JP2011145315A - Imaging lens, imaging optical system and imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve image quality at a periphery part of an image, while maintaining a wide angle and long back focus in an imaging lens. <P>SOLUTION: The imaging lens includes, in the order starting from an object side, a first negative lens L1 having a meniscus shape turning a concave surface to an image side; a first cemented lens LC1 obtained by cementing a second lens L2 and a third lens L3, one of which being positive and the other of which being negative; a fourth positive lens L4 turning a plane or a surface having a larger absolute value of the radius of curvature to the object side; and a second cemented lens LC2m obtained by cementing a fifth lens L5 and a sixth lens L6, one of which is positive and the other of which is negative, and satisfies the conditional expressions, wherein (1): 0.20<f/R1<0.35, expression (2): 0.14<R2/R1≤0.20, and expression (3): Bf/f>1.8, where f: the focal length of the total system, R1: the radius of curvature of an object-side surface of the first lens, R2: the radius of curvature of an image-side surface of the first lens, and Bf: the back focus of the total system. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an imaging lens, an imaging optical system, and an imaging device. More specifically, the imaging lens has a wide angle of view and a long back focus, and imaging in which an optical path conversion member is inserted between the imaging lens and its image plane. The present invention relates to an optical system and an image pickup apparatus including the image pickup lens.

  Conventionally, small and wide-angle imaging lenses are used in the fields of surveillance cameras and endoscopes. For example, as an imaging lens for an endoscope, there are those described in Patent Documents 1 to 5 devised by the present inventors. In such a wide-angle imaging lens, it may be required to photograph a large area near the center of the image while capturing a wide range. For example, an imaging lens for an endoscope is desired to have a wide viewing angle when inserted into the body, but to view the affected area as large as possible when observing the affected area. In order to satisfy such a demand, an imaging lens that has generated large negative distortion has been devised so that the center of the image can be seen large and the object in the periphery of the image can be seen even if it is small. It was.

JP 2008-257108 A JP 2008-257109 A Japanese Patent No. 4265909 JP 63-261213 A Japanese Patent Application No. 2009-130377

  By the way, in recent imaging apparatuses, an imaging lens and a solid-state imaging device are generally used in combination. However, as the development of solid-state imaging devices has progressed and the number of pixels has increased, it has become a problem that it has not been a problem in the past. In other words, in a device that combines a wide-angle lens that generates large negative distortion as described above and a solid-state image sensor, the resolution at the center of the resulting image is sufficiently high, but the object appears too small at the periphery of the image. Therefore, attention has been paid to the point that sufficient resolution cannot be obtained, and improvement of this point has been desired.

  In addition, as the number of pixels of the solid-state image sensor increases, the demand for improving the image quality of the lens system has become stricter. In addition to distortion, a chromatic aberration of magnification is a major factor in image quality degradation of such a wide-angle imaging lens. As the density of solid-state imaging devices increases and the number of pixels increases, it is necessary to sufficiently correct lateral chromatic aberration.

  Another requirement for an imaging lens used in combination with a solid-state imaging device is to have a sufficiently long back focus. When an imaging lens is used in combination with a solid-state imaging device, an optical low-pass filter, an infrared cut filter, or the like is often disposed between the lens system and the solid-state imaging device, which requires sufficient back focus. . Furthermore, there is a type of endoscope in which the imaging surface of the solid-state imaging device is arranged in parallel with the major axis direction of the insertion portion of the endoscope, and in this type, generally between the imaging lens and the solid-state imaging device. In addition, since an optical path conversion member such as an optical path conversion prism for converting the direction of the optical path is inserted and disposed, it is necessary to ensure a sufficiently long back focus.

  The present invention has been made in view of the above circumstances, and includes an imaging lens capable of maintaining a wide angle and a long back focus and improving the image quality of an image peripheral portion, an imaging device including the imaging lens, and the imaging lens. An object of the present invention is to provide an imaging optical system capable of changing the direction of the optical path between the imaging lens and its image plane.

The imaging lens of the present invention includes, in order from the object side, a meniscus negative first lens having a concave surface facing the image side, and a second lens and a third lens, one of which is positive and the other is negative. A first cemented lens, a positive fourth lens having a plane or a surface with a larger absolute value of the radius of curvature facing the object side, and a fifth lens and a sixth lens, one of which is positive and the other is negative And a second cemented lens in which the second cemented lenses are arrayed, and a stop is disposed between the first cemented lens and the fourth lens, and the following conditional expressions (1) to (3) ) Is satisfied.
0.20 <f / R1 <0.35 (1)
0.14 <R2 / R1 ≦ 0.20 (2)
1.8 <Bf / f (3)
However,
f: focal length of entire system R1: radius of curvature of object side surface of first lens R2: radius of curvature of image side surface of first lens Bf: back focus of entire system

  The above-mentioned “first cemented lens formed by cementing a second lens and a third lens, one of which is positive and the other is negative” is a case where the second lens is a positive lens and the third lens is a negative lens. And the case where the second lens is a negative lens and the third lens is a positive lens, and further means that the second lens is disposed on the object side of the third lens. Is. The same applies to the “second cemented lens formed by cementing the fifth lens and the sixth lens, one of which is positive and the other is negative”.

  It should be noted that the sign and surface shape of each lens of the imaging lens of the present invention described above are in the paraxial region when the lens is an aspheric lens, and are described in the conditional expressions (1) and (2) and below. As for the curvature radius of the surface used in the conditional expression (5), when the surface is an aspheric surface, the paraxial curvature radius is used. Regarding the sign of the radius of curvature, a convex surface on the object side is considered positive and a convex surface on the image side is considered negative. The back focus used in the conditional expression (3) and the conditional expression (5) described below uses an air-converted length.

In the imaging lens of the present invention, it is preferable that the following conditional expression (4) is satisfied.
15.0 <| ν2-ν3 | (4)
However,
ν2: Abbe number of the second lens at the d-line ν3: Abbe number of the third lens at the d-line

ただし、
ν５：第５レンズのｄ線におけるアッベ数
ν６：第６レンズのｄ線におけるアッベ数
ＲＡ：第５レンズと第６レンズの接合面の曲率半径
Ｄ１０：第６レンズの中心厚
Ｎ６：第６レンズのｄ線における屈折率 In the imaging lens of the present invention, it is preferable that the following conditional expression (5) is satisfied.

However,
ν5: Abbe number of the fifth lens at the d-line ν6: Abbe number of the sixth lens at the d-line RA: Radius of curvature of the cemented surface of the fifth lens and the sixth lens D10: Center thickness of the sixth lens N6: Sixth lens Refractive index at d-line

  In the imaging lens of the present invention, it is preferable that the refractive index of all of the first lens, the second lens, and the third lens at the d-line is 1.8 or more.

  An imaging optical system of the present invention includes the imaging lens of the present invention described above, and an optical path conversion member that is disposed between the image plane of the imaging lens and the sixth lens and converts the direction of the optical path. It is characterized by.

  An image pickup apparatus of the present invention includes the above-described image pickup lens of the present invention.

  According to the imaging lens of the present invention, the power and shape of each lens are suitably set to satisfy a predetermined conditional expression, so that a wide angle of view and a long back focus are maintained, and an image peripheral portion is The image quality can be improved. According to the image pickup apparatus of the present invention, since the image pickup lens of the present invention having the above-mentioned advantages is provided, an image with good image quality can be obtained in a wide range of angle of view. Further, according to the imaging optical system of the present invention, since the imaging lens of the present invention and the optical path changing member are provided, it is possible to obtain an image with a good image quality in a wide range of angles of view and the degree of freedom of apparatus layout. For example, an endoscope in which the imaging surface of the solid-state imaging device is arranged in parallel with the major axis direction of the insertion portion of the endoscope can be handled.

The figure which shows the structure of the imaging lens and imaging optical system concerning embodiment of this invention Sectional drawing which shows the structure and optical path of the imaging lens of Example 1 of this invention Sectional drawing which shows the structure and optical path of the imaging lens of Example 2 of this invention Sectional drawing which shows a structure and optical path of the imaging lens of Example 3 of this invention. Sectional drawing which shows the structure and optical path of the imaging lens of Example 4 of this invention Sectional drawing which shows a structure and optical path of the imaging lens of Example 5 of this invention. Sectional drawing which shows a structure and optical path of the imaging lens of Example 6 of this invention. Sectional drawing which shows a structure and optical path of the imaging lens of Example 7 of this invention. Sectional drawing which shows the structure and optical path of the imaging lens of Example 8 of this invention. Sectional drawing which shows a structure and optical path of the imaging lens of Example 9 of this invention. Sectional drawing which shows a structure and optical path of the imaging lens of Example 10 of this invention. Sectional drawing which shows a structure and optical path of the imaging lens of Example 11 of this invention. Sectional drawing which shows a structure and optical path of the imaging lens of Example 12 of this invention. Sectional drawing which shows a structure and optical path of the imaging lens of Example 13 of this invention. FIG. 4 is each aberration diagram of the imaging lens of Example 1 of the present invention, (A) is a spherical aberration diagram, (B) is an astigmatism diagram, (C) is a distortion aberration diagram, and (D) is a chromatic aberration diagram of magnification. FIG. 4 is each aberration diagram of the imaging lens of Example 2 of the present invention, (A) is a spherical aberration diagram, (B) is an astigmatism diagram, (C) is a distortion aberration diagram, and (D) is a chromatic aberration diagram of magnification. FIG. 4 is each aberration diagram of the imaging lens of Example 3 of the present invention, (A) is a spherical aberration diagram, (B) is an astigmatism diagram, (C) is a distortion aberration diagram, and (D) is a chromatic aberration diagram of magnification. FIG. 6A is an aberration diagram of the imaging lens of Example 4 of the present invention, where FIG. 6A is a spherical aberration diagram, FIG. 5B is an astigmatism diagram, FIG. C is a distortion aberration diagram, and FIG. FIG. 6A is an aberration diagram of the imaging lens according to the fifth exemplary embodiment of the present invention. FIG. 6A is a spherical aberration diagram, FIG. 5B is an astigmatism diagram, FIG. C is a distortion aberration diagram, and FIG. FIG. 9A is an aberration diagram of the imaging lens of Example 6 of the present invention, where FIG. 9A is a spherical aberration diagram, FIG. 9B is an astigmatism diagram, FIG. C is a distortion aberration diagram, and FIG. FIGS. 9A and 9B are aberration diagrams of the image pickup lens of Example 7 of the present invention, where FIG. 9A is a spherical aberration diagram, FIG. 9B is an astigmatism diagram, FIG. C is a distortion aberration diagram, and FIG. It is each aberration figure of the imaging lens of Example 8 of this invention, (A) is a spherical aberration figure, (B) is an astigmatism figure, (C) is a distortion aberration figure, (D) is a magnification chromatic aberration figure. FIG. 10 is aberration diagrams of the image pickup lens of Example 9 of the present invention, where (A) is a spherical aberration diagram, (B) is an astigmatism diagram, (C) is a distortion aberration diagram, and (D) is a chromatic aberration diagram of magnification. FIG. 11A is a diagram illustrating aberrations of the imaging lens according to the tenth embodiment of the present invention, where FIG. 9A is a spherical aberration diagram, FIG. 9B is an astigmatism diagram, FIG. C is a distortion diagram, and FIG. FIG. 14A is a diagram illustrating aberrations of the imaging lens according to the eleventh embodiment of the present invention, in which (A) is a spherical aberration diagram, (B) is an astigmatism diagram, (C) is a distortion diagram, and (D) is a chromatic aberration diagram of magnification. FIG. 14 is each aberration diagram of the imaging lens of Example 12 of the present invention, (A) is a spherical aberration diagram, (B) is an astigmatism diagram, (C) is a distortion aberration diagram, and (D) is a chromatic aberration diagram of magnification. FIG. 14A is a diagram illustrating aberrations of the imaging lens according to the thirteenth embodiment of the present invention, in which (A) is a spherical aberration diagram, (B) is an astigmatism diagram, (C) is a distortion diagram, and (D) is a chromatic aberration diagram of magnification. Sectional drawing which shows the structure and optical path of the imaging lens of a comparative example It is each aberration figure of the imaging lens of a comparative example, (A) is a spherical aberration figure, (B) is an astigmatism figure, (C) is a distortion aberration figure, (D) is a magnification chromatic aberration figure. Diagram for comparing values of sin θ and tan θ with respect to half angle of view θ The figure which shows schematic structure of the endoscope concerning embodiment of this invention. The figure for demonstrating arrangement | positioning of the vehicle-mounted imaging device concerning embodiment of this invention

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a cross-sectional view of a cross section including an optical axis Z of an imaging optical system 10 according to an embodiment of the present invention. The imaging optical system 10 includes an imaging lens 1 according to an embodiment of the present invention and an optical path conversion member 2 for converting the direction of the optical path.

  The optical path conversion member 2 shown in FIG. 1 is composed of a prism having a reflecting surface. In FIG. 1, the left side of the imaging lens 1 is the object side, and the optical axis Z is indicated by a one-dot chain line. In the imaging optical system 10, light from an object passes through the imaging lens 1, then enters from one surface of the optical path conversion member 2 facing the imaging lens 1, and is a reflection surface formed obliquely with respect to the incident surface. The light path is reflected and bent vertically, and is incident on the solid-state imaging device 3 bonded to the exit surface formed perpendicular to the entrance surface. In the imaging optical system 10 shown in FIG. 1, the imaging lens 1 is arranged so that the image plane of the imaging lens 1 coincides with the imaging plane of the solid-state imaging device 3. In the example shown in FIG. 1, the direction of the optical path is converted substantially vertically by the optical path conversion member 2, but the conversion direction of the optical path of the imaging optical system of the present invention is not necessarily limited to this example, and is arbitrarily set. Is possible. Further, the optical path changing member can be constituted by another optical member such as a mirror or a diffractive optical element.

  Next, a detailed configuration of the imaging lens 1 will be described. The imaging lens 1 has a four-group, six-element configuration, and in order from the object side, a negative first meniscus lens L1 having a concave surface directed toward the image side, and a second lens L2 in which one is positive and the other is negative. And a first cemented lens LC1 formed by cementing the third lens L3 and a positive fourth lens L4 having a plane or a surface with a larger absolute value of the radius of curvature facing the object side, either one is positive A second cemented lens LC2 formed by cementing a negative fifth lens L5 and a sixth lens L6 on the other side is arranged.

  FIG. 1 shows an example in which a negative second lens L2 and a positive third lens L3 are sequentially arranged from the object side and cemented as the cemented lens LC1, but instead a positive second lens L1 is used. The lens L2 and the negative third lens L3 may be arranged in order from the object side and joined. Similarly, FIG. 1 shows an example in which a positive fifth lens L5 and a negative sixth lens L6 are sequentially arranged from the object side and cemented as the cemented lens LC2, but instead, a negative first lens L5 is joined. A configuration may be adopted in which the five lenses L5 and the positive sixth lens L6 are arranged in order from the object side and joined. However, as for the cemented lens LC2, as in the example of FIG. 1, it is more effective to correct the chromatic aberration of magnification by arranging the positive lens and the negative lens in this order from the object side.

  An aperture stop St is disposed between the first cemented lens LC1 and the fourth lens L4. Note that the aperture stop in FIG. 1 does not indicate the shape or size, but indicates the position on the optical axis Z. The imaging lens 1 has a configuration advantageous for correcting chromatic aberration of magnification by disposing a cemented lens composed of positive and negative lenses on both the object side and the image side of the aperture stop St.

The imaging lens 1 is configured to satisfy the following conditional expressions (1) to (3).
0.20 <f / R1 <0.35 (1)
0.14 <R2 / R1 ≦ 0.20 (2)
1.8 <Bf / f (3)
However,
f: focal length of entire system R1: radius of curvature of object-side surface of first lens L1 R2: radius of curvature of image-side surface of first lens L1 Bf: back focus of entire system (air equivalent length)

  Conditional expressions (1) to (3) indicate conditions for favorably correcting distortion while maintaining a wide angle of view and a long back focus. In particular, conditional expressions (1) and (2) The power and shape conditions of the first lens L1 for this purpose are shown.

  If the upper limit of conditional expression (1) is exceeded, it will be difficult to obtain a sufficiently long back focus. If the lower limit of conditional expression (1) is not reached, the effect of suppressing distortion will be weakened and the resolution of the image periphery cannot be improved, making it difficult to meet the demands associated with the recent increase in the number of pixels in solid-state imaging devices. .

  If the upper limit of conditional expression (2) is exceeded, a wide angle of view cannot be obtained. If the lower limit of conditional expression (2) is not reached, the effect of suppressing distortion will be weakened, and the concave surface of the image side surface of the first lens L1 will become deep, and the workability will deteriorate.

  When the object side surface or the image side surface of the first lens L1 is an aspherical surface, a paraxial radius of curvature is used for R1 or R2. In terms of aberration correction, an aspherical surface is more advantageous than a spherical surface, but a spherical surface is preferable to an aspherical surface in terms of productivity and cost. For this reason, the imaging lens 1 of the present embodiment suppresses distortion aberration while ensuring a wide field angle and a practically sufficient back focus even when the first lens L1 is configured as a spherical lens. Therefore, the radius of curvature is selected so as to satisfy the conditional expressions (1) and (2).

  Conditional expression (3) is an expression relating to the back focus. By configuring so as to satisfy the conditional expression (3), it is possible to secure a long back focus as compared with the focal length, and it is possible to arrange various filters, the optical path conversion member 2 and the like between the imaging lens and the image plane. Become. Specifically, for example, a prism having a thickness as shown in FIG.

The imaging lens 1 preferably satisfies the following conditional expression (4).
15.0 <| ν2-ν3 | (4)
However,
ν2: Abbe number of the second lens L2 at the d-line ν3: Abbe number of the third lens L3 at the d-line

  Conditional expression (4) defines the difference in the Abbe number of the materials constituting the first cemented lens LC1 composed of one positive lens and one negative lens on the object side with respect to the aperture stop St. This is a condition necessary for correcting chromatic aberration and axial chromatic aberration. In this lens system, it is preferable that the Abbe number of the material of the negative lens constituting the first cemented lens LC1 is larger than the Abbe number of the material of the positive lens constituting the first cemented lens LC1.

ただし、
ｆ：全系の焦点距離
ν５：第５レンズＬ５のｄ線におけるアッベ数
ν６：第６レンズＬ６のｄ線におけるアッベ数
ＲＡ：第５レンズＬ５と第６レンズＬ６の接合面の曲率半径
Ｂｆ：全系のバックフォーカス（空気換算長）
Ｄ１０：第６レンズＬ６の中心厚
Ｎ６：第６レンズＬ６のｄ線における屈折率 The imaging lens 1 preferably satisfies the following conditional expression (5).

However,
f: focal length ν5 of the entire system: Abbe number ν6 at the d-line of the fifth lens L5: Abbe number RA at the d-line of the sixth lens L6: radius of curvature Bf of the cemented surface between the fifth lens L5 and the sixth lens L6: Back focus of the entire system (air equivalent length)
D10: Center thickness of the sixth lens L6 N6: Refractive index of the sixth lens L6 at the d-line

Conditional expression (5) indicates that the fifth lens L5 and the sixth lens L2 in the second cemented lens LC2 including the fifth lens L5 and the sixth lens L6 that constitute a part of the rear group focusing system on the image side from the aperture stop St. Focusing on the difference between the Abbe number of the lens L6 and the cemented surface, the preferred degree of correction of the lateral chromatic aberration is shown. Conditional expression (5) can be modified as the following expression (5A).

  As can be seen from conditional expression (5A), the right side of conditional expression (5) is the first term consisting of the difference between the Abbe numbers of the fifth lens L5 and the sixth lens L6, and the fifth lens L5 and the sixth lens L6. The sum of the second term obtained by normalizing the absolute value of the radius of curvature of the cemented surface with the focal length and the back focus of the entire system and the air-converted length on the optical axis of the sixth lens L6, that is, the fifth lens L5 The distance from the cemented surface of the sixth lens L6 to the imaging position can be divided into the third term normalized by the focal length.

  These first to third terms indicate three conditions that are advantageous for correcting the lateral chromatic aberration. The correction of the lateral chromatic aberration is such that the difference between the Abbe numbers of the two positive and negative lenses constituting the second cemented lens LC2 (first term) is large and the absolute value of the radius of curvature of the cemented surface (second term) is small. The shorter the distance (third term) from the joint surface to the imaging position, the more advantageous. If the lower limit of conditional expression (5) is not reached, it will be difficult to satisfactorily maintain lateral chromatic aberration while keeping the back focus longer.

  Further, in the imaging lens 1, the larger the dispersion of the negative lens constituting the second cemented lens LC2, the more advantageous for correcting the lateral chromatic aberration. Therefore, the Abbe number of the negative lens constituting the second cemented lens LC2 is 20 The following is desirable.

  Normally, in an imaging lens with insufficient correction of chromatic aberration, the focal length at the short wavelength is shorter than the focal length at the long wavelength, so both the longitudinal chromatic aberration and the lateral chromatic aberration have the short wavelength side aberration of the reference wavelength. Compared to minus (under). When correcting under magnification chromatic aberration, it is preferable that the Abbe number of the positive lens is large and the Abbe number of the negative lens is small on the image side from the aperture stop St.

  Further, in order to correct the lateral chromatic aberration, an optical member responsible for correcting the lateral chromatic aberration is disposed at a position distant from the aperture stop St. In particular, on the image side from the aperture stop St, the optical member is disposed at a position close to the imaging plane. However, in the lens system having a long back focus of the entire system, the optical member cannot be disposed at a position close to the image plane and correction of the lateral chromatic aberration is not easy. However, according to the imaging lens of the present embodiment, by adopting the above preferable configuration, it is possible to achieve both long back focus, for example, back focus longer than 1.8 times the focal length, and good correction of chromatic aberration of magnification. It becomes easy.

  Since many imaging lenses for endoscopes have a large F number in order to increase the depth of field, spherical aberration and coma are rarely important factors in determining image quality. A major factor is lateral chromatic aberration. Since the lateral chromatic aberration appears more prominently toward the image peripheral portion, it is very effective to correct the lateral chromatic aberration well in order to improve the image quality of the image peripheral portion.

  Further, in the imaging lens 1, the refractive indexes of all the lenses located on the object side of the aperture stop St, that is, the first lens L1, the second lens L2, and the third lens L3, in the d-line are 1.8 or more. It is preferable. Selecting such a material is advantageous for widening the angle and correcting distortion.

  When the imaging lens 1 is mounted on an imaging device such as an endoscope or a vehicle-mounted camera without a protective member, the first lens L1 arranged closest to the object side is used for body fluid, cleaning fluid, direct sunlight, wind and rain, oils and fats, etc. Will be exposed. Therefore, it is preferable to use a material having high water resistance, weather resistance, acid resistance, chemical resistance and the like as the material of the first lens L1. As the material of the first lens L1, for example, a material having a powder water resistance, a powder acid resistance standard weight loss rate rank, and a surface method weather resistance rank of 1 defined by the Japan Optical Glass Industry Association is preferably used.

  Next, numerical examples of the imaging lens of the present invention will be described. Lens cross-sectional views of the imaging lenses of Examples 1 to 13 are shown in FIGS. 2 to 14, the left side of the figure is the object side, the right side is the image side, and the on-axis light beam 4 and the off-axis light beam corresponding to the maximum image height from the object-side surface of the first lens L1 to the image surface. The optical path of 5 is also shown. 2 to 9 and FIGS. 11 to 13 also show a parallel plane plate PP that assumes an optical path conversion member, various filters, a cover glass, and the like, and FIGS. 10 and 14 assume various filters, a cover glass, and the like. The parallel plane plate PPa is also shown. The position of the image plane of each imaging lens is the position of the image side surface of the plane parallel plate PP and the position of the image side surface of the plane parallel plate PPa. In FIGS. 2 to 9 and FIGS. 11 to 13, for the sake of simplicity, the parallel plane plate PP is illustrated by developing the optical system so that the optical path from the imaging lens to the image plane is in a straight line.

  Lens data of the imaging lenses of Examples 1 to 13 are shown in Tables 1 to 13, respectively. In the lens data table of each example, the Si column indicates the i-th (i = 1, 2, 3,...) Surface number that sequentially increases toward the image side with the most object-side component surface as the first surface. The Ri column indicates the radius of curvature of the i-th surface, the Di column indicates the surface spacing on the optical axis Z between the i-th surface and the i + 1-th surface, and the Ndj column is the most object side. Indicates the refractive index for the d-line (wavelength 587.6 nm) of the j-th (j = 1, 2, 3,...) Optical element that increases sequentially toward the image side with the optical element being first, and the column of νdj is j The Abbe number with respect to d line of the 2nd optical element is shown. The lens data includes the aperture stop St and the plane parallel plate PP, and the rightmost column of the surface corresponding to the aperture stop St describes the diameter of the aperture stop.

  The sign of the radius of curvature is positive when convex on the object side and negative when convex on the image side. “Mm” is used as the unit of the radius of curvature and the surface interval in the lens data, but this is an example, and the optical system can obtain the same optical performance even when proportionally enlarged or reduced. Appropriate units can also be used.

  It should be noted that “R1”, “R2”, “ν2”, “ν3”, “ν5”, “ν6”, “RA”, “D10”, and “N6” in the above conditional expressions are “R1” in the lens data, respectively. ”,“ R2 ”,“ νd2 ”,“ νd3 ”,“ νd5 ”,“ νd6 ”,“ R10 ”,“ D10 ”, and“ Nd6 ”.

  FIGS. 15A to 15D show aberration diagrams of the spherical aberration, astigmatism, distortion (distortion), and chromatic aberration of magnification (chromatic aberration of magnification) of the imaging lens of Example 1, respectively. The aberration diagrams for spherical aberration, astigmatism, and distortion show aberrations with the d-line (wavelength 587.6 nm) as the reference wavelength, while the spherical aberration diagram shows the F-line (wavelength 486.1 nm) and C-line. The aberration for (wavelength 656.3 nm) is also shown. The lateral chromatic aberration diagram shows aberrations for the F-line and the C-line. Fno. Of spherical aberration diagram. Means F value, and ω in other aberration diagrams means half angle of view.

  Similarly, FIGS. 16 (A) to 16 (D), FIGS. 17 (A) to 17 (D), FIGS. 18 (A) to 18 (D), and FIGS. 19 (A) to 19 (D). 20 (A) to 20 (D), 21 (A) to 21 (D), 22 (A) to 22 (D), 23 (A) to 23 (D), FIG. 24 (A) to FIG. 24 (D), FIG. 25 (A) to FIG. 25 (D), FIG. 26 (A) to FIG. 26 (D), FIG. 27 (A) to FIG. The aberration diagrams of spherical aberration, astigmatism, distortion (distortion), and chromatic aberration of magnification (chromatic aberration of magnification) of the imaging lenses 2 to 13 are shown. As can be seen from the respective aberration diagrams, in the first to thirteenth embodiments, each aberration is corrected well.

  As a comparative example, FIG. 28 shows a lens cross-sectional view of an example of a conventional imaging lens. This comparative example corresponds to the imaging lens of Example 1 of Patent Document 1. The imaging lens of the comparative example shown in FIG. 28 includes, in order from the object side, a negative first lens L1 ′, a cemented lens formed by cementing a negative second lens L2 ′ and a positive third lens L3 ′, and a positive lens. The fourth lens L4 ′ and the cemented lens formed by cementing the positive fifth lens L5 ′ and the negative sixth lens L6 ′ are arranged in a four-group six-lens configuration, and the third lens L3 ′ and the third lens L3 ′ are arranged. An aperture stop St is disposed between the four lenses L4 ′. FIG. 28 also shows the optical paths of the parallel plane plate PP and the on-axis light beam 4 from the object-side surface of the first lens L1 'to the image plane and the off-axis light beam 5 corresponding to the maximum image height.

  Table 14 shows lens data of this comparative example. The meanings of the symbols in Table 14 are the same as those in the lens data of Examples 1 to 13 described above.

  FIGS. 29A to 29D show aberration diagrams of spherical aberration, astigmatism, distortion (distortion), and chromatic aberration of magnification (chromatic aberration of magnification) of this comparative example. The methods shown in FIGS. 29A to 29D are the same as those shown in the aberration diagrams of Examples 1 to 13 described above.

  Tables 15 and 16 show various data of Examples 1 to 13 and the comparative example. The data in Tables 15 and 16 are for the d-line, the unit of length is all mm, and the unit of angle is all degrees.

  Below, the words and phrases described in Tables 15 and 16 will be described together. “Number of components” is the number of lenses and the number of lenses constituting the entire system. “Front group cementing” indicates the power sign and arrangement order of the two lenses constituting the first cemented lens LC1 closer to the object side than the aperture stop St. For example, “concave / convex joining” refers to the negative lens and the positive lens in order from the object side. This means a cemented lens in which lenses are arranged and cemented, and “convex / concave cementing” means a cemented lens in which a positive lens and a negative lens are arranged in order from the object side. The “parallel plane plate thickness” is the thickness of the parallel plane plate PP in the optical axis direction.

  “Object distance” is the distance in the optical axis direction from the lens surface closest to the object to the object. “Object surface radius of curvature” is the radius of curvature of the object surface. “Maximum image height” is the maximum image height. “Viewing angle (degree)” is a viewing angle at all angles of view, and is expressed by 2ω when ω is used as the above-mentioned symbol. “Effective F value” is an effective F value (effective F number). “Aperture stop diameter” is the diameter of the aperture stop St.

  As shown in Tables 15 and 16, Examples 1, 2, 3, 4, 5, 6, 7, 8, 9 and comparative examples are placed at a position 15 mm from the surface vertex of the first lens L1 on the object side. Using the spherical surface with a radius of 15 mm as the object surface, the maximum image height is 1.75 mm, and aberrations are calculated. Then, on the image side of the lens systems of Examples 1, 2, 3, 4, 5, 6, 7, and 8 and the comparative example, the refractive index at the d-line is 1.51680, and the thickness in the optical axis direction is 4.0 mm. The parallel plane plate PP is inserted and the final plane is used as the imaging plane, and the back focus is a value excluding the plane parallel plate PP, that is, an air equivalent value. In Example 9, the plane parallel plate PP has a thickness of 0.2 mm.

  In Examples 10, 11, 12, and 13, a spherical surface with a radius of 10 mm placed at a position 10 mm from the surface apex of the first lens L1 is used as the object surface, and the maximum image height is 1.45 mm. I am doing. Then, on the image side of the lens systems of Examples 10, 11, and 12, a parallel plane plate PP having a refractive index of 1.51680 in the d-line and a thickness of 3.0 mm in the optical axis direction is inserted, and the final surface thereof is As the imaging plane, the back focus is a value excluding the plane parallel plate PP, that is, an air equivalent value. In Example 13, the thickness of the plane parallel plate PPa is 0.2 mm.

  In the comparative example, a spherical surface with a radius of 15 mm placed at a position 15 mm from the surface apex of the first lens L <b> 1 ′ is used as an object surface, and aberrations are calculated with a maximum image height of 1.008 mm. Then, on the image side of the lens system of the comparative example, a parallel plane plate PP having a refractive index of 1.51633 at the d-line and a thickness of 3.0 mm in the optical axis direction is inserted, and its final surface is used as an imaging surface. The back focus is a value obtained by removing the parallel flat plate PP from the parallel flat plate, that is, an air equivalent value.

  In the columns below “Effective F value” in Tables 15 and 16, the values used in the conditional expressions (1) to (5) and the corresponding values of the conditional expressions are shown. Examples 1 to 13 all satisfy conditional expressions (1) to (5), but the comparative example does not satisfy conditional expressions (1), (2), and (5).

  Next, FIGS. 15 (C), 16 (C), 17 (C), 18 (C), 19 (C), 20 (C), 21 (C), and 22 (C). 23 (C), FIG. 24 (C), FIG. 25 (C), FIG. 26 (C), FIG. 27 (C), and FIG. 13 and the distortion in the comparative example are compared. The distortion of the comparative example is almost zero near the 80% image height and has a large negative value at the maximum image height, whereas the distortion aberrations of Examples 1 to 13 are compared near the 80% image height. It takes a small positive value and keeps the positive value almost at the maximum image height, or takes a smaller positive value or a smaller negative value. From this, compared to the comparative example in which the object in the image peripheral portion looks too small and the resolving power is insufficient, Examples 1 to 13 have improved distortion aberration and the resolving power is improved in the image peripheral portion. I understand that.

  Due to the difference in characteristics, if the conditions are the same image size and the same angle of view, the embodiment of the present application has a shorter focal length than the comparative example, and the same F value and the same permissible circle of confusion diameter. If it calculates, the Example of this application can also show the effect that the depth of field can be deepened rather than a comparative example.

  Note that the distortion diagrams of Examples 1 to 13 and Comparative Example of the present invention shown in FIGS. 15 to 27 and 29 both adopt the same projection method, and the focal length f and the half angle of view of the entire system. When θ (variable treatment, 0 ≦ θ ≦ ω) is used and the size of the ideal image height is f × sin θ, the deviation from the ideal image height is shown. The reason for adopting such an ideal image height is to make it easy to recognize the difference between the distortion aberration of Examples 1 to 13 and the distortion aberration of the comparative example at 80% image height. This point will be described below.

In general, distortion Dis is the ideal image height Y _0, represented by the following formula by using the actual image height Y.
Dis = 100 × (Y−Y ₀ ) / Y ₀

Usually, in a general imaging lens system, Y ₀ = −f × tan θ is used as the ideal image height Y ₀ . However, in a wide-angle lens system, θ takes a large value, and θ> 60 ° may be obtained. In such a large θ range, as shown in FIG. 30, the rate of change of tan θ with respect to θ is very large, and the value of tan θ is large.

  In FIG. 30, the horizontal axis represents the half angle of view θ, and the vertical axis represents the value of tan θ or sin θ. Since θ is a half angle of view, θ shown in FIG. 30 is shown only in the angle range of 0 ° to 90 °, and the following description of the distortion aberration diagram is considered only in this angle range. In FIG. 30, in order to help understanding, the range of 60 ° to 70 ° that is the peripheral portion of the image assumed by the imaging lens of the present embodiment is hatched.

The values of tan θ when θ is 60 ° and 70 ° are tan 60 ° = 1.732 and tan 70 ° = 2.747, respectively. For this reason, when −f × tan θ is used as the ideal image height Y ₀ , the distortion aberrations of Examples 1 to 13 and the comparative example both exceed −50% in the image peripheral portion where θ is large. It is difficult to recognize the difference between Examples 1 to 13 and the comparative example.

On the other hand, if the ideal image height Y ₀ is Y ₀ = −f × sin θ, the rate of change of sin θ with respect to θ is small in the range of θ> 60 ° as compared with tan θ as shown in FIG. Thus, the value of sin θ is small. The values of sin θ when θ is 60 ° and 70 ° are sin 60 ° = 0.866 and sin 70 ° = 0.940, respectively. Therefore, when −f × sin θ is used as the ideal image height Y ₀ , the difference in distortion between Examples 1 to 13 and the comparative example becomes remarkable in the peripheral portion of the image where θ is large, and these differences are easily recognized. Become.

  Next, an embodiment of an imaging apparatus to which the imaging lens of the present invention is applied will be described with reference to FIGS. FIG. 31 is a schematic configuration diagram of an endoscope. The endoscope 100 shown in FIG. 31 mainly includes an operation unit 102, an insertion unit 104, and a connector unit (not shown) for pulling out the universal cord 106. An insertion portion 104 to be inserted into the patient's body is connected to the distal end side of the operation portion 102. From the proximal end side of the operation portion 102, a universal cord for connecting to a connector portion for connecting to a light source device or the like. 106 is pulled out.

  Most of the insertion portion 104 is a soft portion 107 that bends in an arbitrary direction along the insertion path. 110 are sequentially connected. The bending portion 108 is provided to direct the distal end hard portion 110 in a desired direction, and the bending operation can be performed by rotating the bending scanning knob 109 provided in the operation portion 102. The imaging optical system of this embodiment is disposed inside the distal end hard portion 110.

  FIG. 32 shows a state where the vehicle-mounted camera of the present embodiment is mounted on the automobile 200. In FIG. 32, an automobile 200 has an on-vehicle camera 201 for imaging the blind spot range on the side surface on the passenger seat side, an on-vehicle camera 202 for imaging the blind spot range on the rear side of the automobile 200, and the rear surface of the rearview mirror. An in-vehicle camera 203 is mounted for photographing the same field of view as the driver. The outside camera 201, the outside camera 202, and the inside camera 203 are imaging devices according to the present embodiment, and convert an imaging lens according to the embodiment of the present invention and an optical image formed by the imaging lens into an electrical signal. And an image sensor.

  The present invention has been described with reference to the embodiments and examples. However, the present invention is not limited to the above embodiments and examples, and various modifications can be made. For example, the values of the radius of curvature, the surface spacing, the refractive index, the Abbe number, etc. of each lens component are not limited to the values shown in the above numerical examples, but can take other values.

  Further, in the embodiment of the imaging device, the example of the endoscope and the vehicle-mounted camera has been described with reference to the drawings. However, the present invention is not limited to this application, for example, a mobile terminal camera, a monitoring camera, or the like It is also applicable to.

DESCRIPTION OF SYMBOLS 1 Imaging lens 2 Optical path conversion member 3 Solid-state image sensor 4 On-axis light beam 5 Off-axis light beam 10 Imaging optical system 100 Endoscope 102 Operation part 104 Insertion part 106 Universal code 107 Soft part 108 Curved part 109 Curved scanning knob 110 Tip hard part 200 Automotive 201, 202 Outside camera 203 Inside camera L1 1st lens L2 2nd lens L3 3rd lens L4 4th lens L5 5th lens L6 6th lens LC1 1st cemented lens LC2 2nd cemented lens PP, PPa optics Member St Aperture stop Z Optical axis

Claims (6)

A first meniscus negative first lens having a concave surface directed toward the image side in order from the object side, and a first cemented lens formed by cementing a second lens and a third lens, one of which is positive and the other is negative A second lens formed by bonding a positive fourth lens having a plane or a surface having a larger absolute value of the radius of curvature toward the object side, and a fifth lens and a sixth lens, one of which is positive and the other is negative. And a cemented lens is arranged between the first cemented lens and the fourth lens, and the following conditional expressions (1) to (3) are satisfied. A characteristic imaging lens.
0.20 <f / R1 <0.35 (1)
0.14 <R2 / R1 ≦ 0.20 (2)
1.8 <Bf / f (3)
However,
f: focal length of entire system R1: radius of curvature of object side surface of the first lens R2: radius of curvature of image side surface of the first lens Bf: back focus of the entire system The imaging lens according to claim 1, wherein the following conditional expression (4) is satisfied.
15.0 <| ν2-ν3 | (4)
However,
ν2: Abbe number of the second lens at the d-line ν3: Abbe number of the third lens at the d-line The imaging lens according to claim 1, wherein the following conditional expression (5) is satisfied.

However,
ν5: Abbe number at the d-line of the fifth lens ν6: Abbe number at the d-line of the sixth lens RA: radius of curvature of the cemented surface of the fifth lens and the sixth lens D10: center thickness of the sixth lens N6: Refractive index at the d-line of the sixth lens   4. The imaging lens according to claim 1, wherein a refractive index in d-line of all of the first lens, the second lens, and the third lens is 1.8 or more. 5. The imaging lens according to any one of claims 1 to 4,
An imaging optical system comprising: an optical path conversion member that is disposed between an image plane of the imaging lens and the sixth lens and converts the direction of the optical path.   An imaging apparatus comprising the imaging lens according to claim 1.

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