>2</SB>>1.57, 40>νd<SB>2</SB>>28, (3) 1.55>Nd<SB>3</SB>>1.45, νd<SB>3</SB>>70. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an imaging lens used in an imaging apparatus such as a digital still camera or a digital video camera, and more particularly to an imaging lens having a three-group three-lens configuration (triplet type).

  Image pickup devices such as CCD (Charge Coupled Device) and CMOS (Complementary Mental Oxide Semiconductor) are miniaturized, and the number of pixels is increased by making the pixel pitch of the image pickup device finer, thereby improving the image quality.

  As described above, an imaging lens that forms an optical image on a small imaging device having a large number of pixels is required to have high imaging performance, and further to be downsized and reduced in cost. Yes. However, if the imaging lens is reduced in size and cost, the imaging performance cannot be obtained. Conversely, if the imaging performance is improved, there is a problem that the imaging lens cannot be reduced in size and cost.

  For example, a method of forming an imaging lens (photographic lens) in a three-group three-lens configuration (triplet type) and molding at least two lenses with a synthetic resin has been devised and implemented (see, for example, Patent Document 1). ).

  However, when an imaging lens having a three-group, three-element configuration is configured with such a low-cost synthetic resin lens, an imaging device with a higher number of pixels, for example, an imaging device with 2 million pixels is accurately used. There is a problem that the image cannot be formed.

Japanese Patent Laid-Open No. 9-133860

  Therefore, the present invention has been devised to solve the above-described problems, and a high-performance imaging lens having a high imaging performance corresponding to a small-sized and high-pixel imaging device is relatively low. It is intended to be offered at a price.

In order to achieve the above-described object, an imaging lens according to the present invention includes, in order from the object side, an optical aperture, a first lens having a positive refractive power having a spherical surface on both sides, and glass. And a second lens having negative refractive power whose both surfaces are aspheric and a third lens having positive refractive power which is made of glass and whose both surfaces are aspheric. The refractive index of the second lens is Nd ₁ , the Abbe number is νd ₁ , the refractive index of the second lens is Nd ₂ , the Abbe number is νd ₂ , the refractive index of the third lens is Nd ₃ , and the Abbe number is When νd ₃ is satisfied, the following conditions (1) to (3) are satisfied.

(1) Nd ₁ > 1.45, νd ₁ > 70
(2) 1.61> Nd ₂ > 1.57, 40> νd ₂ > 28
(3) 1.55> Nd ₃ > 1.45, νd ₃ > 70

  The imaging lens of the present invention has a three-group three-lens configuration (triplet type), and has a positive refractive power in which both surfaces are made spherical by using an optical aperture and glass with low dispersibility in order from the object side. A first lens having a negative refractive power whose both surfaces are aspherical using a highly dispersive glass, and a positive lens whose both surfaces are aspherical using a glass having low dispersibility. And a third lens having a refractive power of 5 mm.

  Such an imaging lens has a low dispersibility because the second lens and the third lens can be formed into an aspheric lens by processing molding by using a relatively inexpensive glass excellent in moldability. Using glass, the first lens can be a spherical lens.

  Therefore, in the imaging lens, the first lens can be a high-performance lens that greatly suppresses chromatic aberration and has high imaging performance, and further, the second lens and the third lens that are aspherical. Therefore, it is possible to provide a high-performance imaging lens with high imaging performance corresponding to an imaging device having a small size and a high number of pixels.

  Further, since the first lens is a spherical lens, and the second lens and the third lens can be easily made an aspherical lens by using glass having excellent moldability, the low cost. In addition, mass productivity can be improved.

  Furthermore, in the imaging lens of the present invention, an optical aperture is disposed between the object side and the first lens, so that the lens thickness of the first lens having imaging performance is reduced. Since the volume of the lens can be reduced, it is possible to further reduce the size and cost of the imaging lens even when the performance of the first lens is increased by using expensive glass. .

  Hereinafter, an imaging lens to which the present invention is applied will be described in detail with reference to the drawings.

As shown in FIG. 1, an imaging lens 10 to which the present invention is applied is used as, for example, an imaging lens of a digital still camera. In order from the object side, an aperture 11, a first lens L <b> 1, 2 of lens L2, the third lens L3, and a cover glass FG are arranged in a state of being matched optical axis Z ₀ of each other. Thus, the lens configuration of the imaging lens 10 is a three-group three-lens configuration (triplet type).

  The imaging lens 10 has an imaging element 12 such as a CCD (Charge Coupled Device) or a CMOS (Complementary Mental Oxide Semiconductor) in which the light beam that has passed from the object side is finally disposed on the imaging surface on the image plane side. The image is formed on the imaging surface 12a.

  The diaphragm 11 adjusts the amount of incident light from the object side that enters the first lens L1 at the subsequent stage. As shown in FIG. 1, the surface of the diaphragm 11 is a surface r1.

  The first lens L1 is made of optical glass with little change in optical performance with respect to environmental changes such as temperature and humidity. The first lens L1 is a glass spherical lens having a positive refractive power (power) for providing an imaging function, and having the surfaces r2 and r3 of the first lens L1 being spherical. The first lens L1 is manufactured by machining and polishing using, for example, FCD1 (manufactured by HOYA) as optical glass.

  The second lens L2 and the third lens L3 are made of optical glass that can be processed and molded, such as molding, because of its low melting point. The second lens L2 and the third lens L3 are glass aspherical lenses in which the surfaces r4 and r5 and the surfaces r6 and r7 are aspherical.

  In order to provide an aberration correction function, the second lens L2 has a negative refractive power to diverge the light beam, while the third lens L3 has a positive refractive power so as to converge the light beam. Has been made. Further, the second lens L2 and the third lens L3 have an image plane correction function in addition to the aberration correction function described above.

  As the second lens L2, for example, FF5 (manufactured by HOYA), S-FTM16 (manufactured by OHARA) or the like is used as the optical glass, and the third lens is an optical glass dedicated to the mold as the optical glass. Supervidron PG325 (manufactured by SUMITA) or the like is used, and each is processed by molding.

  The cover glass FG is a glass for protecting the imaging surface 12a of the imaging element 12 described above, and is made of, for example, a filter cover glass having an infrared cut function. As shown in FIG. 1, the surface of the cover lens FG is defined as surfaces r8 and r9.

  As described above, the image sensor 12 is an image sensor such as a CCD or a CMOS, and is small and has a high pixel count, for example, a pixel count of 2 million pixels or more.

By the way, in this imaging lens 10, the refractive index of the first lens L1 is Nd ₁ , the Abbe number is νd ₁ , the refractive index of the second lens L2 is Nd ₂ , the Abbe number is νd _2, and the third lens When the refractive index of L3 is Nd ₃ and the Abbe number is νd ₃ , the following conditions (1) to (3) are satisfied.

(1) Nd ₁ > 1.45, νd ₁ > 70
(2) 1.61> Nd ₂ > 1.57, 40> νd ₂ > 28
(3) 1.55> Nd ₃ > 1.45, νd ₃ > 70

  Such an imaging lens 10 uses the relatively low-priced optical glass having excellent moldability as described above, so that the second lens L2 and the third lens L3 are aspherical lenses by processing molding. Therefore, the first lens L1 can be a spherical lens by using optical glass with low dispersibility.

  Therefore, in the imaging lens 10, the first lens L1 can be a high-performance lens that greatly suppresses chromatic aberration and has high imaging performance, and further has an aspherical second lens L2 and second lens L2. Thus, a good aberration correction function can be provided by the third lens L3, so that it is possible to provide a high-performance imaging lens that is compact and has high imaging performance corresponding to an imaging device having a large number of pixels.

  In addition, the first lens L1 is a spherical lens, and the second lens L2 and the third lens L3 are aspherical lenses that are easily processed and molded by using optical glass having excellent moldability. Therefore, low cost and mass productivity can be improved.

  Furthermore, the imaging lens 10 of the present invention has a diaphragm 11 disposed between the object side and the first lens L1, thereby reducing the thickness of the first lens L1 having imaging performance, that is, the first lens L1. Since the volume of the first lens L1 can be reduced, the imaging lens 10 can be further downsized and reduced in price even when the first lens L1 is made of high-performance optical glass. can do.

  Next, specific examples of the imaging lens 10 to which the present invention is applied will be described. In Examples 1 to 3 described below, specific materials and numerical values are given, but the present invention is not necessarily limited to the following examples.

  First, before describing the embodiment, Table 1 shows required specifications that are design targets of the imaging lens 10. As the imaging element 12 on which the imaging lens 10 forms an optical image, a CCD having a size of 1 / 2.7 inches and a number of pixels of 2 million pixels is used.

  In Examples 1 to 3 shown below, the imaging lens 10 is designed to satisfy the required specifications shown in Table 1, and various aberration characteristics and MTF characteristics are measured.

  The parameters used in each embodiment will be described. In each embodiment, the radius of curvature of the i-th surface (including the surface of the diaphragm 11, each lens surface, and the surface of the cover glass FG) counted from the object side (the paraxial radius of curvature for a non-curved surface) is Ri ( i = 1 to 9). Ri = ∞ represents a plane.

Further, the axial distance between the i-th surface and the (i + 1) -th surface on the optical axis Z ₀ is di (i = 1 to 10), and the j-th lens is counted from the object side. The refractive index and Abbe number of the material are represented by Nd _j and νd _j (j = 1 to 3), respectively.

  Also, the axial plane distance d0 is the distance from the object that is the subject to the stop 11, IMG is the imaging plane 12a of the image sensor 12, and the axial plane distance d10 is the distance from the plane r9 to the IMG, f Indicates a focal length, FNo indicates an F number (F number), and ω indicates a half angle of view.

  For an aspherical surface, X is the coordinate in the optical axis direction, h is the coordinate in the optical axis orthogonal direction, Ri is the paraxial radius of curvature, K is the conic constant, and A, B, C, D, E,. As a spherical coefficient, a shape is specified using a well-known aspherical expression shown as an expression (A) below.

X = (h ² / Ri) / [1 + √ {1− (K + 1) (h / Ri) ² }] + A · h ⁴ + B · h ⁶ + C · h ⁸ + D · h ¹⁰ + E · h ^12.・ ・ (A)

{Example 1}
FIG. 2 shows the imaging lens 10 according to the first embodiment. In FIG. 2, as the incident light beam incident on the imaging lens 10, the center position (0.00 mm) of the imaging surface 12 a of the imaging element 12, the position of 30% image height (1.05 mm), and the image height 50%. Incident light beam that forms an image at a position (1.75 mm), an image height of 70% (2.45 mm), an image height of 90% (3.15 mm), and an image height of 100% (3.50 mm). Is shown. The design data of the imaging lens 10 shown in FIG. 2 as Example 1 is as shown in Table 2 below.

  As shown in Table 2, the focal length f of the entire system in the imaging lens 10 shown as Example 1 is 5.75 mm, the F number and FNo are 3.5, and the half angle of view ω is 30. 979 °. As shown in Table 2, the imaging lens 10 shown as Example 1 is designed so as to satisfy all the conditions (1) to (3) described above.

  In addition, it is necessary when calculating the aspherical shapes of the surfaces r4 and r5 of the second lens L2, which are glass aspherical lenses, and the surfaces r6 and r7 of the third lens L3, using the above-described equation (A). Table 3 shows the conic constant K and A, B, C, D, and E, which are fourth-order, sixth-order, eighth-order, tenth-order, and twelfth-order aspherical coefficients.

  FIGS. 3 and 4 show various aberrations due to the imaging lens 10 of Example 1 configured as described above.

  FIG. 3 shows the incident light rays having wavelengths λ1 (0.436 μm), λ2 (0.486 μm), λ3 (0.546 μm), λ4 (0.588 μm), and λ5 (0.656 μm) incident on the imaging lens 10. The aberrations of the sagittal image plane (indicated by the solid line S) and the tangential image plane (indicated by the broken line T) are shown. As shown in FIG. 3, the aberration due to the imaging lens 10 is uniform in each wavelength region, and the sagittal image plane and the tangential image plane are located at a position where the image height is 80% to 90%. Astigmatism is widened and the astigmatism is large, but it can be seen that the astigmatism is corrected again at a position near 100% of the image height.

  FIG. 4 shows distortions (distortions) of the incident light beams having wavelengths λ1, λ2, λ3, λ4, and λ5 incident on the imaging lens 10. As shown in FIG. 4, the distortion aberration of each wavelength is within 1%, and generally satisfies 1 to 2%, which is generally regarded as a distortion allowable amount of a photographic lens. A sufficient function as an imaging lens for a digital still camera can be achieved.

  As described above, as can be seen from the aberration diagrams shown in FIGS. 3 and 4, the imaging lens 10 of Example 1 has a good aberration correction function.

  Next, FIGS. 5 and 6 show MTF characteristics measured using the imaging lens 10 of Example 1. FIG.

  FIG. 5 shows the sagittal image plane (solid line) at the center position of the image plane 12a of the image sensor 12, the position of 30% of the image height, the position of 50% of the image height, the position of 70% of the image height, and the position of 100% of the image height. FIG. 4 is a diagram showing MTF characteristics of a tangential image plane (indicated by a broken line T) and a tangential image plane. In FIG. 5, the horizontal axis represents the spatial frequency and the vertical axis represents the MTF value. Note that at the center position, the sagittal image plane and the tangential image plane are rotationally symmetric, so the MTF characteristics shown in FIG. 5 are the same.

  FIG. 6 shows the MTF characteristics showing the change of the MTF value with respect to the image height at a spatial frequency fixed at 60 lines / mm and 120 lines / mm. FIG. 6 shows a sagittal image plane (shown by a solid line S1) and a tangential image plane when the horizontal axis represents the image height, the vertical axis represents the MTF value, and the spatial frequency is 60 lines / mm. The MTF value (indicated by the broken line T1) and the MTF values of the sagittal image plane (indicated by the solid line S2) and the tangential image plane (indicated by the broken line T2) when the spatial frequency is 120 lines / mm are shown. Yes.

  As shown in FIG. 5, in the imaging lens 10 of Example 1, the MTF value at a spatial frequency of 160 lines / mm at the center position is about 0.4 (40%), and the MTF value at a spatial frequency of 80 lines / mm is 0. 7 (70%), which is a design value shown in Table 1, which greatly exceeds 30% at a spatial frequency of 160 lines / mm and 50% at a spatial frequency of 80 lines / mm.

  As shown in FIGS. 5 and 6, the MTF value at a spatial frequency of 120 lines / mm at a position where the image height is 70% is 0.25 (25%) or more in the sagittal image plane and the tangential image plane. The MTF value at a frequency of 60 lines / mm is 0.60 (60%) or more on the sagittal image plane and the tangential image plane, and the design values shown in Table 1 are 25% at a spatial frequency of 120 lines / mm. It can be seen that 40% is greatly exceeded at 60 lines / mm.

  Furthermore, as shown in FIG. 6, there is no problem such that the MTF value suddenly decreases between the center position and the image height of 70%. Rather, the spatial frequency is 60 lines / mm, 120 lines / In both cases, the difference between the MTF values of the sagittal image plane and the tangential image plane once increases, but it can be seen that the correction functions again in such a direction that the MTF values coincide.

  Therefore, the resolving power of the imaging lens 10 of Example 1 sufficiently satisfies the design requirement specifications shown in Table 1, and uses the imaging element 12 having a size of 1 / 2.7 inches and a number of pixels of 2 million pixels. Even in such a case, a clear optical image can be formed on the imaging surface 12a.

{Example 2}
FIG. 7 shows an imaging lens 10 according to the second embodiment. In FIG. 7, as the incident light beam incident on the imaging lens 10, the center position (0.00 mm) of the image pickup surface 12 a of the image pickup device 12, the position of 30% image height (1.05 mm), and the image height 50%. Incident light beam that forms an image at a position (1.75 mm), an image height of 70% (2.45 mm), an image height of 90% (3.15 mm), and an image height of 100% (3.50 mm). Is shown. The design data of the imaging lens 10 shown in FIG. 7 as Example 2 is as shown in Table 4 below.

  As shown in Table 4, the focal length f of the entire system in the imaging lens 10 shown as Example 2 is 5.78 mm, the F number and FNo are 3.5, and the half angle of view ω is 31. It is 160 °. Further, as shown in Table 4, the imaging lens 10 shown as Example 2 is designed so as to satisfy all the conditions (1) to (3) described above.

  In addition, it is necessary when calculating the aspherical shapes of the surfaces r4 and r5 of the second lens L2, which are glass aspherical lenses, and the surfaces r6 and r7 of the third lens L3, using the above-described equation (A). Table 5 shows the conic constant K and A, B, C, D, and E, which are fourth-order, sixth-order, eighth-order, tenth-order, and twelfth-order aspherical coefficients.

  FIGS. 8 and 9 show various aberrations due to the imaging lens 10 of Example 2 configured as described above.

  FIG. 8 shows the incident light rays having wavelengths λ1 (0.436 μm), λ2 (0.486 μm), λ3 (0.546 μm), λ4 (0.588 μm), and λ5 (0.656 μm) incident on the imaging lens 10. The aberrations of the sagittal image plane (indicated by the solid line S) and the tangential image plane (indicated by the broken line T) are shown. As shown in FIG. 8, the aberration due to the imaging lens 10 is uniform in each wavelength region, and the sagittal image plane and the tangential image plane are located at a position where the image height is 80% to 90%. Astigmatism is widened and the astigmatism is large, but it can be seen that the astigmatism is corrected again at a position near 100% of the image height.

  FIG. 9 shows the distortion (distortion) of each incident ray of wavelengths λ1, λ2, λ3, λ4, and λ5 that has entered the imaging lens 10. As shown in FIG. 9, the distortion aberration of each wavelength is within 1% and sufficiently satisfies 1 to 2%, which is generally regarded as a distortion allowable amount of a photographic lens. A sufficient function as an imaging lens for a digital still camera can be achieved.

  Thus, as can be seen from the aberration diagrams shown in FIGS. 8 and 9, the imaging lens 10 of Example 2 has a good aberration correction function.

  Next, FIGS. 10 and 11 show MTF characteristics measured using the imaging lens 10 of Example 2. FIG.

  10 shows a sagittal image plane (solid line) at the center position of the image plane 12a of the image sensor 12, the position of 30% of the image height, the position of 50% of the image height, the position of 70% of the image height, and the position of 100% of the image height. FIG. 4 is a diagram showing MTF characteristics of a tangential image plane (indicated by a broken line T) and a tangential image plane. In FIG. 10, the horizontal axis represents the spatial frequency and the vertical axis represents the MTF value. Note that at the center position, the sagittal image plane and the tangential image plane are rotationally symmetric, so the MTF characteristics shown in FIG. 10 are the same.

  FIG. 11 shows MTF characteristics in which the spatial frequency is fixed at 60 lines / mm and 120 lines / mm, and the change in the MTF value with respect to the image height at each spatial frequency. FIG. 11 shows the sagittal image plane (indicated by the solid line S1) and the tangential image plane when the horizontal axis represents the image height, the vertical axis represents the MTF value, and the spatial frequency is 60 lines / mm. The MTF value (indicated by the broken line T1) and the MTF values of the sagittal image plane (indicated by the solid line S2) and the tangential image plane (indicated by the broken line T2) when the spatial frequency is 120 lines / mm are shown. Yes.

  As shown in FIG. 10, in the imaging lens 10 of Example 2, the MTF value at a spatial frequency of 160 lines / mm at the center position is about 0.37 (37%), and the MTF value at a spatial frequency of 80 lines / mm is 0. 7 (70%), which is a design value shown in Table 1, which greatly exceeds 30% at a spatial frequency of 160 lines / mm and 50% at a spatial frequency of 80 lines / mm.

  As shown in FIGS. 10 and 11, the MTF value at a spatial frequency of 120 lines / mm at a position where the image height is 70% is about 0.32 (32%) on the sagittal and tangential image planes. The MTF value at a frequency of 60 lines / mm is 0.60 (60%) or more on the sagittal image plane and the tangential image plane, and the design values shown in Table 1 are 25% at a spatial frequency of 120 lines / mm. It can be seen that 40% is greatly exceeded at 60 lines / mm.

  Furthermore, as shown in FIG. 11, there is no problem that the MTF value sharply decreases from the center position to the image height of 70%, rather, the spatial frequency is 60 lines / mm, 120 lines / In both cases, the difference between the MTF values of the sagittal image plane and the tangential image plane once increases, but it can be seen that the correction functions again in such a direction that the MTF values coincide.

  Therefore, the resolving power of the imaging lens 10 of Example 2 sufficiently satisfies the design requirement specifications shown in Table 1, and uses the imaging element 12 having a size of 1 / 2.7 inches and a number of pixels of 2 million pixels. Even in such a case, a clear optical image can be formed on the imaging surface 12a.

{Example 3}
FIG. 12 shows an imaging lens 10 according to the third embodiment. In FIG. 12, as the incident light beam incident on the imaging lens 10, the center position (0.00 mm) of the image pickup surface 12 a of the image pickup device 12, the position of 30% image height (1.05 mm), and the image height 50%. Incident light beam that forms an image at a position (1.75 mm), an image height of 70% (2.45 mm), an image height of 90% (3.15 mm), and an image height of 100% (3.50 mm). Is shown. The design data of the imaging lens 10 shown in FIG. 12 as Example 3 is as shown in Table 6 below.

  As shown in Table 6, the focal length f of the entire system in the imaging lens 10 shown as Example 3 is 5.78 mm, the F number and FNo are 3.5, and the half angle of view ω is 31. 066 °. Further, as shown in Table 6, the imaging lens 10 shown as Example 3 is designed so as to satisfy all the conditions (1) to (3) described above.

  When calculating the aspherical shapes of the surfaces r4 and r5 of the second lens L2, which are glass aspherical lenses, and the surfaces r6 and r7 of the third lens L3, using the above-described equation (A). Table 7 shows the necessary conic constant K and A, B, C, D, and E, which are the fourth, sixth, eighth, tenth, and twelfth aspherical coefficients.

  FIGS. 13 and 14 show various aberrations due to the imaging lens 10 of Example 3 configured as described above.

  FIG. 13 illustrates the incident light rays having wavelengths λ1 (0.436 μm), λ2 (0.486 μm), λ3 (0.546 μm), λ4 (0.588 μm), and λ5 (0.656 μm) incident on the imaging lens 10. The aberrations of the sagittal image plane (indicated by the solid line S) and the tangential image plane (indicated by the broken line T) are shown. As shown in FIG. 13, the aberration due to the imaging lens 10 is uniform in each wavelength region, and the sagittal image plane and the tangential image plane are located at a position where the image height is 80% to 90%. Astigmatism is widened and the astigmatism is large, but it can be seen that the astigmatism is corrected again at a position near 100% of the image height.

  FIG. 14 shows the distortion (distortion) of each incident ray of wavelengths λ1, λ2, λ3, λ4, and λ5 incident on the imaging lens 10. As shown in FIG. 14, the distortion aberration of each wavelength is within 1%, and generally satisfies 1 to 2%, which is generally regarded as a distortion allowable amount of a photographic lens. A sufficient function as an imaging lens for a digital still camera can be achieved.

  As can be seen from the aberration diagrams shown in FIGS. 13 and 14, the imaging lens 10 of Example 3 has a good aberration correction function.

  Next, FIGS. 15 and 16 show the MTF characteristics measured using the imaging lens 10 of Example 3. FIG.

  FIG. 15 illustrates the sagittal image plane (solid line) at the center position of the image plane 12a of the image sensor 12, the position of 30% of the image height, the position of 50% of the image height, the position of 70% of the image height, and the position of 100% of the image height. FIG. 4 is a diagram showing MTF characteristics of a tangential image plane (indicated by a broken line T) and a tangential image plane. In FIG. 15, the horizontal axis represents the spatial frequency and the vertical axis represents the MTF value. Note that at the center position, the sagittal image plane and the tangential image plane are rotationally symmetric, so the MTF characteristics shown in FIG. 15 are the same.

  FIG. 16 is an MTF characteristic showing the change of the MTF value with respect to the image height at each spatial frequency with the spatial frequency fixed at 60 lines / mm and 120 lines / mm. In FIG. 16, the horizontal axis represents the image height, the vertical axis represents the MTF value, the sagittal image plane (indicated by the solid line S1) and the tangential image plane when the spatial frequency is 60 lines / mm. The MTF value (indicated by the broken line T1) and the MTF values of the sagittal image plane (indicated by the solid line S2) and the tangential image plane (indicated by the broken line T2) when the spatial frequency is 120 lines / mm are shown. Yes.

  As shown in FIG. 15, in the imaging lens 10 of Example 3, the MTF value at a spatial frequency of 160 lines / mm at the center position is about 0.3 (30%), and the MTF value at a spatial frequency of 80 lines / mm is 0. .6 (60%), which is the design value shown in Table 1, exceeding 30% at a spatial frequency of 160 lines / mm and exceeding 50% at a spatial frequency of 80 lines / mm.

  As shown in FIGS. 15 and 16, the MTF value at a spatial frequency of 120 lines / mm at a position where the image height is 70% is 0.35 (35%) or more in the sagittal image plane and the tangential image plane. The MTF value at a frequency of 60 lines / mm is 0.45 (45%) or more on the sagittal image plane and the tangential image plane, and the design values shown in Table 1 are 25% at a spatial frequency of 120 lines / mm. It can be seen that 40% is greatly exceeded at 60 lines / mm.

  Furthermore, as shown in FIG. 16, there is no problem that the MTF value sharply decreases from the center position to the image height of 70%. Rather, the spatial frequency is 60 lines / mm, 120 lines / In both cases, the difference between the MTF values of the sagittal image plane and the tangential image plane once increases, but it can be seen that the correction functions again in such a direction that the MTF values coincide.

  Therefore, the resolving power of the imaging lens 10 of Example 3 sufficiently satisfies the design requirement specifications shown in Table 1, and uses the imaging element 12 having a size of 1 / 2.7 inches and a number of pixels of 2 million pixels. Even in such a case, a clear optical image can be formed on the imaging surface 12a.

  In the first to third embodiments described above, the case where the imaging lens 10 is applied to a digital still camera has been described. However, the present invention is not limited to this and can also be applied to a digital video camera.

It is a figure for demonstrating the structure of the imaging lens shown as the best form for implementing this invention. 4 is a diagram for explaining a configuration of an imaging lens shown as Example 1. FIG. It is an astigmatism figure of the imaging lens. It is a distortion aberration figure of the imaging lens. It is the figure which showed the MTF characteristic of the same imaging lens. It is another figure which showed the MTF characteristic of the imaging lens. 6 is a diagram for describing a configuration of an imaging lens shown as Example 2. FIG. It is an astigmatism figure of the imaging lens. It is a distortion aberration figure of the imaging lens. It is the figure which showed the MTF characteristic of the same imaging lens. It is another figure which showed the MTF characteristic of the imaging lens. 6 is a diagram for explaining a configuration of an imaging lens shown as Example 3. FIG. It is an astigmatism figure of the imaging lens. It is a distortion aberration figure of the imaging lens. It is the figure which showed the MTF characteristic of the same imaging lens. It is another figure which showed the MTF characteristic of the imaging lens.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Imaging lens, 11 Aperture, 12 Imaging element, L1 1st lens, L2 2nd lens, L3 3rd lens, FG cover glass

Claims (1)

In order from the object side, optical aperture,
A first lens having a positive refractive power made of glass and having both surfaces spherical;
A second lens having negative refractive power made of glass and having both surfaces aspherical;
A third lens made of glass and having a positive refractive power whose both surfaces are aspheric surfaces;
The refractive index of the first lens is Nd ₁ , the Abbe number is νd ₁ ,
The refractive index of the second lens is Nd ₂ , the Abbe number is νd ₂ ,
An imaging lens characterized by satisfying the following conditions (1) to (3) when the refractive index of the third lens is Nd ₃ and the Abbe number is νd ₃ :
(1) Nd ₁ > 1.45, νd ₁ > 70
(2) 1.61> Nd ₂ > 1.57, 40> νd ₂ > 28
(3) 1.55> Nd ₃ > 1.45, νd ₃ > 70

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